Q. What do you mean by Isotopes?
Isotopes and mass number although all atoms of an element have the same number of protons most elements have atoms with different numbers of neutrons. For instance because the atomic number of hydrogen is one all atoms of hydrogen must have one proton (and one electron). Nevertheless hydrogen atoms are able to have different numbers of neutrons. Around 99985 out of every 100000 naturally occurring hydrogen atoms have no neutrons. But the remaining 15 out of every 100000 hydrogen atoms have one neutron. Atoms with the similar number of protons but different numbers of neutrons are called isotopes. Hydrogen has two naturally taking place isotopes.