Page 23 - Radiochemistry and nuclear chemistry

12 Radiochemistry and Nuclear Chemistry

A= N+ Z (2.1)

where Z is the number of protons (= the atomic number) and N is the number of neutrons.
The elemental identity and the chemical properties are determined by the atomic number.
As we have seen in Chapter 1, an dement may be composed of atoms that, while having
the same number of protons in the nuclei, have different mass numbers and, therefore,
different numbers of neutrons. Neon, for example, has an atomic number of 10, which
means that the number of protons in the nuclei of all neon atoms is 10; however, 90 % of
the neon atoms in nature have 10 neutrons present in their nuclei while 10 % of the atoms
have 12 neutrons. Such atoms of constant Z but different A are called isotopes. The heavy
hydrogen isotopes 2H and 3H are used so often in nuclear science that they have been given
special names and symbols, deuterium (D) and tritium (T), respectively.
The word isotope is often misused to desi~aate any particular nuclear species, such as
160, 14C, 12C. It is correct to call 12C and '4C isotopes of carbon since they are nuclear
species of the same dement. However, 160 and 12C are not isotopic since they belong to
different elemental species. The more general word nuclide is used to designate any specific
nuclear species; e.g. 160, 14C, and 12C are nuclides. The term radionuclide should be used
to designate any radioactive nuclear species, although radioisotope is a common term used
for the same purpose.
In addition to being classified into isotopic groups, nuclides may also be divided into
groupings with common mass numbers and common neutron numbers. Isotopes are nuclides
with a common number of protons (Z), whereas isobar is the term used to designate
nuclides with a common number of nucleons (A), i.e. the same mass number. Nuclei with
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the same number of neutrons (N) but different atomic numbers are termed isotones 19 K and
40
1BAr are examples of isobars, while 3H and ~He are examples of isotones.
In some cases a nucleus may exist for some time in one or more excited states and it is
differentiated on this basis. Such nuclei that necessarily have the same atomic number and
mass number are called isomers. 60mCo and 6~ are isomers; the 60mCo nuclide exists
in a high energy (excited) state and decays spontaneously by emission of a "y-ray with a
half-life of 10.5 rnin to the lowest energy, ground state, designated by 6~
60m.-, 3' 60
27 ~o ........ > 27Co
10.5 min
The symbol m stands for metastable, while g (or no symbol) refers to the ground state.

2.2. Atomic masses and atomic weights

The universal mass unit, abbreviated u (sometimes amu for atomic mass unit), is defined
as one-twelfth of the mass of the 12C atom which has been defined to be exactly 12 u. The
absolute mass of a 12C atom is obtained by dividing the value 12 by the Avogadro number
(N A = 6.022 137 x 1023). The value for the mass of a 12C atom, i.e. the nucleus plus the
6 extranuclear electrons, is thus 1.992 648 x 10 -23 g. Atomic masses are expressed in units
of u relative to the 12C standard. This text uses M to indicate masses in units of u, and m
in units of kilograms; m = M/103 N A.