If you want to refer to a certain isotope, you write it like this: AXZ. 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 is 2H1, and tritium is
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're unstable.
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 are radioactive: their nuclei change or decay by spitting out
radiation, in the form of particles or electromagnetic waves.