The nuclear composition is expressed in terms of the neutrons and protons in the nucleus, or, as the atomic number (Z) and mass number (A).

Atomic number

The number of protons present in the nucleus of an atom represents the atomic number of the atom. It is denoted by the capital letter, Z.

Z = p

Or, Z = e

Since the number of protons (p) is equal to the number of electrons (e), atomic number can also be taken as the number of electrons in the orbits of an atom.

Mass number

The mass number of an atom is equal to the sum of the number of neutron (n) and protons (p) present in the atom. It is denoted by the capital letter, A.

A = n + p

You know that protons are positively charged. Their number is equal to the number of negatively charged electrons. Thus, their opposite charges cancel each other, making the atom, electrically neutral.

To represent the nuclear contents of an atom, the symbol of the element (say X) is written with its atomic number and mass number to the left of the symbol as follows.

a/z X