What is absolute atomic mass in chemistry. Relative atomic and molecular masses

What a molecule consists of and find their relative atomic masses in the periodic system chemical elements. If one atom occurs n times, multiply it mass for this number. Then add the found values and get the molecular mass given substances, which is equal to its molar mass in g/mol. Find mass one by dividing the molar mass substances M by the Avogadro constant NА=6.022∙10^23 1/mol, m0=M/NA.

Example Find mass one molecules water. The water molecule (H2O) consists of two hydrogen atoms and one oxygen atom. The relative atomic mass of hydrogen is 1, for two atoms we get the number 2, and the atomic mass of oxygen is 16. Then molar mass water will be equal to 2+16=18 g/mol. Determine mass one molecules: m0=18/(6.022^23)≈3∙10^(-23)

mass molecules can be calculated if the number of molecules in a given substance is known. To do this, divide the total mass substances m by the number of particles N (m0=m/N). For example, if it is known that in 240 g substances contains 6∙10^24 molecules, then the mass of one molecules will be m0=240/(6∙10^24)=4∙10^(-23) g.

Determine mass one molecules substances with sufficient accuracy, knowing the number and neutrons that are included in its nuclei of atoms of which it consists. The mass of the electron shell and the mass defect in this case should be neglected. Take the mass and equal to 1.67∙10^(-24) g. For example, if it is known that a molecule consists of two oxygen atoms, what is its mass? The nucleus of an oxygen atom has 8 protons and 8 neutrons. Total nucleons 8+8=16. Then the mass of the atom is 16∙1.67∙10^(-24)=2.672∙10^(-23) g. Since the molecule consists of two atoms, its mass is 2∙2.672∙10^(-23)=5.344 ∙10^(-23)

You can calculate the mass of any molecule, knowing its chemical formula. For example, we calculate the relative mamolecular mass of an alcohol molecule.

You will need

periodic table

Instruction

Consider the chemical formula of the molecule. Determine the atoms of which chemical elements are included in its composition.

The alcohol formula is C2H5OH. An alcohol molecule consists of 2 atoms, 6 hydrogen atoms and 1 oxygen atom.

If the mass of a molecule is in grams rather than in atomic mass units, remember that one atomic mass unit is the mass of 1/12 carbon atom. Numerically 1 a.m.u. \u003d 1.66 * 10 ^ -27 kg.

Then the mass of an alcohol molecule is 46*1.66*10^-27 kg = 7.636*10^-26 kg.

note

In Mendeleev's periodic table, the chemical elements are arranged in order of increasing atomic mass. Experimental methods for determining the molecular weight have been developed mainly for solutions of substances and for gases. There is also a method of mass spectrometry. The concept of molecular weight is of great practical importance for polymers. Polymers are substances consisting of repeating groups of atoms, but the number of these groups is not the same, so for polymers there is the concept of average molecular weight. By average Molecular weight refers to the degree of polymerization of a substance.

Helpful advice

Molecular weight is an important quantity for physicists and chemists. Knowing the molecular weight of a substance, you can immediately determine the density of the gas, find out the molarity of the substance in solution, determine the composition and formula of the substance.

Sources:

Molecular mass
how to calculate the mass of a molecule

Molecular weight is the molecular weight, which can also be called the value of the mass of a molecule. The molecular weight is expressed in atomic mass units. If we disassemble the value of the molecular weight in parts, it turns out that the sum of the masses of all the atoms that make up the molecule is its molecular weight. mass. If we talk about units of mass, then mostly all measurements are made in grams.

Instruction

Molecular weight itself is related to the concept of a molecule. But it cannot be said that this condition can be applied only to those where the molecule, for example, hydrogen, is located separately. For cases where the molecules are not separate from the rest, but in close relationship, all of the above conditions and definitions are also valid.

To start, to define mass hydrogen, you will need - either, in which hydrogen is composed and from which it can be easily isolated. This can be some kind of alcohol solution or another mixture, some of the components of which, under certain conditions, change their state and easily free the solution from its presence. Find a solution from which you can vaporize the necessary or unnecessary substances by heating. This is the most easy way. Now decide whether you will evaporate a substance that you do not need or whether it will be hydrogen, molecular mass which you plan to measure. If an unnecessary substance evaporates, it's okay that it is not toxic. in the case of the evaporation of the desired substance, you need equipment so that all the evaporation is preserved in the flask.

After you have separated everything unnecessary from the composition, proceed to measurements. For this, Avogadro's number will suit you. It is with its help that you can calculate the relative atomic and molecular mass hydrogen. Find all the options you need hydrogen which are present in any table, determine the density of the resulting gas, as it will come in handy for one of the formulas. Then substitute all the results obtained and, if necessary, change the unit of measurement to , as already mentioned above.

The concept of molecular weight is most relevant when it comes to polymers. It is for them that it is more important to introduce the concept of average molecular weight, due to the heterogeneity of the molecules that make up their composition. Also, by the average molecular weight, one can judge how high the degree of polymerization of a particular substance is.

Related videos

Weight substances is found using a device called a balance. You can also calculate mass body, if known. number substances and its molar mass or its density and volume. The amount of pure substances You can find it by its mass or the number of molecules it contains.

§6.

^ Absolute molecular weight – mass of the molecule expressed in mass units: g, kg . It is denoted by m M (X), where X is the formula of the substance. For example, the mass of an oxygen molecule is

M M (O 2) \u003d 53.2 10 -24 g \u003d 53.2 10 -27 kg.

^ About

The relative molecular weight is molecular weight ratio substances to atomic mass isotope carbon With mass number 12(
). It is denoted by Mr(X), where X is the formula of the substance.

Relative molecular weight shows how many times the mass of a molecule is greater the mass of a carbon atom. For example, the mass of a water molecule m M (H 2 O) \u003d 28.95 10 -24 g.

The relative molecular weight of a water molecule H 2 O is equal to the ratio of the mass of the H 2 O molecule to the value of the atomic mass unit:

Mr(H 2 O) = 18. The mass of a water molecule is 18 times greater than the mass of a carbon atom.

A molecule is made up of atoms.

Relative molecular weight is equal to the sum of the relative atomic masses of the elements that make up the molecule.

For example, the relative molecular mass of water is equal to the sum of the relative atomic masses of hydrogen and oxygen:

Key words and terms

^ Note!

1) what equals what

Relative molecular weight is equal to the sum of the relative atomic masses of the elements that make up the molecule.

test questions

What is absolute molecular weight?
What is called relative molecular weight?
What does relative molecular weight show?
What is the relative molecular weight?

Calculate the relative molecular masses of substances:

a) H 2 SO 4;

D) Fe 2 (SO 4) 3;

E) Ca 3 (PO 4) 2;

I) Al(OH) 3 .

§ 7. Moth. Molar mass

A substance is characterized by mass (m), measured in kg (g, mg), volume (V), measured in m 3 (l, ml), and the number of particles contained in it. Chemists use for calculations physical quantity- amount of substance.

