Atomic Theory

                           Protons       17                             17

                   Electrons     17                             17

                   Neutrons     20                             18

The atoms :     37Cl         and            35Cl  are therefore said to be isotopes of the element     17                              17      chlorine.

There are sometimes more them two types of atoms – isotopes for a particular element. 

 For example, carbon has three isotopes:   12C   ,      13C    and      14C

                                                                    6         6             6

The abundance of the isotopes vary. For example, for the element chlorine,  35Cl  isotope is 75% abundant and 37Cl is 25% abundant.                                                       17                                                                                                                 17