Isotopes

Isotopes are versions of the same chemical element that have different numbers of neutrons in their nuclei. They have the same number of protons (which is what makes them the same element) but different total masses. Most elements occur naturally as a mixture of two or more isotopes. Isotopes are not just a chemistry curiosity. They power nuclear reactors, date ancient bones, help doctors find tumours and even tell us where a piece of meat came from.

• What it isSame element, different neutronsSame protons, different mass
• Stable isotopesAround 254 knownLast forever, never decay
• Radioactive isotopesThousands knownDecay over time, release radiation
• Carbon isotopesC-12, C-13, C-14C-14 is used in dating fossils
• Uranium isotope used in reactorsU-235Splits to make energy
• Hydrogen isotopesH, D, TTritium (T) is radioactive

Same element, different mass

The number of protons in an atom decides which element it is. Carbon always has 6 protons. Oxygen always has 8. Hydrogen always has 1.

The number of neutrons, however, can vary. Carbon atoms in nature usually have 6 neutrons (which adds to 6 protons to give a mass of 12). These are called carbon-12. But about 1 per cent of natural carbon atoms have 7 neutrons (carbon-13). And a tiny fraction have 8 neutrons (carbon-14).

All three are still carbon. They behave almost the same in chemistry. But they have slightly different masses, and one of them (carbon-14) is radioactive.

How to read an isotope name

Isotopes are usually written as element-name + mass number. The mass number is protons + neutrons.

• Carbon-12: 6 protons + 6 neutrons
• Carbon-14: 6 protons + 8 neutrons
• Oxygen-16: 8 protons + 8 neutrons
• Uranium-235: 92 protons + 143 neutrons
• Uranium-238: 92 protons + 146 neutrons

Fact Hydrogen has three named isotopes because they are so important: protium (just 1 proton, no neutrons), deuterium (1 proton + 1 neutron, symbol D), and tritium (1 proton + 2 neutrons, symbol T). Deuterium combined with oxygen makes "heavy water" (D2O), which looks like regular water but is denser and used in some nuclear reactors.

Stable and radioactive isotopes

Some isotopes are stable: their nuclei sit happily together forever. Others are radioactive: their nuclei are unstable and break down over time, throwing out particles or energy. This breakdown is called radioactive decay.

The time it takes for half of a sample of radioactive atoms to decay is called its half-life. Half-lives range from tiny fractions of a second (some lab-made elements) to billions of years (uranium-238 has a half-life of 4.5 billion years).

Radioactive isotopes give out three main types of radiation:

• Alpha particles: helium-4 nuclei (2 protons + 2 neutrons). Travel only a few centimetres in air and can be stopped by paper. Harmful only if swallowed or breathed in.
• Beta particles: fast-moving electrons. Travel further than alpha but can be stopped by aluminium foil.
• Gamma rays: super-high energy light (like extreme X-rays). Travel through almost anything and need thick lead or concrete to stop.

Carbon dating

One of the most famous uses of isotopes is carbon dating. Cosmic rays from space hit the atmosphere and produce small amounts of radioactive carbon-14. This mixes into the carbon dioxide in the air and is absorbed by plants through photosynthesis. Animals eat the plants and become slightly carbon-14 themselves.

While they are alive, plants and animals contain roughly the same proportion of carbon-14 as the atmosphere. When they die, they stop taking in fresh carbon. The carbon-14 they already had slowly decays into nitrogen-14, with a half-life of about 5,730 years. By measuring how much carbon-14 is left in a sample (bone, wood, leather, cloth), scientists can work out how long ago the plant or animal died, up to around 50,000 years ago.

Carbon dating revolutionised archaeology. It helped pin down the ages of Stonehenge, mummies in Egypt, cave paintings in France and ancient bones from around the world.

Did you know? The atomic weight you see on the periodic table is not a whole number for most elements. That is because it is the average mass of all the natural isotopes of that element, weighted by how common each one is. Chlorine, for example, is listed as 35.45. That is because natural chlorine is about 76 per cent chlorine-35 and 24 per cent chlorine-37. Average them out and you get 35.45.

Isotopes in medicine

Many medical scans rely on isotopes. Doctors inject a tiny amount of a safe radioactive isotope into the patient. Special cameras then track where the isotope goes inside the body, revealing how organs work.

• Technetium-99m: used in bone, heart, brain and tumour scans. Has a half-life of just 6 hours, so it disappears quickly with little lasting radiation.
• Iodine-131: used to treat overactive thyroid glands and certain thyroid cancers. The thyroid naturally absorbs iodine, so this radioactive form goes straight there.
• Fluorine-18: used in PET scans, which show where the body is using sugar (often a clue to cancer activity).

Isotopes in power and weapons

Nuclear reactors use a special isotope of uranium called uranium-235. Its nucleus is just unstable enough that, when hit by a neutron, it splits in two and releases huge amounts of energy plus more neutrons. Those neutrons go on to split more U-235 nuclei: a chain reaction. Controlled, this powers cities. Uncontrolled, it makes an atomic bomb.

Natural uranium is mostly the heavier, more stable isotope U-238 (99.3 per cent), with only about 0.7 per cent of the useful U-235. Reactors and weapons need uranium that has been "enriched" so the U-235 fraction is much higher.

Try this Look up the isotopes of an element you are interested in (try carbon, oxygen, iron or uranium). For each, write down the number of protons (always the same) and neutrons. Notice how the mass number changes but the element does not. Then look up the half-life of any radioactive isotopes you find. You will see they range from microseconds to billions of years, depending on how unstable each combination is.

Deeper dive: tracing a stolen ivory tusk back to its elephant

Every living thing absorbs a slightly different mix of isotopes depending on what it eats, drinks and breathes. Plants growing in dry deserts use slightly different isotopes of carbon than those in wet forests. Animals drinking from a river in the north use different oxygen and hydrogen isotopes than those drinking from a southern river. The ratios get locked into bones, teeth, hair and tusks.

This means scientists can analyse an isotope "fingerprint" in a sample and work out roughly where the animal lived and what it ate. Forensic scientists now use this to fight wildlife crime. When customs officers seize illegal ivory tusks at airports, scientists test small samples for the ratios of carbon, nitrogen, oxygen and strontium isotopes inside. By matching the fingerprint to maps of African isotopes, they can identify which forest or savanna the elephant came from to within a few hundred kilometres.

The same approach has been used to catch counterfeit honey makers, track stolen wine, find the home country of human trafficking victims (from their hair), and check whether expensive olive oils really come from where the label claims. Isotopes leave a fingerprint in everything that grew on Earth, and scientists are learning to read it.

For more, see protons, neutrons and electrons and what is an atom.