^ Amount of substance - this is number of structural particles (molecules, atoms, ions and others) given substance .

The amount of a substance is denoted (X) (υ - read “nu”), or n (X) (n - read “en”), where X is the formula of the particle.

The unit for measuring the amount of a substance is the mole.

^ Mol – the amount of substance that contains so many particles (atoms, molecules and other particles), how many atoms are there in 12 g of carbon .

Let us calculate how many carbon atoms are contained in carbon weighing 12 g. To do this, we divide the mass of 12 g by the mass of one carbon atom, equal to 19.93 10 -24 g.

12 g / mol: 19.93 10 -24 g \u003d 6.02 10 23 1 / mol.

Number 6.02 10 23 mole -1 called constant Avogadro and denote N A . The Avogadro constant N A shows the number of structural particles in 1 mol of a substance.

^ Mole is the amount of substance that contains 6.02 10 23 structural particles (atoms, molecules, ions or others).

Figure 4

1 mol of water H 2 O contains 6.02 10 23 water molecules;

1 mole of O 2 oxygen molecules contains 6.02 10 23 O 2 oxygen molecules;

1 mol of SO 4 2- ions contains 6.02 10 23 SO 4 2- ions;

1 mol of carbon atoms C contains 6.02 10 23 carbon atoms;

1 mol of CO 2 molecules contains 6.02 10 23 CO 2 molecules (Fig. 5)

About 2 CO 2

O 2 O 2 CO 2 CO 2

Figure 5

Mass 1 mol, or 6.02 10 23 molecules, called molar mass of the substance , weight 6.02 10 23 atoms - the molar mass of the element, mass 6.02 10 23 ions - molar mass of ions .

Molar mass - this attitude masses of matter m(X) to his quantity υ(X).

The mass of a substance m is measured in kilograms (or grams), the amount of a substance υ is measured in moles. The molar mass of a substance M is expressed in kilograms per mole (kg/mol) or grams per mole (g/mol).

The molar mass of atoms is numerically equal to the relative atomic mass of the element, and the molar mass of molecules is numerically equal to the relative molecular mass of the substance(Table 5).

For example:

the mass of 1 mol of carbon atoms is 12 g/mol;

the molar mass of NaOH is 40 g/mol;

the mass of 1 mol of iron atoms is 56 g/mol

Figure 6

Table 5 - Numeric value molar masses of some substances

^ Amount of substance (υ(X), n(X)) can be calculated if known weight substances and molar mass .

Example1. How much substance is contained in 54 g of water?

M
Decision:

M(H) = 1 g/mol, M(O) = 16 g/mol,

M (H 2 O) \u003d 1 2 + 16 \u003d 18 g / mol,

.
(H 2 O) \u003d 54 g

υ(H 2 O) \u003d?

Answer: 54 g of water contains 3 moles of water.

mass substances are calculated if known number and his molar mass.

Example 2. Determine the mass of 5 mol KI

D
Decision:

1) M(K) = 39 g/mol, M(I) = 127 g/mol,

M(KI) = 39 + 127 = 166 g/mol;

2) m(KI) = υ(KI) M(KI) = 5 mol 166 g/mol = 830 g.
ano:

Answer: The mass of 5 moles of KI is 830 g.

^ Massu one atom or one molecules substances can be calculated by dividing it molar mass per Avogadro number.

Example 3. What is the mass of a chlorine Cl 2 molecule?

D
Decision:

M (Cl) \u003d 35.5 g / mol, M (Cl 2) \u003d 2 35.5 \u003d 71 g / mol;

2) m M (Cl 2) =

N A \u003d 6.02 10 23 mol -1

m M (Cl 2) \u003d?

Answer: The mass of a chlorine molecule is 11.79 10 -23 g.

Number (amount) of structural particles N(X) of this substance is calculated by the formula

Example 4. How many molecules are contained in 0.3 mol of nitrogen?

D
Decision:

N A \u003d 6.02 10 23 mol -1,

N (N 2) \u003d υ (N 2) N A \u003d 0.3 mol 6.02 10 23 mol -1 \u003d 1.8 10 23.
ano:

υ (N 2) \u003d 0.3 mol

Answer: 0.3 mole of nitrogen contains 1.8 10 23 molecules.

Example 5. How many nitrogen atoms are there in 0.3 mole of nitrogen?

D
Decision:

N(N) = υ(N) N A .

1 mol of N 2 molecules contains 2 mol of nitrogen atoms N, hence

υ (N) \u003d 2υ (N 2) \u003d 2 0.3 mol \u003d 0.6 mol.

N(N) \u003d 0.6 mol 6.02 10 23 mol -1 \u003d 36.12 10 22.
ano:

υ (N 2) \u003d 0.3 mol

Answer: 0.3 mole of nitrogen contains 36.12 10 22 nitrogen atoms.

Example 6. What mass of ammonia NH 3 contains as many molecules as there are in water H 2 O weighing 54 g?

D
Decision:

1) M (H 2 O) \u003d 18 g / mol, M (NH 3) \u003d 17 g / mol;

2)
;

3) υ (NH 3) = υ (H 2 O) = 3 mol;

4) m (NH 3) \u003d υ (NH 3) M (NH 3) \u003d 3 mol 17 g / mol \u003d 51 g.
ano:

m(H 2 O) = 54g

υ (NH 3) \u003d υ (H 2 O)

Answer: Ammonia with a mass of 51 g contains as many molecules as there are in water with a mass of 54 g.

Key words and terms

Russians	English	French	Arabic
to measure	to measure	mesurer	يقيس
And he	ion	ion	أيون
number	quantity	quantity	كمية
mole	mole	mole	مول
constant	constant	constant	ثابت؛دائم؛مستمر
divide	to divide	divisor, separator	يقسم
chemical formula	chemical formula	formula chimique	قانون؛
number	number	nombre	رقم ؛عدد

^ Note!

1) what contains what

mole contains 6.02 10 23 particles.

2) as much as

A mole contains as many particles as there are atoms in 12 g of carbon.

3) what expressed what

Molar mass of a substance expressed in grams per mol (g/mol).

test questions

What is the amount of substance?
In what units is the amount of a substance expressed?
What is a mole?
What does the Avogadro constant show?
What is called molar mass? In what units is molar mass expressed?
How is it calculated: a) molar mass; b) the amount of substance;

c) the mass of the substance; d) mass of a molecule, atom; e) the number of atoms,

Molecules?

Tasks for independent work

1. Calculate the molar masses of substances: a) I 2, b) O 3, c) P 2 O 5, d) HCl,

E) Cl 2, f) H 3 PO 4, g) NH 4 NO 3, h) Mg (NO 3) 2.

2. How much substance is contained: a) in sulfuric acid H 2 SO 4

Weighing 9.8 g; b) in KOH weighing 11.2 g; c) in iron weighing 0.56 g?

