ISOTOPES

          All atoms having different atomic weights but belonging to the same element are termed as isotopes, i.e. atomic number of isotopes of an element remains the same. Obviously, isotopes contain same number of protons and electrons. Thus the isotopes are atoms of different weight belonging to the same element and having the same atomic number. The difference in the masses of the isotopes of the same element is due tot he different number of neutrons contained in the nucleii. For example, hydrogen exists in three isotopic forms. Atomic number of hydrogen is 1. Three isotopes of hydrogen are:

a) Ordinary hydrogen (1H1) with atomic mass equal to 1.

b) Deuterium (1D2) with atomic mass equal to 2.

c) Tritium (1T3) with atomic number equal to 3.

similarly, chlorine has two isotopes, 17Cl35 and 17Cl37. Those isotopes are available in the ratio of 3;1. Their average atomic weight is 

ISOBARS

          Atoms with the same mass but belonging to different chemical elements are called isobars. Obviously, isobars possess different number of protons and electrons in their atoms. Total number of protons and neutrons in each of their nuclei is also same. The example of first pair of isobars is argon and calcium. Argon (atomic number 18) has 18p, 18e and 22n in its atom. Calcium (atomic number 20) has 20p, 20e and 20n in its atom.

ISOTONES

          These are the nuclides having the same numbers of neutrons (N) but a different Z and A. Example of Isotones are 136 C and 147 N. Isotones having a given value of N, obviously do not all corresponding to the same chemical element.

          The analysis of the properties of isotones and isobars helps us to disclose several features of atomic nuclei. Such analysis helps us to predict that what will happen to the stability of a nucleus when an extra n or p is needed to the nucleus.