An atom is the smallest particle of an element that has no change in its chemical identity during any physical or chemical change. Effectively an atom is the smallest unit of matter. Note: The term “atom” comes from the Greek word ‘Atomos’, which means that which cannot be divided because atoms are the smallest and minutest things in the universe which cannot be divided. Atoms comprise three subatomic particles like protons, neutrons and electrons.

Isotopes are the atoms that have the same atomic number but different mass number. As a matter of fact, isotopes have the same number of protons but different numbers of neutrons in it. Some common examples of isotopes include carbon has three isotopes, namely, C - 12 , C - 13 , C - 14 . The isotopes have the same proton number but different neutron number.