From the given examples, form the pair of isotopes and the pair of isobars:

₁₈Ar⁴⁰, ₁₇Cl³⁵, ₂₀Ca⁴⁰, ₁₇Cl³⁷

● Isotopes are the atoms of same chemical element having different

atomic mass. This means they differ in neutron number. For ex- ₁₇Cl³⁵ and ₁₇Cl³⁷. Here both atoms are chlorine atoms and have atomic number 17. So, each atom has 17 electrons and 17 protons. But the former one has atomic mass 35 and the latter one has atomic mass 37. This implies that former one has 18 neutrons (35-17) and latter one has 20 neutrons (37-17). Hence, they are isotopes.

● Isobars are the atoms of different elements having same atomic mass.

This implies that the sum of protons and neutrons in each atom is same. For ex- ₁₈Ar⁴⁰ and ₂₀Ca⁴⁰. Here, the former one is argon atom and latter one is calcium. Their atomic numbers are different. But their atomic mass is same i.e. 40. This implies that the number of nucleons in both atoms is same, even though the proton, electron and neutron numbers are different.