What is an isotope?

Every carbon atom has 6 protons, and that is what makes it carbon. Yet if you could weigh carbon atoms one by one, you would find that they don’t all weigh the same. A few are a little heavier than the rest.

How can two atoms of the same element have different weights? The answer is hiding in the nucleus.

Same protons, different neutrons

The nucleus of an atom holds two kinds of particles: protons and neutrons. The number of protons decides which element an atom is. But the number of neutrons can differ, even between atoms of the same element.

Versions of one element with different numbers of neutrons are called isotopes. The name comes from Greek words meaning “same place”: all the isotopes of an element share one box in the periodic table.

Neutrons have no electric charge, but they do have weight. So an isotope with more neutrons is a little heavier. Apart from that, the atoms are almost exactly alike.

How do isotopes get their names?

To name an isotope, add up its protons and neutrons, and put the total after the element’s name:

• Carbon-12 has 6 protons and 6 neutrons: 6 + 6 = 12.
• Carbon-14 has 6 protons and 8 neutrons: 6 + 8 = 14.

This total is called the mass number. Scientists also write it as a small number in front of the element’s symbol: ¹²C and ¹⁴C.

Hydrogen’s three natural isotopes

Hydrogen, the lightest element, has three isotopes found in nature, and two of them even have their own names:

• Ordinary hydrogen, or hydrogen-1, has 1 proton and no neutrons. Almost all hydrogen is like this.
• Deuterium, or hydrogen-2, has 1 proton and 1 neutron. It is also called heavy hydrogen. About 1 in every 6,400 hydrogen atoms in water is deuterium.
• Tritium, or hydrogen-3, has 1 proton and 2 neutrons. It is radioactive: sooner or later, its nucleus changes by itself.

Do isotopes behave differently?

Isotopes of one element have the same number of protons, so their atoms also have the same number of electrons. And chemistry is all about electrons. They decide how atoms join together with chemical bonds.

That is why the isotopes of an element behave almost exactly alike in chemistry. Plants take in carbon-12 and carbon-13 from the air in almost the same way, and your body uses both to build you.

Stable and radioactive isotopes

Many isotopes are stable: their nuclei stay the same forever. Carbon-12 and carbon-13 are stable.

Other isotopes are radioactive. Their nuclei are unsteady, and sooner or later they change by themselves and give off radiation.

Carbon-14 is radioactive. In about 5,700 years, half of any amount of carbon-14 turns into nitrogen. This time is called its half-life.

The lab below starts with carbon-12. Add two neutrons: the element stays the same, but the nucleus becomes radioactive.

Build an atom

Keep the protons and change the neutrons: which nuclei hold together?

Not to scale. Real electrons move in fuzzy clouds, not on rings.

Protons − 6 +

Neutrons − 6 +

Electrons − 6 +

Element

Carbon (C) Atomic number 6: every carbon atom has 6 protons.

Isotope

Carbon-12: stable nucleus 6 protons + 6 neutrons = 12 particles in the nucleus. It does not change by itself.

Charge

Neutral atom 6 protons (+) and 6 electrons (−) cancel out.

Outer shell

4 of 8 Shares 4 electrons. Shells from the inside: 2, 4.

Pick an element

1H2He3Li4Be5B6C7N8O9F10Ne11Na12Mg13Al14Si15P16S17Cl18Ar19K20Ca

Elements built: 0 of 20

Quests

0 of 7 done

1. Hydrogen-1, ordinary hydrogen
2. Deuterium, heavy hydrogen
3. Tritium, radioactive hydrogen
4. Carbon-12
5. Carbon-14, used to date old things
6. A nucleus that falls apart within a day
7. Potassium-40, found in bananas

A clock inside old things

Carbon-14 is made all the time, high up in the air, when rays from space hit nitrogen atoms. Plants take it in from the air together with ordinary carbon, and animals eat the plants. So every living thing has a tiny, steady amount of carbon-14 inside it.

When a plant or an animal dies, it stops taking in new carbon. From then on, its carbon-14 slowly disappears. After about 5,700 years only half of it is left, and after about 11,400 years only a quarter.

