What is an Isotope?

isotopes of hydrogen

What is an isotope?

An isotope is an atom that has the same number of protons and a different number of neutrons. The atom is still considered the same element (still contains the same number of protons) as before but will have a different mass. The change in mass may change the atom’s properties. For a refresher on the parts of an atom—protons, neutrons, and electrons—see this article!

Image depicting the isotopes of hydrogen. Deuterium, protium and tritium.
Image of the three main isotopes of hydrogen. Notice that the number of neutrons (Blue) changes, but the number of protons (Pink) and electrons (Red) stays the same in each.

There are 254 known stable isotopes. There are many more unstable isotopes. Unstable isotopes are radioactive and decay to a more stable form over time. This decay is not always fast. For example, carbon-14 decays to half of the initial amount (defined as a half-life) over 5,730 years.

Isotope Notation

There are several ways scientists denote an isotope in writing.

One way is to write the element name followed by the number of neutrons and protons added together. The number of protons plus neutrons also equals the mass number. For example, a carbon with 7 neutrons and 6 protons (instead of 6 neutrons and 6 protons) would be denoted as carbon-13. And a carbon with 8 neutrons would be labeled carbon-14 because the total mass number is now 14.

A second notation for isotopes is to write it as the element symbol with the mass number on the upper left side and the number of protons on the bottom left side. For carbon, the notation would be 126C for carbon-12 and 136C for carbon-13.

Isotope notation includes the mass number

Sometimes the atomic number is dropped, and the isotope is just written as the mass number and the element symbol. For example, carbon-13 would be written at 13C in this notation.

Common Isotopes and Their Uses

All elements have isotopes. However, some have more uses than others. Some of these isotopes are discussed below.


Hydrogen has three main isotopes. These are protium, deuterium and tritium. Protium is 1H or hydrogen-1. Protium accounts for 99.98% of all hydrogen atoms. Deuterium (2H) and tritium (3H) comprise the last 0.02%. There are other isotopes, but these are all manmade.

One primary use of deuterium (hydrogen-2) is nuclear magnetic resonance spectroscopy (NMR spectroscopy) for solvents. The NMR measures hydrogen-1, so solvents that dissolve the sample of interest only have hydrogen-2. By using hydrogen-2 in the solvent, the spectrum generated does not measure the spectrum of the solvent.


Carbon has 15 known isotopes with various half-lives. The longest half-life is carbon-14. Carbon-14 is commonly used for radiometric dating. This technique allows for the approximation of the age of a sample based on the amount of carbon-14 present.

The other natural isotopes of carbon are carbon-12 and carbon-13. The other known isotopes are all human-made.


Oxygen‘s main isotope is oxygen-16. The other most common isotopes are oxygen-17 and oxygen-18.

The isotopes of oxygen are often used to study the history of the climate. Using ice cores and water samples the ratio of the different isotopes of oxygen in these samples tells scientists about previous climates. As the temperature decreases, the proportion of oxygen-18 present will also decrease.

For more information on how oxygen’s isotopes can inform us about the earth in the past, check out this page from NASA.

Fun Facts About Isotopes

  • Every element has an isotope, they may just be very uncommon.
  • Deuterium and tritium are the only isotopes with names. They refer to hydrogen with a single neutron and two neutrons respectively.
  • The word isotope has Greek origin. ‘Iso’ means same and ‘topos’ refers to place.
  • Isotopes are great resources for learning about the history of the earth. They can inform us about the climate and tell us how old a sample is.

Isotope Formation

Isotopes can either form naturally or through laboratory experiments.

When isotopes form naturally it can occur through radioactive decay. That is an unstable isotope decays to form a new more stable isotope.

Laboratories also make isotopes. Charged particles bombard an atom causing it to become an isotope. It may be an isotope of the same element or a different element.

List of Uses of Isotopes

Here is a list of a few of the numerous uses for isotopes

  • Medical imaging
  • NMR Spectroscopy
  • Power source for space missions
  • Smoke detectors
  • Cancer therapy
  • Explosives detection
  • Nuclear reactors
  • Scientific research
  • Climate modeling
  • Radiocarbon dating
  • Sample dating

Radioactive Decay

Radioactive decay is the process by which an unstable isotope becomes a more stable isotope. A radioisotope refers to an unstable isotope. The new atom after decay may be the same element or different. During the process the atom losses energy to the environment in the form of ionizing radiation. Radiation types are alpha particles, gamma rays, and beta particles.

An unstable isotope will decay several times until it reaches a stable atom. Some common radioactive elements are uranium, thorium, and radium.

For more information on radioactive decay, see this page.