3. Calculate the mass of: a) molecular hydrogen H 2 by the amount

Substances 2 mol; b) atomic oxygen by the amount of substance

3 mol; c) water in the amount of substance 0.3 mol.

4. Calculate the mass of molecules: a) O 3, b) O 2, c) H 2 SO 4.

5. How many molecules are contained: a) in ammonia NH 3 weighing 3.4 g; b) c

Hydrogen H 2 weighing 4 g; c) in sulfuric acid H 2 SO 4 weighing 49 g?

6. How many atoms of all elements are contained: a) in ammonia NH 3

Weighing 3.4 g; b) in hydrogen H 2 weighing 4 g; c) in sulfuric acid H 2 SO 4

Weighing 49 g?

7. What mass of hydrogen chloride HCl contains as many molecules as

Them in water weighing 49 g?

8. What mass of hydrogen H 2 contains as many atoms as there are

Sere S weighing 6.4 g?

§ 8. Chemical formulas. Mass fraction of a substance.

Calculations by chemical formulas

The composition of a substance is expressed using chemical formulas.

^ Chemical formula - it's conditional writing the composition of a substance using chemical symbols and (if needed) indexes.

We read: “five-ash-two-o”.

Denoting: five molecules of water.

index (shows the number of atoms of a given element in a molecule)

coefficient (shows number of molecules)

Chemical formula shows:

Qualitative composition (what elements does matter consist of);
Quantitative composition (how many atoms of each element are included in the molecule);
One molecule of a substance .

For example, the formula H 2 SO 4 (ash-two-es-o-four) shows:

the sulfuric acid molecule consists of hydrogen, sulfur and oxygen atoms;
the molecule contains two hydrogen atoms, one sulfur atom and four oxygen atoms;
one molecule of sulfuric acid;
H 2 SO 4 is a complex substance, because it consists of atoms of different chemical elements.

Formula O 3 (o-three) shows:

the ozone molecule is made up of oxygen atoms;
the molecule contains three oxygen atoms;
one ozone molecule;
O 3 is a simple substance, because it consists of atoms of one element.

By chemical formula can calculate:

the relative molecular weight of the substance;
the mass fraction of each element in the substance (in fractions of a unit or in percent).

Mass fraction of a substance- it's an attitude masses given substances in the system to the mass of the whole system .

where ω(X) (ω - read "omega") - mass fraction of substance X; m(X) is the mass of substance X; m is the mass of the entire system.

^ Mass fraction of the element - it's an attitude total atomic mass of an element to relative molecular weight .

where n is the number of element atoms; Ar is the relative atomic mass of the element;

Mr is the relative molecular weight.

The mass fraction is expressed in fractions of a unit or as a percentage.

Example 1 Calculate the relative molecular weight of CaCO 3 calcium carbonate. Determine the mass fraction of each element in CaCO 3.

D
Decision:

Ar(Ca) = 40, Ar(O) = 16, Ar(C) = 12

Mr(CaCO 3) \u003d 40 + 12 + 3 16 \u003d 100

ano:

ω
or 40%;
(Ca) = ?

or 12%;

or 48%.

Answer: Mr(CaCO 3) = 100; mass fraction of calcium 0.4; carbon - 0.12;

Oxygen - 0.48.

Key words and terms

Russians	English	French	Arabic
calculate	to calculate	calculator	يحسب؛يعد
Share	part, portion	part, party	جزء
quality	qualitative	qualified	نوعي؛ذو علاقة بالنوع
quantitative	quantitative	quantitatif	كمي؛مقداري
mass	mass	masse	كتلي
definition	definition	definition	تعريف؛تحديد
percentage	percentage	pourcentage	نسبة مئوية
system	system	systeme	نظام
formula	formula	formula	قانون؛

^ Note!

1) what express (depict) With using what

The composition of a substance is shown by using chemical

formulas.

2) what shows what

Chemical formula shows molecular composition.

3) what contains what

Molecule contains two hydrogen atoms, one sulfur atom and

four oxygen atoms.

4) part what included what

Part molecules are included three oxygen atoms.

5) why ( dative) can + infinitive + what

According to the chemical formula can be calculated relative

molecular weight.

test questions

What is called chemical formula?
What shows: a) chemical formula; b) index;

c) coefficient?

What is mass fraction?

Tasks for independent work

1. Write formulas:

Sodium - two - o

Potassium - two - es

Ash - en - o - three

Ash - two - es - o - four

Aluminum - two - es - o - four - three times

Ferum - oh - ash - three times

Zinc - o - ash - twice

Manganese - about

2. Read and write down the name of the formulas:

P2O5

BaSO4

3. Write the formulas of substances that contain: a) one atom

Sulfur and three oxygen atoms; b) two sodium atoms and one sulfur atom;

C) two hydrogen atoms, one sulfur atom and three oxygen atoms;

D) one lead atom, two nitrogen atoms and six oxygen atoms;

D) one calcium atom and two chlorine atoms.

4. Designate chemical symbols or formulas: a) two atoms

Sulfur; b) three nitrogen atoms; c) seven water molecules; d) one chlorine atom;

E) five copper atoms; e) three molecules of sulfuric acid.

5. Calculate the relative molecular weights of substances: a) H 3 AsO 4 ;

B) MgCl 2 ; c) Fe 2 (SO 4) 3; d) Al 2 O 3; e) Ca 3 (PO 4) 2. Determine the mass

The proportion of each element in these substances.

atomic mass is the sum of the masses of all protons, neutrons and electrons that make up an atom or molecule. Compared to protons and neutrons, the mass of electrons is very small, so it is not taken into account in the calculations. Although this is incorrect from a formal point of view, it is often this term used to denote the average atomic mass of all isotopes of an element. In fact, this is the relative atomic mass, also called atomic weight element. Atomic weight is the average of the atomic masses of all naturally occurring isotopes of an element. Chemists must distinguish between these two types of atomic mass when doing their job - an incorrect value for atomic mass can, for example, lead to an incorrect result for the yield of a reaction product.

Steps

Finding the atomic mass according to the periodic table of elements

Learn how atomic mass is written. Atomic mass, that is, the mass of a given atom or molecule, can be expressed in standard SI units - grams, kilograms, and so on. However, due to the fact that atomic masses expressed in these units are extremely small, they are often written in unified atomic mass units, or a.u.m. for short. are atomic mass units. One atomic mass unit is equal to 1/12 the mass of the standard carbon-12 isotope.

atomic unit mass characterizes the mass one mole of the given element in grams. This value is very useful in practical calculations, since it can be used to easily convert the mass of a given number of atoms or molecules of a given substance into moles, and vice versa.

Find the atomic mass in Mendeleev's periodic table. Most standard periodic tables contain the atomic masses (atomic weights) of each element. As a rule, they are given as a number at the bottom of the cell with the element, under the letters denoting the chemical element. This is usually not an integer, but a decimal.

