Two atoms of chlorine, A and B have the same nuclear charge. Their mass numbers are 35 and 37 respectively. What are the atoms collectively known as? What is the difference between the two ? Do they differ in their chemical properties?

(i) Two atoms having same nuclear charge have same no. of protons. But as atoms A and B have different mass numbers, these have different no. of neutrons. Such atoms are called isotopes.

(ii) They differ in their mass numbers and so have different physical properties.

(iii) They do not differ in their chemical properties—as they have same number of protons and electrons. The chemical properties of any element depend on the electrons.