4. Chlorine-35 atom and Chlorine-37 atom are called isotopes...Use these two examples to explain what is 'isotopes'.

Isotopes are atoms of the same element that have different nucleon number (mass no.) or different number of neutrons.
Isotopes  of the same element have similar chemical properties but different physical properties.

Since Chlorine-35 and Chlorine-37 have the same number of proton and  electron but different number of neutron, they are called isotopes of chlorine. Thus, the average nucleon number of chlorine will be  [(75/100) x35 ] + [(25/100) x37] =  35.5

Chlorine Atoms	No. of Proton	No. of Electron	No. of Neutron
Chlorine-35	17	17	18
Chlorine-37	17	17	20

http://alexteoh.com/Atomic%20Structure_files/image003.jpg


http://www.wordwizz.com/images/isotopes.gif