Remember that the periodic table shows the average atomic masses of the elements. As noted earlier, the relative atomic masses given for each element in the periodic table are the averages of the masses of all the isotopes of an atom. This average value is valuable for many practical purposes: for example, it is used in calculating the molar mass of molecules consisting of several atoms. However, when you are dealing with individual atoms, this value is usually not enough.
- Since the average atomic mass is an average of several isotopes, the value given in the periodic table is not accurate the value of the atomic mass of any single atom.
- The atomic masses of individual atoms must be calculated taking into account the exact number of protons and neutrons in a single atom.

Calculation of the atomic mass of an individual atom

Find the atomic number of a given element or its isotope. The atomic number is the number of protons in an element's atoms and never changes. For example, all hydrogen atoms, and only they have one proton. Sodium has an atomic number of 11 because it has eleven protons, while oxygen has an atomic number of eight because it has eight protons. You can find the atomic number of any element in the periodic table of Mendeleev - in almost all of its standard versions, this number is indicated above the letter designation of the chemical element. The atomic number is always a positive integer.
- Suppose we are interested in a carbon atom. There are always six protons in carbon atoms, so we know that its atomic number is 6. In addition, we see that in the periodic table, at the top of the cell with carbon (C) is the number "6", indicating that the atomic carbon number is six.
- Note that the atomic number of an element is not uniquely related to its relative atomic mass in the periodic table. Although, especially for the elements at the top of the table, it may seem that the atomic mass of an element is twice its atomic number, it is never calculated by multiplying the atomic number by two.
Find the number of neutrons in the nucleus. The number of neutrons can be different for different atoms of the same element. When two atoms of the same element with the same number of protons have different amount neutrons, they are different isotopes of this element. Unlike the number of protons, which never changes, the number of neutrons in the atoms of a particular element can often change, so the average atomic mass of an element is written as a decimal fraction between two adjacent whole numbers.

Add up the number of protons and neutrons. This will be the atomic mass of this atom. Ignore the number of electrons that surround the nucleus - their total mass is extremely small, so they have little to no effect on your calculations.

Calculating the relative atomic mass (atomic weight) of an element

Determine which isotopes are in the sample. Chemists often determine the ratio of isotopes in a particular sample using a special instrument called a mass spectrometer. However, during training, this data will be provided to you in the conditions of tasks, control, and so on in the form of values taken from the scientific literature.
- In our case, let's say that we are dealing with two isotopes: carbon-12 and carbon-13.
Determine the relative abundance of each isotope in the sample. For each element various isotopes meet in different ratios. These ratios are almost always expressed as a percentage. Some isotopes are very common, while others are very rare—sometimes so rare that they are difficult to detect. These values can be determined using mass spectrometry or found in a reference book.
- Assume that the concentration of carbon-12 is 99% and carbon-13 is 1%. Other isotopes of carbon really exist, but in quantities so small that in this case they can be neglected.
Multiply the atomic mass of each isotope by its concentration in the sample. Multiply the atomic mass of each isotope by its percentage(expressed as a decimal). To convert percentages to decimal, just divide them by 100. The resulting concentrations should always add up to 1.
- Our sample contains carbon-12 and carbon-13. If carbon-12 is 99% of the sample and carbon-13 is 1%, then multiply 12 (atomic mass of carbon-12) by 0.99 and 13 (atomic mass of carbon-13) by 0.01.
- The handbooks give percentages, based on the known quantities of all the isotopes of an element. Most chemistry textbooks include this information in a table at the end of the book. For the sample under study, the relative concentrations of isotopes can also be determined using a mass spectrometer.
Add up the results. Sum the multiplication results you got in the previous step. As a result of this operation, you will find the relative atomic mass of your element - the average value of the atomic masses of the isotopes of the element in question. When an element is considered as a whole, and not a specific isotope of a given element, it is this value that is used.
- In our example, 12 x 0.99 = 11.88 for carbon-12, and 13 x 0.01 = 0.13 for carbon-13. The relative atomic mass in our case is 11.88 + 0.13 = 12,01 .

Some isotopes are less stable than others: they decay into atoms of elements with fewer protons and neutrons in the nucleus, releasing particles that make up atomic nucleus. Such isotopes are called radioactive.

Absolute mass molecules of substance B can be calculated by the equation

Absolute masses of atoms and molecules. Atomic mass unit. Relative atomic mass. Relative molecular weight and its calculation.

Task 5. Determine the absolute mass (gPsch) of a water molecule.

It is easy to replace the absolute masses of molecules in terms of relative molecular masses (see , 3, Ch. I). The molecular weight of the first gas is

Calculate the absolute mass of one molecule of Br3, Oj, NH3, H2SO4, H2O, I2.

Based on the molar mass and the Avogadro number, one can calculate the absolute masses of atoms and molecules using the following formula-

Answer The absolute mass of a water molecule is ZX X 10-" g \u003d 3-10- kg.

The number of molecules in one mole of a substance, called the Avogadro number, Nf, = 6.0240-Yu. Dividing the mass of one mole of any substance by the Avogadro number, we get the absolute mass of the molecule in grams. For example, the mass of a molecule is Hg 2.016 6.02-10 = 3.35-10 "g. Similarly, the absolute mass of an atom is calculated. Molecules have a diameter of approximately one to tens of angstroms (1 A = 10" cm).

Depending on the size and shape of the unit cell, as well as possible sizes and symmetries of the molecule decide how many molecules can fit in a given unit cell. When solving this problem, one always takes into account the rule that molecules are closely packed in a crystal, i.e., the protrusions of one molecule enter the depressions of another, etc. (Fig. 16). Thus, the shape of the elementary cell often makes it possible to judge about general form molecules. The absolute mass of a molecule (from which it is easy to calculate the molecular mass) based on X-ray diffraction data is determined as follows

Knowing the Avogadro number, it is easy to find the absolute mass of a particle of any substance. Indeed, the mass in grams of a molecule (atom) of a substance is equal to the molar mass divided by the Avogadro number. For example, the absolute mass of a hydrogen atom (the molar mass of hydrogen atoms is 1.008 g / mol) is 1.67-10-g. It is approximately as many times less than the mass of a small pellet, how many times the mass of a person is less than the mass of the entire globe ..

In this way, one can calculate the absolute masses of molecules and atoms of other elements. Since these quantities are negligible and inconvenient for calculations, they use the concept of atomic (molecular) weight, which corresponds to the mass of atoms (molecules), expressed in relative units. Per atomic mass unit (a.m.u.)

The number of molecules in 1 mol of a substance, called the Avogadro constant VA, is 6.0220-10. By dividing the mass of 1 mole of any substance by the Avogadro constant, we get the absolute mass of the molecule / ly in grams. For example, the mass of the molecule H 2.016 6.02-10 3 \u003d 3.35-g. Similarly, the absolute mass of an atom is calculated. The molecules have a diameter of about 0.1 to 1 nm.

