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Physical chemistry 204

number, N A. Since the mass of one atom is equal to 12×1.660540×10 −24 g, the value
of the Avogadro number is:

The Avogadro number of any entity is called a mole of that entity. Therefore, one mole
23
of atoms contains 6.0221×10 atoms of carbon-12 and has a mass of 12.000 g.
The properties of some sub-atomic particles and stable nuclei are given in Table 1.

Binding energy

The total nuclear mass of an atom is always slightly less than the sum of the individual
masses of the constituent protons and neutrons because energy is released when neutrons
and protons combine to form the nucleus. The energy loss is called the binding energy of
the nucleus. The magnitude of mass loss, m, is quantitatively related to the energy
released, E, according to Einstein’s equation:
2
E=mc
where c is the velocity of light. Conversely, energy equal to, or greater than, the

Table 1. Properties of some sub-atomic particles
and stable nuclei

Symbol Z A Mass/amu Isotopic abundance/%

(electron) −1 0 5.4858×10 −4 –
(neutron) 0 1 1.0086650 –

(proton) 1 1 1.0072765 –
1 1 1.0078250 99.985

(deuterium) 1 2 2.0141018 0.015
(α-particle) 2 4 4.0026033 100

6 12 12 (exactly) 98.90
6 13 13.0033548 1.10

8 16 15.9949146 99.762
8 17 16.9991360 0.038

8 18 17.9991594 0.200