There are just over 100 elements, but over 1500 ISOTOPES. Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. It’s rare in chemistry, but just for a moment, let’s forget about the electrons and concentrate on the tiny, massive nucleus in the middle.
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. These are both found in the Sun.
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 or proton 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’re unstable.
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.
Carbon is a good example. Most of the carbon that we come across is carbon 12, with 6 neutrons. The isotope carbon 14 is much rarer, with 8 neutrons and is radioactive, decaying very slowly over thousands of years to nitrogen 14 with a neutron mysteriously turning into a proton and firing out a high energy electron from the nucleus (yes, that’s right) which we call a beta particle. This is the basis of carbon dating. Plants trap or ‘fix’ atmospheric carbon during photosynthesis, so the level of 14C in plants and animals when they die approximately equals the level of 14C in the atmosphere at that time. However, it decreases thereafter from radioactive decay, allowing the date of death or fixation to be estimated. It works OK for things up to about 60,000 years old, assuming the tiny percentage of carbon 14 in the atmosphere has stayed the same for this time. This site’s about carbon dating amongst other things and is quite funny. Have a look at it. It’s got some experiments to try on it as well as cartoons. ASK BEFORE YOU TRY ANYTHING AT HOME!