How the absolute mass of atoms and molecules is calculated Calculate the absolute masses of a copper atom and a molecule of hydrogen phosphide.

The kinetic energy e of two molecules with masses W] and W2 can be expressed as through their common absolute speeds C and Cr in space, and through the components of these velocities

Calculation of absolute masses and volumes of atoms and molecules

The quotient of dividing the absolute mass of a molecule of a compound or element by one twelfth of the absolute mass of an atom of a carbon isotope. The sum of the atomic masses of all the elements of a molecule.

The masses of other atoms, as well as molecules (absolute molecular mass is denoted by tm), turn out to be just as extremely small, for example, the mass of a water molecule is

According to the chemical formula gaseous substance it is possible to determine some of its quantitative characteristics percentage composition, molecular weight, density, relative density for any gas, the absolute mass of the molecule.

Test questions. 1. What is an atom molecule atomic weight molecular weight mass of an atom mass of a molecule gram-atom gram-molecule 2. What is the molecular weight of CO2 and the absolute mass of a COa molecule, expressed in grams any gas at normal conditions 5. What is the Avogadro number What is it equal to 6. According to the formula of acetylene CsHa

For example, a relative molecular weight of water of 18 (rounded) means that a water molecule is 18 times heavier than 12 parts of the absolute mass of a carbon atom.

Define the concepts a) element, atom, molecule b) simple and complex substance c) relative atomic and molecular masses, absolute masses of an atom and a molecule. What should be understood by the conditional particle UCH

Even much earlier, in the second half of the 19th century, the first attempts were made to approach the question of the absolute mass and size of atoms and molecules. Although it is obviously impossible to weigh a single molecule, the theory opened up another way - it was necessary to somehow determine the number of particles in a mole of molecules or atoms - the so-called Avogadro number (La). It is just as impossible to directly count the molecules as it is to weigh them, but the Avogadro number is included in many equations of various departments of physics, and it can be calculated from these equations. Obviously, if the results of such calculations, performed in several independent ways, coincide, then this can serve as proof of the correctness of the found value.

Since the absolute masses of atoms and molecules are small, relative masses are usually used.

The kinetic energy of two molecules with masses and can be expressed in terms of the velocity components or in terms of the absolute velocities themselves as follows

As you know, heat is a measure of the kinetic energy of the particles that form a given substance. It has been found that at a temperature much higher than the temperature absolute zero, average kinetic energy molecules is proportional absolute temperature T. For a molecule with mass m and average speed and

Example 8 Calculate the absolute mass of a sulfuric acid molecule in grams.

All compounds under study are subdivided into a training array containing molecules with known properties and a predictable group of molecules. The analyzed learning array for the studied property is divided into two alternative groups (active - inactive). The created models represent equations of the logical form L = 7 (3), where L is the activity, (8) is the decisive set of features (CRF) - the complex of fragments structural formulas and their various combinations, the so-called sub-structural descriptors. The assessment of the effect of fragments and their combinations on activity is carried out on the basis of the information content coefficient, which varies from minus 1 to plus 1. The higher absolute value information content, the higher the probability of the influence of this feature on the properties. A plus sign characterizes a positive effect, a minus characterizes a negative one. P is an algorithm by which the properties of the studied substances are recognized. Two algorithms are used in the forecasting process - geometry (I) and voting (II). The first of them is based on determining the distance in the Euclidean metric between the substance under study and the calculated hypothetical standard of the property under study. The second method involves the analysis of the number of features (votes) in the structure of compounds, with positive and negative informativeness. Molecular design procedures are described further in Section 5.

The relative molecular weight Mr is the ratio of the absolute mass of a molecule to Vi2 of the mass of an atom of a carbon isotope. Note that relative masses are, by definition, dimensionless quantities.

Becker nozzle. Various kinetic methods for solving the problem of isotope separation can be classified into methods that use the difference in transfer coefficients for molecules of different masses, and methods that use the movement of a separated mixture in a potential field. The most characteristic method of the second class is precisely the gas centrifuge method, which, however, requires very impressive development work even for a laboratory demonstration of its grandiose capabilities, due to the absolute engineering non-standard nature of the gas centrifuge. Proposed, presumably by Dirac, at about the same time as the gas centrifuge method, the separation nozzle method (Becker nozzles, after the leader of the first successful experimental work)

Atoms of elements and molecules of substances are characterized by a certain physical (absolute) mass m, for example, the mass of a hydrogen atom H is 1.67-g, the mass of the P4 molecule is 2.06-10 g, the mass of the H,0 molecule is 2.99-10 g, the mass of the molecule H2804 1.63 K) d. The absolute masses of atoms of elements and molecules of substances are extremely small, and it is inconvenient to use such values. Therefore, the concept of the relative mass of atoms and molecules was introduced.

Relative molecular weight chemical compound- an atom number showing how many times the absolute mass of one molecule of an atom compound is greater than an atomic unit of mass.

Determination of the absolute masses of atoms (as well as the masses of molecules and their fragments) mass spectroscopy.

Of great value is the determination of the absolute mass of the contents of one elementary cell crystal structure. The dimensions of the unit cell can be measured, if necessary, with very high precision(the error is less than 0.01%). It is more difficult to measure densities, but the total measurement error can be up to 0.1% of the unit cell mass (without too much experimental work). In addition to determining the absolute mass of a cell, information on the possible content of a cell can be obtained from crystal structures in another way. space group symmetries, the nature and variety of equivalent acceptable node positions, and the basic requirements that the intensity of the observed X-ray reflections must match, within acceptable limits, the intensity calculated for the proposed crystal structure all this gives a certain amount of information that must be consistent with some putative chemical formula. So, regardless of the presence of other molecules, 46 water molecules per unit should be included in any formula. cell structure hydrates of type I. If the dimensions of the unit cell

The Avogadro number is the number of molecules in a gram-molecule of any substance. This value can be determined various methods, while the results obtained different ways, coincide within the measurement accuracy. Currently, the value of Avogadro's number is taken to be 6.023-10. Avogadro's number is a universal constant; it does not depend on the nature of the substance and its state of aggregation. To calculate the absolute mass of an atom or molecule, you need to divide the gram-atomic or gram-molecular mass by the Avogadro number. For example,

One of the most important properties substance is its molecular weight. Since the absolute masses of molecules are very small, relative masses are used in the calculations. The molecular weight of a substance is usually understood as the decrease in the mass of a molecule of a given substance to 1/12 of the mass of a carbon atom. Accordingly, the masses of atoms of chemical elements are also compared with 1/12 of the mass of a carbon atom. Then the atomic mass of carbon is 12, other elements (rounded) hydrogen - 1, oxygen-16, nitrogen-14. The mass of a molecule of a chemical compound is determined by adding the atomic masses of the elements that make up the molecule. For example, the molecular weight carbon dioxide CO2 is 12 + 2-16 = 44 (1 carbon atom with mass 12 and 2 oxygen atoms with mass 16). The molecular weight of methane CH is 12 + 4-1 = 16. The molecular weight of some of the most commonly used combustible gases and their combustion products is given in Table. 1.1.

