Chemistry, Third edition

ATOMIC STRUCTURE

3.Protons and neutrons are very much heavier than electrons.This means that

most of the mass of an atom lies in its nucleus. The number of protons added to

the number of neutrons is termed the mass number(symbolizedA) of the atom.

4.Most of the atom is empty space.The picture we should keep in our mind is of

minute ‘specks’ (the electrons) circling huge distances from a small (but relatively

heavy) nucleus.

5.Atoms are electrically neutral.This is because the number of protons and

electrons are equal. The number of protons in an atom is called its atomic number

(symbolizedZ).

6.The atoms of each element are different because they have different numbers of

protons(i.e. different atomic numbers). The helium atom consists of a nucleus of

two protons and two neutrons surrounded by two electrons, so we can write

Z 2 A 4

Such an atom is shown diagrammatically in Fig. 3.2. The position of the electrons

is indicated by ‘e’. This diagram is not to scale – if it were, the electrons would be

several hundred metres from the symbols we have used to represent the nucleus.

The helium atom is represented as 24 He. This symbol is built up as follows:

●He is the chemical symbol for helium.

●The superscript ‘4’ is the mass number.

●The subscript ‘2’ is the atomic number.

Generalizing:

The number of neutrons Nis the number of protons Zsubtracted from the

number of protons and neutrons A:

NAZ

Atomic mass scale

Table 3.1 gives more information about subatomic particles. In the fifth column,

the mass of the particles is given in ‘atomic mass units’, symbolised as u, where

1u 1.660 54  10 ^24 g

31

Structure of the

atom

Atoms of carbon are

represented as^126 C. Sketch

the atom, following the

pattern of Fig. 3.2.

Exercise 3A

Table 3.1The mass and charge of subatomic particles

Particle Symbol Charge* Mass/g Mass/u Approx. mass/u

Proton p  1 1.672 62  10 ^24 1.007 27 1
Electron e  1 9.109 39  10 ^28 0.000 54 0
Neutron n  0 1.674 93  10 ^24 1.008 67 1
Alpha  2 6.644 66  10 ^24 4.001 51 4

• Relative to the electron, which has an absolute charge of 1.602  10 ^19 coulombs.

+ proton

e– electron

n neutron

+ n

n +

e–

e–

4

2 He

Fig. 3.2Helium atom.