Isotopes

Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

Hydrogen		Deuterium		Tritium

If you want to refer to a certain isotope, you write it like this: ^AX_Z. Here X is the chemical symbol for the element, Z is the atomic number, and A is the number of neutrons and protons combined, called the mass number. For instance, ordinary hydrogen is written ¹H₁, deuterium is²H₁, and tritium is ³H₁.

How many isotopes can one element have? Can an atom have just any number of neutrons?

No; there are "preferred" combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they'reunstable.

I'm not sure what you mean by "unstable." Do atoms just fall apart if they don't have the right number of neutrons?

Well, yes, in a way. Unstable atoms areradioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.