The composition of a nucleus can be described in terms of its constituents. Atomic nucleus is composed of elementary particles called protons and neutrons. These two particles are collectively called nucleons. The protons are positively charged particles and neutrons are electrically neutral. The charge of proton being positive and equal to the charge of electron (-e) in magnitude can be represented as (+e). The mass of the neutron is slightly more that of the proton.
The number of protons in the nucleus is called atomic number (Z). The total number of protons and neutrons is called mass number (A). Hence (A – Z) gives the number of neutrons and is called neutron number (N). If X is the chemical symbol of the element then it is represented by ZXA.
The nuclei having the same atomic number (Z), but different mass numbers (A) are called isotopes. The nuclei having the same neutron number (N) but different atomic numbers (Z) are called isotones. The nuclei having the same mass number (A), but different atomic numbers (Z) are called isobars. Nuclei having the same atomic number (Z) and mass number (A) but with different nuclear properties such as radioactive decay and magnetic moments are called isomers.