Illustrated Glossary of Organic Chemistry

Isotope: One of a group of atoms that have the same number of protons (i.e., the have the same atomic number and are the same element) but differ in the number of neutrons (i.e., they have different mass).

Isotopes of Hydrogen
P = proton, N = neutron, E = electron

One proton
No neutrons
2H (deuterium)
One proton
One neutron
3H (tritium)
One proton
Two neutrons