Atoms are made up of a nucleus containing protons and neutrons, surrounded by electrons. But how many of each?

The number of protons determines which element it is: an atom with six protons is an atom of carbon; seven makes it nitrogen; eight, oxygen. The number of electrons generally matches the number of protons, to make the atom electrically neutral. But how many neutrons are in the nucleus? Does it even matter? … It turns out that it matters a lot. - How counting neutrons explains nuclear waste