Isotopes are probably the most important idea in geochemistry. To explain what they are, let’s use oxygen as an example.
All oxygen atoms have the same chemical properties – for instance, they can all make water, H2O, by combining with two hydrogen atoms – because they all have the same number of electrons (and protons: 8 of each). Almost all oxygen atoms on Earth also have 8 neutrons, but a very small proportion have two extra neutrons, making them slightly heavier.
We work out how heavy atoms are by adding the number of protons and neutrons, so most oxygen atoms have a mass of 16 (8 protons + 8 neutrons), but the other type mentioned earlier have a mass of 18 (8 protons + 10 neutrons). We call these oxygen-16 and oxygen-18 (or 16O and 18O), and they’re both isotopes of oxygen.
Some isotopes of other elements are unstable and lose combinations of protons, neutrons, and electrons to get to a more stable atom – this is radioactivity. Our two isotopes of oxygen, though, are stable – they don’t decay, and stay just the way they are pretty much forever.
That’s what makes them so useful in my work. I can use them to tell how much ice there was on Earth millions of years ago.