Of course, states II and III are not absolutely stable, and as a result of thermal motion, fluctuations around these positions or even rotations can take place. As the temperature rises, the relative number of molecules in the mass of a substance that do not correspond to the most stable state increases, but cannot exceed the number of molecules in the ground state.

Dalton did not see a qualitative difference between simple and complex atoms, therefore, did not recognize two steps (atoms and molecules) in the structure of matter. In this sense, Dalton's atomism was a step backwards in comparison with Lomonosov's elemental-corpuscular concept. However, the rational grain of Dalton's atomistics was his doctrine of the mass of atoms. Considering quite correctly that the absolute masses of atoms are extremely small, Dalton proposed to determine the relative atomic masses. In this case, the mass of the hydrogen atom, as the lightest of all atoms, was taken as unity. Thus, for the first time, Dalton defined the atomic mass of an element as the ratio of the mass of an atom of a given element to the mass of a hydrogen atom. He also compiled the first table of atomic masses of 14 elements. Dalton's doctrine of atomic masses played an invaluable role in the transformation of chemistry into a scientific science and the discovery Periodic Law. therefore

It is necessary to distinguish between the concepts of the absolute mass of a molecule and the gram-molecule. So, 10 grams of water molecules is 18 X 10 \u003d 180g, that is, approximately a glass of water, and 10 water molecules is a negligible amount that cannot be weighed.

What are molecular. mass of CO2 absolute mass of a CO2 molecule, expressed in primes

On the basis of the experiments carried out, a clear relationship was established between the absolute mass of diffused amino acid molecules and their molecular weights.

See pages where the term is mentioned Molecule absolute mass: Basics general chemistry Volume 2 Edition 3 (1973) -- [

Relative atomic mass

Atoms of elements are characterized by a certain (only their inherent) mass. For example, the mass of an H atom is 1.67 . 10 −23 g, C atom − 1.995 . 10 −23 g, O atom − 2.66 . 10 −23

It is inconvenient to use such small values, so the concept of relative atomic mass AND r is the ratio of the mass of an atom of a given element to an atomic mass unit (1.6605 . 10 −24 g).

Molecule - smallest particle substances that keep Chemical properties this substance. All molecules are built from atoms and are therefore also electrically neutral.

The composition of the molecule is transferred molecular formula , which reflects both the qualitative composition of the substance (symbols of the chemical elements included in its molecule) and its quantitative composition (lower numerical indices corresponding to the number of atoms of each element in the molecule).

Mass of atoms and molecules

To measure the masses of atoms and molecules in physics and chemistry, it is accepted one system measurements. These quantities are measured in relative units.

The atomic mass unit (a.m.u.) is equal to 1/12 of the mass m carbon atom 12 C ( m one atom 12 C is equal to 1.993×10 -26 kg).

Relative atomic mass of an element (A r) is a dimensionless quantity equal to the ratio medium weight of an element atom to 1/12 of the mass of a 12 C atom. When calculating the relative atomic mass, the isotopic composition of the element is taken into account. Quantities A r determined according to the table D.I. Mendeleev

Absolute mass of an atom (m) is equal to the relative atomic mass multiplied by 1 a.m.u. For example, for a hydrogen atom, the absolute mass is defined as follows:

m(H) = 1.008×1.661×10 -27 kg = 1.674×10 -27 kg

Relative molecular weight of the compound (M r) is a dimensionless quantity equal to the ratio of the mass m molecules of a substance to 1/12 of the mass of an atom 12 C:

The relative molecular weight is equal to the sum relative masses atoms that make up the molecule. For example:

M r(C 2 H 6) \u003d 2H A r(C) + 6H A r(H) = 2×12 + 6 = 30.

The absolute mass of a molecule is equal to the relative molecular mass times 1 amu.

2. What is called the molar mass of the equivalent?

con equivalents discovered by Richter in 1791. Atoms of elements interact with each other in strictly defined ratios - equivalents.

In SI, the equivalent is the 1/z part of the (imaginary) particle X. X is an atom, molecule, ion, etc. Z- is equal to the number the number of protons that the X particle binds or donates (neutralization equivalent) or the number of electrons that the X particle donates or accepts (oxidation-reduction equivalent) or the charge of the X ion (ion equivalent).

The molar mass of the equivalent, the dimension is g / mol, is the ratio of the molar mass of the particle X to the number Z.

For example, the molar mass of an element's equivalent is determined by the ratio of the molar mass of an element to its valency.

The law of equivalents: The masses of the reactants are related to each other as the molar masses of their equivalents.

mathematical expression

where m 1 and m 2 are the masses of the reactants,

Molar masses of their equivalents.

If the reacting portion of a substance is characterized not by mass, but by volume V(x), then in the expression of the law of equivalents its molar mass of the equivalent is replaced by the molar volume of the equivalent.

3. What are the basic laws of chemistry?

Basic laws of chemistry. The law of conservation of mass and energy was formulated by M. V. Lomonosov in 1748. The mass of substances involved in chemical reactions does not change. In 1905, Einstein believed that the relationship between energy and mass

E \u003d m × c 2, c \u003d 3 × 10 8 m / s

Mass and energy are properties of matter. Mass is a measure of energy. Energy is a measure of motion, so they are not equivalent and do not turn into each other, however, whenever the energy of the body changes E, its mass changes m. Perceptible mass changes occur in nuclear chemistry.

From the point of view of atomic-molecular theory, atoms having constant mass do not disappear and do not arise from nothing, this leads to the conservation of the mass of substances. The law has been proven experimentally. Based on this law, chemical equations. Quantitative calculations using reaction equations are called stoichiometric calculations. The basis of all quantitative calculations is the law of conservation of mass, and therefore, it is possible to plan and control production.

4. What are the main classes inorganic compounds exist? Give a definition, give examples.

Simple substances. Molecules are made up of atoms of the same kind (atoms of the same element). In chemical reactions, they cannot decompose to form other substances.

Complex substances (or chemical compounds). Molecules are made up of atoms different kind(atoms of various chemical elements). In chemical reactions, they decompose to form several other substances.

There is no sharp boundary between metals and non-metals, because there are simple substances that exhibit dual properties.

5. What are the main types of chemical reactions?

Exist great multitude various chemical reactions and several ways to classify them. Most often, chemical reactions are classified according to the number and composition of the reactants and reaction products. According to this classification, four types of chemical reactions are distinguished - these are reactions of combination, decomposition, substitution, exchange.

