#4

CHLORINE-35 ATOM AND CHLORINE-37 ATOM ARE ISOTOPES. 

Use these two examples to explain what isotopes are.

Firstly, isotopes are atoms of the same element with the same number of protons but different number of neutrons. 

Because of this, isotopes have the same proton number but different nucleon number.

As for Chlorine, it has 2 isotopes. Cl gas consists of 75% abundance of Cl-35 and the remaining 25% abundance is Cl-37. They have the same number of protons (17) however, their neutrons have different numbers. One has 18 (Cl-35), and the other; 20 (Cl-37). 

This is why they are isotopes.