Isotopes

The number of protons in an atom determines what element it is. However, all atoms of the same element have the same number of protons.

An isotope is a variation of an element with the same number of protons, but a different number of neutrons. This means that isotopes of the same element have different atomic masses.

For example, carbon is an element that can exist as different isotopes, such as carbon-12, carbon-13 and carbon-14. These isotopes have the same number of protons (6) but a different number of neutrons, leading to a difference in atomic mass.

Illustrations of three isotopes of carbon. Carbon-12 is depicted with a nucleus containing 6 neutrons and 6 protons, totalling 12 particles, surrounded by electron orbits. Carbon-13 shows a nucleus with 7 neutrons and 6 protons, totalling 13 particles. Carbon-14 displays a nucleus with 8 neutrons and 6 protons, totalling 14 particles. Each isotope is represented by a red circle with the respective carbon number and the 'C' symbol.

You can also write isotopes like this:

  • Carbon-12 → \mathrm{}^{12}_{6}C
  • Carbon-13 → \mathrm{}^{13}_{6}C
  • Carbon-14 → \mathrm{}^{14}_{6}C

Despite having different numbers of neutrons, isotopes have the same number of electrons and therefore have the same chemical properties and reactivity. Although, some isotopes are unstable due to an imbalance of protons and neutrons in their nucleus.

Let’s look at some examples of isotopes.

Examples of Isotopes

Hydrogen is the lightest and most abundant element in the universe. It has three isotopes: protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3). Protium is the most abundant isotope of hydrogen, making up more than 99% of all hydrogen atoms.

Illustrations of three isotopes of hydrogen. From left to right: Protium, represented with a single red '+' symbol indicating 1 proton, having an atomic weight of 1 and no neutrons; Deuterium with 1 proton and 1 neutron (marked with 'n'), resulting in an atomic weight of 2; and Tritium with 1 proton and 2 neutrons, totalling an atomic weight of 3. All isotopes have electron orbits depicted with grey spheres symbolising electrons.

Lithium is a metal that has two stable isotopes: lithium-6 and lithium-7.

Illustrations of two isotopes of lithium. On the left, Lithium-6 is depicted with a nucleus containing 3 protons (shown as '+' symbols) and 3 neutrons (marked with 'n'), totalling an atomic weight of 6, surrounded by electron orbits. On the right, Lithium-7 shows a nucleus with 3 protons and 4 neutrons, having an atomic weight of 7. Both isotopes have electron orbits with grey spheres representing electrons.

Oxygen is a non-metal that has three stable isotopes: oxygen-16, oxygen-17, and oxygen-18. Oxygen-16 is the most abundant isotope, making up more than 99% of all oxygen atoms.

Illustrations of three isotopes of oxygen. From left to right: Oxygen-16 with a nucleus containing 8 protons (shown as '+' symbols) and 8 neutrons (marked with 'n'), totalling an atomic weight of 16; Oxygen-17 with 8 protons and 9 neutrons, with an atomic weight of 17; and Oxygen-18 with 8 protons and 10 neutrons, giving an atomic weight of 18. All isotopes are surrounded by electron orbits with grey spheres indicating electrons.

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