Explainer: what is an isotope?

If you’re a keen reader of science news, you may have seen the words “isotope” or “isotopic analysis” pop up in a few wildly different articles.

In the past year at Cosmos alone, we’ve mentioned isotopes in articles on geology, astronomy, palaeontology, nuclear science, history, ecology and chemistry.

So what exactly are isotopes, and why are they useful across so many different fields?

What is an isotope?

The answer lies with chemistry – and atomic structure.

Not all atoms are created equal. While all atoms of the same element have the same number of protons (hydrogen has one proton, helium two, lithium three, and so on as per the Periodic Table), they don’t always have the same number of neutrons.

Roughly 99.98% of hydrogen atoms in the universe have no neutrons, but most of the other 0.02% have one neutron. Because neutrons have a similar amount of mass to protons, this makes these 0.02% of hydrogen atoms slightly more massive – or, on Earth, slightly heavier.

These two different types of hydrogen atoms are referred to as isotopes of one another.

Two lithium atom nuclei, each with three protons represented by orange balls, and three or four neutrons represented by grey balls
Two lithium atom nuclei. While both have three protons, one isotope has three neutrons while the other has four. Credit: Anynobody, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=74851735

Most other elements also have several isotopes. Around 92% of silicon atoms have 14 neutrons, for instance, while 5% have 15 neutrons and 3% have 16. (There are also some trace amounts of other isotopes of silicon).

How can we use them?

Because there are so many atoms, even slight differences in the concentrations of various isotopes can be used to get new information.

The concentration of strontium isotopes, for instance, differs between rocks – meaning that different areas have different strontium “signatures”. Last year, a group of Australian scientists used these strontium signatures to locate the origins of koala teeth around the Adelaide region.

This isn’t an Earth-specific thing, either – some US scientists have used isotopes to figure out the origins of different parts of meteorites, for example. This isotopic signature work can be very, very precise.

When we start to look at the physical properties of isotopes, we can learn even more. Because all of these atoms have slightly different masses, there are situations where they behave slightly differently.

On Earth, water (H2O) containing a lighter isotope of oxygen, for instance, evaporates better, while H2O with the heavier oxygen isotope tends to precipitate faster.

This means that, depending on evaporation and rain levels around the globe, the concentration of oxygen isotopes in the rain is slightly different. We can use this information to draw conclusions about the climate, and climate history, in an area – all by looking at oxygen isotopes.

Living things also distinguish between isotopes. Some plants, for instance, prefer lighter carbon atoms when absorbing CO2 for photosynthesis. Researchers can learn about how much the plant breathes by checking its carbon isotopes.

Finally, some isotopes – radioactive isotopes – aren’t stable, and break down. Occasionally, they release a lot of energy while doing this: this is where nuclear energy comes from.

Other isotopes break down very, very slowly, taking thousands to billions of years to deplete. If we can figure out how long these radioactive isotopes take to break down, we can use the concentration of isotopes to figure out how old things are.

This is the cornerstone of radiometric dating, which is used to determine the age of everything from rocks to human remains.

How do we identify different isotopes?

Atoms can be ‘weighed’ on a device called a mass spectrometer. This machine throws atoms at a detector through a magnetic field that curves their paths slightly. Atoms with more mass are less inclined to bend their path, meaning they hit the detector at different points.

When writing about isotopes, researchers tend to use numbers to distinguish them. “Lighter oxygen isotope” and “heavier oxygen isotope” aren’t really precise enough terms – especially if you’re working with an element that has more than two naturally occurring isotopes. (And weight and mass are not quite the same thing.)

Isotopes are usually referred to with a number that adds their protons and neutrons together – so the neutron-less hydrogen is called 1H to denote its single proton, while the heaver version is 2H to indicate there’s a neutron there as well.

The isotope 16O has 8 protons and 8 neutrons, while the rarer 17O and 18O have 9 and 10 neutrons, respectively. Occasionally, this is also written as oxygen-16 or oxygen-18.

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