If you look up the definition of isotope, using Google, it provides you with the following: "each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element."
The defining characteristic of each element is the number of protons in its nucleus. Hydrogen has one, helium has two, lithium has three and so on. There are, as a consequence, 92 natural elements which can be found in nature.
In reality, technetium - element 43 - is not naturally occurring and the elements from polonium to actinium are so scarce that, for all intents and purposes, they are non-existent. The set of possible elements which chemistry has to play with is limited.
But add neutrons into the mix and things get a little more interesting.
A hundred years before atomic theory was developed, Amedeo Avogadro proposed a rather unusual hypothesis - that equal volumes of gas would contain equal numbers of particles. As it turned out, he was right and from his work the mole or Avogadro's number was born.
It is a very large number - 6.022 x 1023 - but it does allow chemists to bunch atoms together in a useful way. It is Avogadro's number of atoms which gives us the atomic mass of an element in grams. That is, one mole of hydrogen atoms weighs 1.00794 grams, provided we take the isotopes of hydrogen into account.
Hydrogen is unique on the periodic table because we have named its isotopes. The term hydrogen is applied to the atom consisting of one proton, no neutrons, and one electron although this is also called protium (and I will use that for clarity). Add a neutron and you get deuterium. Add a second neutron and you get tritium.
Tritium is radioactive with a half-life of 12.3 years, but the first two are stable. This means if you select a mole of hydrogen atoms, it will consist of a mixture of protium and deuterium atoms, but each has its own unique atomic mass. For protium, the value is 1.007825 while for deuterium it is 2.014102.
Each of the two isotopes contributes to the mass we measure for hydrogen, according to their respective atomic abundance.
As protium dominates, making up 99.9885 per cent of all hydrogen atoms, it is perhaps not surprising the mass of hydrogen is close to that of protium. Indeed, it is why the term hydrogen is essentially synonymous with protium and chemists refer to all hydrogen atoms as hydrogen.
But other than being an interesting intellectual exercise, do isotopes matter?
The simple answer is yes. For example, the difference in mass between the two stable isotopes of hydrogen means water made with deuterium is heavier and boils at a slightly higher temperature. The simple act of boiling water on a stove results in isotopic enrichment. Boil water repeatedly and eventually you will get heavy water.
The difference in the mass of isotopes for carbon results in segregation of the elements in biochemical processes. Both carbon-13 and carbon-14 do not move through biosynthesis at the same pace as carbon-12.
This results in both isotope enrichment by some reactions and isotope depletion for others.
The various isotopes of cobalt or technetium can be used for radiation therapy in treating diseases such as cancer. Isotopes play a big role in medical imaging.
One of the most interesting roles for isotopes is in dating ancient objects. A few weeks ago, we were discussing carbon dating. The amount of carbon-14 remaining in a once living object can be used to determine just how long ago the material died.
But if you want to examine the age of the Earth, carbon dating simple doesn't extend far enough into the past.
To do that, scientists use the isotopes of lead. Specifically, the different isotopes of lead generated by the radioactive decay of isotopes of uranium - specifically lead-206 and lead-207. Furthermore, radioactive thorium decays via a separate process with its own half-life into lead-208. The isotopic composition of lead is not static, but dynamic. It is constantly changing as the Earth grows older.
The ratio of lead isotopes is a fairly good clock for the age of the Earth.
Determining the age of the Earth relied upon having a sample of lead from the primordial solar system. Such samples were found in iron meteorites. By measuring the isotopic abundance of lead in zircon crystals and in iron meteorites, Claire Patterson was able to determine the Earth and its solar system are 4.5 billion years old.
Isotopes are variations of each element generated by changing the number of neutrons but they tell us a great deal about the world around us - even how old it is.