UPSC 2017 CDS I GK Paper

The mass number of both the elements – calcium (atomic number 20) and argon (atomic number 18) – is 40. They are, therefore, known as isobars. Isobars differ in atomic number (or number of protons) but have the same mass number. An example of a series of isobars is 40S, 40Cl, 40Ar, 40K and 40Ca. The nuclei of all these nuclides contain 40 atomic masses. However, they contain varying numbers of protons and neutrons.