Connection reaction is a reaction in which the reactants are two or more simple or complex substances, and the product is one complex substance. Examples of compound reactions:

Oxide formation from simple substances- C + O 2 \u003d CO 2, 2Mg + O 2 \u003d 2MgO

The interaction of a metal with a non-metal and obtaining a salt - 2Fe + 3Cl 2 \u003d 2FeCl 3

Interaction of oxide with water - CaO + H 2 O \u003d Ca (OH) 2

decomposition reaction A reaction in which the reactant is one complex substance and the product is two or more simple or complex substances. Most often, decomposition reactions proceed when heated. Examples of decomposition reactions:

Chalk decomposition when heated: CaCO 3 \u003d CaO + CO 2

The decomposition of water under the action electric current: 2H 2 O \u003d 2H 2 + O 2

Decomposition of mercury oxide when heated - 2HgO = 2Hg + O 2

substitution reaction- this is a reaction, the reactants of which are simple and complex substances, and the products are also simple and complex substances, but the atoms of one of the elements in the complex substance are replaced by atoms of a simple reagent. Examples:

Substitution of hydrogen in acids - Zn + H 2 SO 4 \u003d ZnSO 4 + H 2

Displacement of metal from salt - Fe + CuSO 4 \u003d FeSO 4 + Cu

Alkali formation - 2Na + 2H 2 O \u003d 2NaOH + H 2

Exchange reaction- this is a reaction, the reactants and products of which are two complex substances, during the reaction, the reactants exchange their constituent parts, resulting in other complex substances. Examples:

The interaction of salt with acid: FeS + 2HCl \u003d FeCl 2 + H 2 S

Interaction of two salts: 2K 3 PO 4 + 3MgSO 4 = Mg 3 (PO 4) 2 + 3K 2 SO 4

There are chemical reactions that cannot be attributed to any of the listed types.

6. By whom, when and by what experiments was the nucleus of the atom discovered and the nuclear model of the atom created?

Nuclear model of the atom. One of the first models of the structure of the atom was proposed by the English physicist E. Rutherford. In experiments on the scattering of a-particles, it was shown that almost the entire mass of an atom is concentrated in a very small volume - a positively charged nucleus. According to Rutherford's model, electrons move continuously around the nucleus at a relatively large distance, and their number is such that, as a whole, the atom is electrically neutral. Later, the presence in the atom of a heavy nucleus surrounded by electrons was confirmed by other scientists. The first attempt to create a model of the atom based on the accumulated experimental data (1903) belongs to J. Thomson. He believed that the atom is an electrically neutral system spherical shape with a radius of approximately 10–10 m. positive charge atom is evenly distributed throughout the volume of the ball, and negatively charged electrons are inside it (Fig. 6.1.1). To explain the line emission spectra of atoms, Thomson tried to determine the location of electrons in an atom and calculate the frequencies of their oscillations around equilibrium positions. However, these attempts were not successful. A few years later, in the experiments of the great English physics E. Rutherford proved that Thomson's model is incorrect.

7. What new did N. Bohr introduce in the concept of the atom? Give summary Bohr's postulates as applied to the hydrogen atom.

Bohr's theory for the hydrogen atom

Following the Bohr theory for the hydrogen atom, Sommerfeld proposed such a quantization rule that, when applied to the hydrogen atom, the Bohr model does not contradict wave nature electron postulated by de Broglie. Derive an expression for the energy levels of the hydrogen atom using the Sommerfeld rule, according to which the allowed electron orbitals are circles with a length that is a multiple of the electron wavelength.

Since the quantum numbers I, m do not contribute anything to the energy of the electronic state, then all possible states in a given radial level are energetically equal. This means that only single lines will be observed in the spectrum, such as Bohr predicted. However, it is well known that in the spectrum of hydrogen there is fine structure, the study of which was the impetus for the development of the Bohr-Sommerfeld theory for the hydrogen atom. It's obvious that simple form wave equation does not quite adequately describe the hydrogen atom, and thus we are in-position, only slightly best addition when based on the Bohr model of the atom.

8. What is determined and what values \u200b\u200bcan have: the main quantum number n, secondary (orbital) - l, magnetic - m l and spin - m s?

Quantum new numbers.

1. Principal quantum number, n– accepts integer values from 1 to ¥ (n=1 2 3 4 5 6 7…) or letters (K L M N O P Q).

max value n corresponds to the number of energy levels in the atom and corresponds to the period number in the table of D.I. Mendeleev, characterizes the value of the electron energy, the size of the orbital. An element with n=3 has 3 energy levels, is in the third period, has a larger electron cloud and energy than an element with n=1.

2. Orbital quantum number l takes values depending on the principal quantum number and has corresponding literal meanings.

l=0, 1, 2, 3… n-1

l - characterizes the shape of the orbitals:

Orbitals with the same value n, but with different values l differ somewhat in energy, i.e. the levels are divided into sublevels.

The number of possible sublevels is equal to the main quantum number.

3. Magnetic quantum number m l takes values from -l,…0…,+l.

The number of possible values of the magnetic quantum number determines the number of orbitals of a given type. Within each level there can only be:

one s is an orbital, because m l=0 for l=0

three p - orbitals, m l= -1 0 +1, with l=1

five d orbitals m l=-2 –1 0 +1 +2, with l=2

seven f orbitals.

The magnetic quantum number determines the orientation of the orbitals in space.

4. Spin quantum number (spin), m s.

Spin characterizes magnetic moment electron, due to the rotation of the electron around own axis clockwise and counterclockwise.

By denoting an electron with an arrow, and an orbital with a dash or a cell, you can show

Rules characterizing the order in which orbitals are filled.

Pauli principle:

ll n 2, and at the levels - 2n 2

n+l), if equal, with n- the least.

Gund's rule

9. How does Bohr's theory explain the origin and line structure atomic spectra?

N. Bohr's theory was proposed in 1913, it used Rutherford's planetary model and Planck-Einstein's quantum theory. Planck believed that along with the limit of divisibility of matter - an atom, there is a limit of divisibility of energy - a quantum. Atoms do not radiate energy continuously, but in certain portions of quanta

The first postulate of N. Bohr: there are strictly defined allowed, so-called stationary orbits; being on which the electron does not absorb and does not radiate energy. Allowed are only those orbits for which the angular momentum equal to the product m e ×V×r, can change in certain portions (quanta), i.e. is quantized.

The state of an atom with n=1 is called normal, with n=2.3… - excited.

The speed of the electron decreases with increasing radius, the kinetic and total energy increases.

Bohr's second postulate: when moving from one orbit to another, an electron absorbs or emits a quantum of energy.

E far -E near =h×V. E \u003d -21.76 × 10 -19 / n 2 J / atom \u003d -1310 kJ / mol.

Such energy must be expended in order to transfer an electron in a hydrogen atom from the first Bohr orbit (n=1) to an infinitely distant one, i.e. remove an electron from an atom, turning it into a positively charged ion.