Scientists measure how much carbon-14 is left in an old bone, a piece of wood or a scrap of cloth. From that, they work out how long ago it died. This is called radiocarbon dating, and it works for things up to about 50,000 years old.

Isotopes that help us

• In hospitals, doctors give patients a tiny amount of a radioactive isotope. It travels around the body and shows up on a special camera, so doctors can see how organs are working. The most common one, technetium-99m, is used in tens of millions of scans every year.
• In power stations, uranium-235 is the fuel. Its nuclei can split and give out lots of heat.
• In science, isotopes reveal how old rocks are, and even how warm the Earth was long ago.

Go deeper: Numbers and notation for grown-ups and the extra curious

• The atomic number $Z$ counts protons, the neutron number $N$ counts neutrons, and the mass number $A = Z + N$ counts both. Carbon-14 has $Z = 6$, $N = 8$ and $A = 14$, written $^{14}_{6}\mathrm{C}$.
• One kind of nucleus, with a definite $Z$ and $N$, is called a nuclide. Thousands of nuclides are known, but only 251 have never been seen to decay. 80 elements have at least one stable isotope, and tin has the most: ten.
• Hydrogen-4 to hydrogen-7 have also been made in laboratories, but they fall apart almost at once. Only the three lightest isotopes of hydrogen occur in nature.
• Most elements in nature are a mix of isotopes. Natural carbon is about 98.9% carbon-12 and 1.1% carbon-13, with about one carbon-14 atom in every trillion. That is why the periodic table gives carbon an atomic mass of 12.011 and not exactly 12: it is an average.
• Isotopes are not perfectly alike in chemistry. Heavier isotopes usually react a little more slowly. The effect is biggest for hydrogen, because deuterium is twice as heavy as ordinary hydrogen.

Go deeper: Why the number of neutrons matters for grown-ups and the extra curious

Protons all have a plus charge, so they push each other apart. The strong nuclear force holds the nucleus together anyway, and neutrons add to this glue without adding any push. Light stable nuclei have about as many neutrons as protons: carbon-12 has 6 of each, oxygen-16 has 8 and calcium-40 has 20. Heavy nuclei need extra neutrons to hold their many protons together. Lead-208, the heaviest stable nuclide, has 82 protons and 126 neutrons.

A nucleus with too few or too many neutrons for its protons is unstable. It changes by radioactive decay: a neutron turns into a proton, or the other way round, until the mix is better. Carbon-14 has too many neutrons for its 6 protons. One of them turns into a proton, and the nucleus becomes nitrogen-14, with 7 protons and 7 neutrons.

Go deeper: Uranium-235 and uranium-238 for grown-ups and the extra curious

Natural uranium is about 99.3% uranium-238 and only 0.7% uranium-235. The difference matters. A uranium-235 nucleus splits easily when a slow neutron hits it, and the split frees more neutrons, which can split more nuclei. Uranium-238 hardly ever splits this way. Most power reactors need fuel with about 3–5% uranium-235, so the uranium is first enriched.

The two isotopes behave almost exactly alike in chemistry, so no chemical trick can separate them. Enrichment uses their tiny difference in weight instead. The uranium is turned into a gas and spun in very fast centrifuges, where the heavier uranium-238 drifts slightly outwards.

Both isotopes are radioactive, but very slow. Uranium-238 has a half-life of about 4.5 billion years, about the age of the Earth. Uranium-235 has a half-life of about 700 million years.

Check yourself

Carbon-12 and carbon-14 are both carbon. What is different about them?

The number of protons The number of neutrons The number of electrons

Show the answer

The number of neutrons — Both have 6 protons, so both are carbon. Carbon-12 has 6 neutrons, and carbon-14 has 8.

Check yourself

A nucleus has 1 proton and 2 neutrons. What is it?

Helium Tritium, a kind of hydrogen Carbon

Show the answer

Tritium, a kind of hydrogen — One proton always means hydrogen. With 2 neutrons it is hydrogen-3, which is called tritium.