ABSTRACT
The atomic mass number is the number of protons plus the number of neutrons in the nucleus, and is given by the symbol A. Thus, the number of neutrons can be given by A-Z. An element can therefore be defined as a substance consisting of atoms with the same atomic number. This does not apply to the atomic mass. Neutrons present in the nucleus do not affect the atomic number or the chemical reactivity of that element. However, with a mass of 1, the number of neutrons present in the nucleus will affect the mass of the atom. Chemically identical atoms that have different numbers of neutrons are termed isotopes. The simplest isotopes are those of hydrogen and are shown in Fig. 6.1.
