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LECTURE 2. ATOM STRUCTURE. PERIODIC LAW AND

PERIODIC SYSTEM OF ELEMENTS

PROGRAM QUESTIONS:

2.1 PERIODIC LAW – THE FUNDAMENTAL LAW OF NATURE

2.2 THE STRUCTURE OF THE ATOM AND THE PERIODICITY OF THE

PROPERTIES OF CHEMICAL ELEMENTS

2.1 Periodic law - the fundamental law of nature

Atoms consist of a positively charged nucleus and electrons located around the

nucleus at certain energy levels and sublevels.

The atomic nucleus is approximately 100,000 times smaller than an atom. The
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size of the atom is approximately 10-10 m, the size of the nucleus is 10 ÷ 10

m. If we imagine that the nucleus has a diameter of 1 mm (the size of a poppy
seed), then the diameter of the atom will be 105 mm = 100 m.

The nucleus consists of positively charged particles - protons and neutral
particles - neutrons. The number of protons is equal to the number of electrons and

coincides with the ordinal number of the element in the periodic system

(supplement A). Consequently, the ordinal number of an element determines the

charge of the nucleus and the number of electrons in the atom. For example, for an

element with the ordinal number of 13 (Al), the nucleus charge is +13, around

which there are 13. The atomic mass is mainly concentrated in the nucleus and

consists of masses of protons and neutrons. Since the masses of the proton (p) and

the neutron (n) are approximately the same and equal to 1 atomic mass unit, then

the mass of the atom is an integer. The total number of protons (Z) and neutrons

(N) in an atom of an element is called the mass number A

A = Z + N

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