2. General notions of atomic-molecular studies

The main notions of chemistry are:

• Atom;

• Molecule;

• Simple and complicated (complex) substances;

• Atomic and molecular mass;

• Mole and molecular volume;

• Valency of element and chemical equivalent.

According to advanced doctrines atom is a chemically indivisible participle of substance consisted from positive charged nucleus and negative changed electrons.

The main characteristics of atom are its weight (mass), size, composition, nucleus charge and structure of electronic shells.

Atomic mass changes from 1,67·10^-27 kg for Hydrogen H to 4,35·10^-25 kg for Kurchatovyi Ku; atomic nucleus consists from protons and neutrons, its radius are 10^-14-10^-15 m.

Quantitative characteristics of atom are:

1. a nuclear charge (which is equal to number of chemical element in Periodical Table of Chemical Elements);

2. a relative atomic mass.

Chemical element is a kind of atom identical in nuclear charge.

Every element has own name and chemical symbol in Periodical Table of Chemical elements named D.I. Mendeleev. Chemical element saves without changes in chemical reactions crossing from one substance to other. Chemical elements existing in the form of simple and complicated (complex) substances.

Simple substances are the substances form from the atom of one chemical element oft the form of existence of chemical element in the free state.

For example, Oxygen O₂, Hydrogen H₂, Silver Ag are the simple substances, while water H₂O, Silver Oxide Ag₂O are the complicated substances.

Now it is known 113-115 chemical elements with well-known properties and more than 500 simple substances. Why quantities of last one are more in some times that first one? Because there is a phenomenon of allotropy. Allotropy is a possibility of chemical elements to exist in the form at least of two simple substances with different properties. This phenomenon is caused by two reasons:

1. Different number of atoms in molecule (for example Oxygen O₂ and Ozone O₃);

2. Formation of different crystalline forms (for example diamond, graphite and a few new artificial substances for Carbon C).

Molecule is a smallest participle of substance, which has its chemical properties. During chemical transformation molecules saves but during chemical reactions decomposed on atoms or groups of atoms forming new substances.

Relative atomic mass (A_r) of chemical element is named a physical value that equal to ratio of medium weight of element to 1/12 part of atomic weight of Carbon atom 12C. One-twelfth part of Carbon atomic weight ¹²C is advanced off-system unit for atomic and molecular weights that named atomic mass unit (a.m.u.) or Carbonic unit:

1 a.m.u. = (19,93·10^-27 kg) :12 = 1,66·10^-27 kg or 1,66·10^-24 g,

where 19,93·10^-27 kg – an absolute mass of Carbon atom ¹²C. So calculating relative atomic masses it used next ratio:

A_r = m_a: 1 a.m.u. = m_a: 1,66·10^-27, where m_a- atomic mass in kg.

Relative molecular mass (M_r) is a physical value equal to ratio of medium isotope mass of molecule into 1/12 part of atomic mass of Carbon atom ¹²C. Practically molecular mass is equal to sum of atomic masses of elements included to molecule.