Isotopes and isobars

These are the elements having same atomic number but different mass number. They have the same atomic number because the number of protons inside their nuclei remains the same. The difference in their mass number is due to the difference in their number of neutrons. Since they are neutral isotopes are elements having same number of electrons, which make them to possess identical chemical properties.

Let us see some examples 1H1, 1H2, 1H3 are all isotopes of hydrogen. They all have their atomic number to be unity but the number of neutrons are 0, 1, 2 and z respectively. 17Cl37, 17Cl35 are isotopes of chlorine. They have 17 protons in the nucleus but have number of neutrons equal to 20 and 18 respectively.

Practically every element consists of a mixture of several isotopes. The relative abundance of different isotopes differs from element to element. For example chlorine is composed of two isotopes of masses 34.98U and 36.98U, which are nearly integral multiples of the mass of hydrogen atom. Their relative abundances are 75.4 and 24.6 percent respectively.


Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number (or number of protons) but have the same mass number. An example of a series of isobars would be 40S, 40Cl, 40Ar, 40K, and 40Ca.

The nuclei of these nuclides all contain 40 nucleons; however, they contain varying numbers of protons and neutrons. The term "isobars" (originally "isobares") for nuclides was suggested by Alfred Walter Stewart in 1918. It is derived from the Greek word isos, meaning "equal" and baros, meaning "weight".

Mass Number

It is the total number of protons and neutrons present in the nucleus of an atom. Protons and neutrons present in the nucleus of an atom are together called as nucleons and hence mass number is the number of nucleons present in an atom. Mass number is represented by A. Example: Nitrogen has a mass number of 14 as it has 7 protons and 7 neutrons in its atom.