Science Basics: Isotopes

Published 2009-10-04 by Chris

I’ve written a few articles on my MSc research and other topics of geochemistry, radioactive decay and radiogentic dating in my time running this blog. Something I’ve been guilty of is assuming a lot of knowledge when it comes to my topics. For example, what an isotope is. I’ve often talked about ²⁶Al, decaying to ²⁶Mg or the ¹⁴⁶Sm→¹⁴²Nd isotopic system, all while assuming you, the reader, knows what I’m on about.

Now I realise that many of my readers are experienced scientists with either undergraduate degrees or published research papers behind them, and this article is not for you. For those of you out there who may have a passing interest in science or geology and find yourself intimidated by the overwhelming barrage of terminology I tend to stuff into my articles, I thought I’d lay out some basics for future reference.

Today, it’s a pretty easy concept with far reaching and extremely important consequences, and that is the concept of an isotope.

Hopefully you know what an atom is. If not, there’s plenty of resources online to help you. Every atom, except your standard Hydrogen nucleus, contains both positively charged protons and neutrally charged neutrons. The chemical element an atom is, is dependent entirely on the number of protons. Hydrogen has one proton, Helium 2, Lithium 3, for example. What can vary, however, is the number of neutrons, and this variation in neutrons in an atomic nucleus is what defines an atom’s isotope.

Take Helium for example. Your standard Helium atom, such as the one many people would have inhaled, contains two protons (making it Helium) and two neutrons. This combination makes it a ⁴He, pronounced “Helium 4,” isotope. It’s 4 because that’s the total count of protons and neutrons in the nucleus: 2 protons + 2 neutrons = ⁴He. The nuclei of these atoms can be visualised like this (blue are neutrons, orange are protons):

Helium also comes in the form of ³He, which has two protons and one neutron. How and why there are different isotopes is due to the process which formed the atom in the first place, be it a big bang, the nuclear decay of bigger atoms, or the fusion of smaller atoms (in this case, two hydrogen atoms will fuse, in a process, to form Helium). ³He can be visualised like this:

Both ³He and ⁴He are what’s know as stable isotopes. This means they’re quite happy sitting in the 3 or 4 isotopic states for ever and ever without decaying, i.e, having sections of the atomic nucleus splitting away. Helium has 8 know isotopic states, but none of these 6 other isotopic states are physically stable, and tend to split into lighter Helium isotopes and eventually Hydrogen and a hail of neutrons after a very short amount of time. ⁵He, for example, has a halflife (the time it takes half of any given quantity to decay away) of 0.7 zeptoseconds, which is 7×10^-22, or 0.0000000000000000000007 seconds. That’s incredibly quick, meaning ⁵He, whenever it happens to form, isn’t around long enough to do anything before effectively exploding into two Hydrogen atoms.

These unstable isotopes, such as ⁵He or ²⁶Al (Aluminium is stable with 13 protons and 14 neutrons, i.e, ²⁷Al) and big, heavy elements like Uranium and Plutonium (which are unstable simply due to their large nucleus size) are not able to hold together their atomic nuclei and after a time, each atom does something called decay – i.e, split into smaller atoms by radiating away parts of their atomic nucleus (hence, nuclear radiation). This is the basis of nuclear power – as the splitting of Plutonium, Uranium and Thorium atoms into smaller atoms releases a lot of energy.

And this is where the really useful part of isotopes comes from. The decay of unstable isotopes is constant, predictable and measurable. After 0.7 zeptoseconds, half of all ⁵He atoms produced 0.7 zs previously, would have decayed. Guaranteed. So if you know the ratio of ⁵He produced in a reaction in relation to the quantity of something stable like ⁴He, produced in that same reaction, and you can measure them both, you can very easily work out how long it’s been since the reaction took place. That’s the basis radiometric dating, and the subject of a future science basics post.

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