The composition of any atom can be represented by using the normal element symbol (X) with super-script on the left hand side as the atomic mass number (A) and subscript (Z) on the left hand side as the atomic number (i.e., ^A _Z X).

Isobars are the atoms with same mass number but different atomic number for

example, ⁶₁₄C and ⁷₁₄N. On the other hand, atoms with identical atomic number but different atomic mass number are known as Isotopes. In other words (according to equation 2.4), it is evident that difference between the isotopes is due to the presence of different number of neutrons present in the nucleus. For example, considering of hydrogen atom again, 99.985% of hydrogen atoms contain only one proton. This isotope is called protium( ₁¹H).

 Rest of the percentage of hydrogen atom contains two other isotopes, the one containing 1 proton and 1 neutron is called deuterium (₁²D, 0.015%) and the other one possessing 1 proton and 2 neutrons is called tritium (₁²T ).

 The latter isotope is found in trace amounts on the earth. Other examples of commonly occurring isotopes are: carbon atoms containing 6, 7 and 8 neutrons

besides 6 protons ( ₆¹² C, ₆¹³C, ₆¹⁴C); chlorine atoms containing 18 and 20 neutrons besides 17 protons ( ₁₇³⁵Cl, ₁₇ ³⁷Cl ).

Lastly an important point to mention regarding isotopes is that chemical properties of atoms are controlled by the number of electrons, which are determined by the number of protons in the nucleus. Number of neutrons present in the nucleus have very little effect on the chemical properties of an element. Therefore, all the isotopes of a given element show same chemical behavior.