¿Qué es un isótopo? Una explicación sencilla con ejemplos

Los isótopos son átomos del mismo elemento con diferentes números de neutrones. Descubre qué significa eso, por qué algunos isótopos son radiactivos y construye uno tú mismo en 3D.

What is an isotope? An isotope is a version of an element whose atoms have a different number of neutrons. Every atom of an element has the same number of protons, but the number of neutrons can vary. Carbon-12 and carbon-14 are both carbon (6 protons each), but carbon-12 has 6 neutrons and carbon-14 has 8.

Protons decide the element, neutrons decide the isotope

An atom's nucleus contains two kinds of particles:

Protons carry a positive charge. Their number, the atomic number, decides which element the atom is. Six protons is always carbon; eight is always oxygen.
Neutrons have no charge. Adding or removing neutrons doesn't change the element, only its mass.

So if you take a carbon atom and add a neutron, you still have carbon, just a heavier isotope of it.

How isotopes are named

Isotopes are named by their mass number: protons plus neutrons.

Isotope	Protons	Neutrons	Mass number
Hydrogen-1 (protium)	1	0	1
Hydrogen-2 (deuterium)	1	1	2
Hydrogen-3 (tritium)	1	2	3
Carbon-12	6	6	12
Carbon-14	6	8	14

You will also see them written with symbols, like ¹⁴C or C-14.

Do isotopes behave differently?

Chemically, hardly at all. Chemical reactions are about electrons, and isotopes of an element have the same number of electrons. Water made with deuterium ("heavy water") still looks and behaves like water, though it is slightly denser.

The big differences are physical: mass, and above all stability.

Why some isotopes are radioactive

A nucleus needs a balance of protons and neutrons to hold together. With too many or too few neutrons, it becomes unstable and eventually decays, releasing radiation and turning into a different nucleus. These unstable isotopes are called radioisotopes.

Carbon-12 and carbon-13 are stable. Carbon-14 is radioactive, with a half-life of about 5,730 years, which is what makes radiocarbon dating possible.
Uranium-235 can split in a chain reaction, which is why it is used in nuclear reactors.
Deuterium and tritium fuse together in nuclear fusion experiments, releasing huge amounts of energy.

What is relative atomic mass?

Most elements in nature are a mix of isotopes. Chlorine is about 76% chlorine-35 and 24% chlorine-37, so its average atomic mass is about 35.5, which is the number you see on the periodic table.

Frequently asked questions

Are isotopes the same element?

Yes. Isotopes of an element all have the same number of protons, and the number of protons is what decides the element. Carbon-12, carbon-13 and carbon-14 are all carbon, with 6 protons each. They have the same electrons too, so they react in the same way: see how those electrons form bonds in ionic vs covalent bonds.

What are the three isotopes of hydrogen?

Hydrogen has three natural isotopes. Protium (hydrogen-1) has no neutrons and makes up more than 99.98% of all hydrogen. Deuterium (hydrogen-2) has one neutron and is stable. Tritium (hydrogen-3) has two neutrons and is radioactive, with a half-life of about 12.3 years.

How does carbon-14 dating work?

Living things keep taking in carbon, including a tiny, steady share of radioactive carbon-14. When an organism dies, it stops, and its carbon-14 decays with a half-life of about 5,730 years. Measuring how much carbon-14 is left in wood, bone or cloth tells you roughly how long ago it died, back to about 50,000 years.

Try it yourself

Open the Atom Builder, make carbon with 6 protons and 6 electrons, then add neutrons one by one. Watch the mass number change while the element stays the same, and see which isotopes the builder marks as stable.