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

Chlorine-35 has 35 as its nucleon number. Therefore, (35-17=18) it has 17 protons and 18 neutrons

Chlorine-37 has 37 as its nucleon number. Therefore, (37-17=20) it has 17 protons and 20 neutrons.

Isotopes are different atoms of the same element which have the same number of protons but different number of neutrons.
Chlorine in this case has 18 and 20 neutrons respectively. As chlorine has two different number of neutrons but same number of protons, thus, they are called isotopes.

http://alexteoh.com/Atomic%20Structure.htm