The nuclei have been separated on the basis of the number of protons (atomic number) or the total number of nucleons (mass number) as given below:

(i) Isotopes: The atoms of element having similar atomic number but different mass number are called as isotopes. All isotopes have the same chemical properties.

(ii) Isobars: The nuclei which have the similar mass number (A) but different atomic number (Z) are called isobars. Isobars occupy dissimilar positions in periodic table so all isobars have dissimilar chemical characteristics.

(iii) Isotones: The nuclei having equal number of neutrons are known as isotones. For them both the atomic number (Z) and mass number (A) are dissimilar, but the value of (A - Z) is same.

(iv) Mirror nuclei: Nuclei having the similar mass number A but with the proton number (Z) and neutron number (A - Z) interchanged (or whose atomic number vary by 1) are known as mirror nuclei