Background Review, Continued
We know that all atoms of the same element have the same number of protons. But atoms of the same element can have a different number of neutrons. Look at the image just below. It shows 3 different atoms of hydrogen. They are all hydrogen because they all have 1 proton; but a hydrogen atom may have zero, one, or two neutrons.
The number of protons plus the number of neutrons in an atom’s nucleus is called the mass number for an atom. Atoms that have the same number of protons but different numbers of neutrons are called isotopes. In the image you see 3 isotopes of hydrogen. Each one has its own name (protium, deuterium, tritium) but we refer to them collectively as isotopes of hydrogen.
Scientists identify isotopes by attaching their mass number to the name or symbol of the element. For example, the symbol for hydrogen is H, and we can represent tritium by writing it as H-3.
