What are isotopes, and why do they matter?

Same protons, different neutrons

Every carbon atom has 6 protons. That is what makes it carbon. But carbon atoms do not all have the same number of neutrons. Most have 6, about one in a hundred has 7, and a very tiny number have 8.

These different versions are called isotopes. The word comes from Greek words meaning “same place,” because all the isotopes of an element share one box on the periodic table.

Neutrons have no charge, so adding or removing them does not change the number of electrons. Chemistry depends on electrons, so the isotopes of an element behave almost the same in reactions. The main difference between them is their mass.

How isotopes are named

An isotope is named after its element plus its mass number, the total of its protons and neutrons. Carbon-12 has 6 protons and 6 neutrons. Carbon-13 has 6 protons and 7 neutrons. Carbon-14 has 6 protons and 8 neutrons.

Scientists also write the mass number at the upper left of the symbol, as in ¹²C and ¹⁴C. To find the neutrons, subtract the atomic number from the mass number: 14 − 6 = 8 for carbon-14.

Hydrogen’s isotopes are special because they have their own names. Ordinary hydrogen (hydrogen-1) has no neutrons, deuterium (hydrogen-2) has one and tritium (hydrogen-3) has two.

Why atomic masses are decimals

Most elements found in nature are a mix of isotopes. The atomic mass on the periodic table is the average mass of all those atoms, weighted by how common each isotope is.

Chlorine is a clear example. About three out of every four chlorine atoms are chlorine-35, and about one in four is chlorine-37. The average lands between the two but closer to 35: 35.45, the number in chlorine’s box.

Carbon works the same way. About 98.9% of carbon atoms are carbon-12 and about 1.1% are carbon-13, so carbon’s atomic mass is 12.011, just above 12.

17ClChlorine35.45

Stable and radioactive isotopes

Many isotopes are stable, which means they never change on their own. Others are radioactive. Their nuclei are unstable, so sooner or later they break apart and give off radiation. This is called radioactive decay, and it often turns the atom into a different element.

Carbon-14 is radioactive. When it decays, one of its neutrons turns into a proton, and the atom becomes nitrogen-14. The time it takes for half of a sample to decay is called its half-life. For carbon-14, the half-life is about 5,730 years.

Some elements have no stable isotopes at all. Technetium (43), promethium (61) and every element heavier than lead (82) are radioactive.

How isotopes are used

Plants take in a little carbon-14 from the air, and animals take it in by eating plants. When a living thing dies, it stops taking in carbon, and its carbon-14 slowly decays. By measuring how much is left, scientists can date bones, wood and cloth up to about 50,000 years old. This is called radiocarbon dating.

Doctors use radioactive isotopes too. Technetium-99m helps create images of bones and organs, and iodine-131 treats some thyroid diseases. Uranium-235, which makes up only about 0.7% of natural uranium, is the fuel in most nuclear power plants.