Bohr's quantum theory explained the linear nature of the spectrum of hydrogen atoms.

Disadvantages:

1. It is postulated that the electron stays only in stationary orbits, how does the transition of electrons take place in this case?

2. All the details of the spectra are not explained, their different thicknesses.

What is called an energy level and an energy sublevel in an atom?

Number energy levels atom equal to the number of the period in which it is located. For example, potassium (K) - an element of the fourth period, has 4 energy levels(n = 4). Energy sublevel- a set of orbitals with the same values of the principal and orbital quantum numbers.

11. What shape do they have s-, p- and d- electronic clouds.

During chemical reactions, the nuclei of atoms remain unchanged, only the structure changes electron shells due to the redistribution of electrons between atoms. The ability of an atom to donate or accept electrons determines its chemical properties.

The electron has a dual (corpuscular-wave) nature. Thanks to wave properties electrons in an atom can only have strictly certain values energy that depends on the distance to the nucleus. Electrons with similar energy values form an energy level. It contains a strictly defined number of electrons - maximum 2n 2 . Energy levels are subdivided into s-, p-, d- and f- sublevels; their number is equal to the level number.

Quantum numbers of electrons

The state of each electron in an atom is usually described using four quantum numbers: principal (n), orbital (l), magnetic (m), and spin (s). The first three characterize the motion of an electron in space, and the fourth - around its own axis.

Principal quantum number(n). Determines the energy level of the electron, the distance of the level from the nucleus, the size of the electron cloud. It takes integer values (n = 1, 2, 3 ...) and corresponds to the period number. From periodic system for any element, by the number of the period, you can determine the number of energy levels of the atom and which energy level is external.

The element cadmium Cd is located in the fifth period, which means n = 5. In its atom, electrons are distributed over five energy levels (n = 1, n = 2, n = 3, n = 4, n = 5); the fifth level will be external (n = 5).

Orbital quantum number(l) characterizes geometric shape orbitals. Takes an integer value from 0 to (n - 1). Regardless of the number of the energy level, each value of the orbital quantum number corresponds to an orbital of a special shape. A set of orbitals with the same values of n is called an energy level, with the same n and l - a sublevel.

l=0 s-sublevel, s-orbital - sphere orbital

l=1 p- sublevel, p-orbital – dumbbell orbital

l=2 d- sublevel, d- orbital - orbital complex shape

f-sublevel, f-orbital - an orbital of even more complex shape

On the first energy level(n = 1) the orbital quantum number l takes on a single value l = (n - 1) = 0. The shape of the inhabited is spherical; on the first energy level there is only one sublevel - 1s. For the second energy level (n = 2), the orbital quantum number can take two values: l = 0, s-orbital - a sphere of a larger size than at the first energy level; l = 1, p-orbital - dumbbell. Thus, at the second energy level there are two sublevels - 2s and 2p. For the third energy level (n = 3), the orbital quantum number l takes three values: l = 0, s-orbital - a sphere of a larger size than at the second energy level; l \u003d 1, p-orbital - a dumbbell of a larger size than at the second energy level; l = 2, d is an orbital of complex shape.

Thus, at the third energy level there can be three energy sublevels - 3s, 3p and 3d.

12. Give the formulation of the Pauli principle and Gund's rule.

Pauli principle: An atom cannot have two or more electrons with the same set of all four quantum numbers. From which it follows that two electrons with oppositely directed spins can be in the same orbital.

Maximum possible number of electrons:

on the s - sublevel - one orbital - 2 electrons, i.e. s2;

into p- - -three orbitals - 6 electrons, i.e. p 6 ;

on d - - - five orbitals - 10 electrons, i.e. d10;

on f- –– - seven orbitals – 14 electrons, i.e. f 14 .

The number of orbitals at sublevels is determined by 2 l+1, and the number of electrons on them will be 2×(2 l+1), the number of orbitals at sublevels is equal to the square of the main quantum number n 2, and at the levels - 2n 2, then. in the first period of the periodic system of elements, there can be a maximum of 2 elements, in the second - 8, in the third - 18 elements, in the fourth - 32.

In accordance with the I and II rules of M.V. Klechkovsky, the filling of orbitals occurs in ascending order of the sum ( n+l), if equal, with n- the least.

Electronic formulas are written as follows:

1. In the form of a numerical coefficient indicate the number of the energy level.

2. Give the letter designations of the sublevel.

3. The number of electrons in a given energy sublevel is indicated as an exponent, with all electrons in a given sublevel summed up.

The placement of electrons within a given sublevel is subject to Gund's rule: at a given sublevel, electrons tend to occupy the maximum number of free orbitals, so that the total spin is maximum.

13. Give the formulation of Klechkovsky's rules. How do they determine the order of filling the AO?

In accordance with the I and II rules of M.V. Klechkovsky, the filling of orbitals occurs in ascending order of the sum ( n+l), if equal, with n- the least.

Electronic formulas are written as follows:

1. In the form of a numerical coefficient indicate the number of the energy level.

2. Give the letter designations of the sublevel.

3. The number of electrons in a given energy sublevel is indicated as an exponent, with all electrons in a given sublevel summed up.

14. What is called ionization energy, electron affinity, electronegativity and in what units are they measured?

Atomic characteristics. Chemical nature element is determined by the ability of its atom to lose or gain electrons. This ability can be quantified ionization energy atom and his electron affinity.

Ionization energy called the energy that must be expended to detach an electron from an atom (ion or molecule). It is expressed in joules or electron volts. 1 EV \u003d 1.6 × 10 -19 J.

The ionization energy, I, is a measure of the reducing power of an atom. The smaller I, the greater the reducing power of the atom.

The smallest values I have s elements of the first group. The values of I 2 for them sharply increase. Similarly, for s elements of group II, I 3 increases sharply.

The highest values I 1 have p-elements of group VIII. This increase in the ionization energy when going from s elements of group I to p elements of group VIII is due to an increase in the effective charge of the nucleus.

electron affinity called the energy that is released when an electron is attached to an atom (ion or molecule). It is also expressed in J or eV. We can say that electron affinity is a measure of the oxidizing ability of particles. Reliable values of E have been found for only a small number of elements.

Group VII p-elements (halogens) have the highest electron affinity, since by attaching one electron to a neutral atom they acquire a complete octet of electrons.

E (F) = 3.58 eV, E (Cl) = 3.76 eV

smallest and even negative values E have atoms with the configuration s 2 and s 2 p 6 or a half-filled p-sublevel.

E (Mg) = -0.32 eV, E (Ne) = -0.57 eV, E (N) = 0.05 eV

Attachment of subsequent electrons is impossible. So, multiply charged anions O 2-, N 3- do not exist.

Electronegativity called quantitative characteristic the ability of an atom in a molecule to attract electrons to itself. This ability depends on I and E. According to Mulliken: EO = (I + E) / 2.

The electronegativities of the elements increase over the period, and decrease over the group.