Understanding Isotopes

Although atoms of the same element will always have the same number of protons, they may have different numbers of neutrons. If the number of neutrons is different to the number of protons, we call the atom an isotope.

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Isotopes of Hydrogen

The table below shows three isotopes of hydrogen. Remember that the mass number (top left on the symbol) indicates the number of protons and neutrons, while the atomic number (bottom left on the symbol) indicates only the number of protons. Hydrogen always has an atomic number of 1.

NameIsotopeSymbolProtonsElectronsNeutrons
ProtiumHydrogen-1{}_{1}^{1}\mathrm{H}111 − 1 = 0
DeuteriumHydrogen-2{}_{1}^{2}\mathrm{H}112 − 1 = 1
TritiumHydrogen-3{}_{1}^{3}\mathrm{H}113 − 1 = 2
Three diagrams representing the isotopes of hydrogen: Protium, Deuterium and Tritium. The first diagram on the left is labelled "Protium," with the atomic symbol above. It shows a nucleus with one red proton inside an oval representing the electron path. Below the diagram, text states: "Atomic Weight = 1, Protons = 1, Neutrons = 0." The middle diagram is labelled "Deuterium," with the atomic symbol above. It shows a nucleus with one red proton and one blue neutron, also within an electron path oval. The text below reads: "Atomic Weight = 2, Protons = 1, Neutrons = 1." The third diagram on the right is labelled "Tritium," with the atomic symbol above. The nucleus has one red proton and two blue neutrons, with the electron path oval around them. Below, the text says: "Atomic Weight = 3, Protons = 1, Neutrons = 2." Each isotope diagram illustrates the different number of neutrons contributing to the varying atomic weights of hydrogen's isotopes.

The three isotopes of hydrogen — protium, deuterium, and tritium — share identical chemical properties because they have the same number of electrons and protons, which are responsible for the element’s chemical behaviour. However, the stability of an isotope is influenced by the ratio of neutrons to protons in its nucleus. When this ratio is imbalanced, the isotope can be unstable.

  • Protium (¹H), with one proton and no neutrons, is a stable isotope.
  • Deuterium (²H), with one proton and one neutron, is also stable.
  • Tritium (³H), with one proton and two neutrons, is unstable and radioactive.

Isotopes of Lithium

Let’s look at helium and its stable isotopes. Like hydrogen, the number of neutrons can be calculated by subtracting the atomic number from the mass number.

IsotopeSymbolProtonsElectronsNeutrons
Lithium-6{}_{3}^{6}\mathrm{Li}336 − 3 = 3
Lithium-7{}_{3}^{7}\mathrm{Li}337 − 3 = 4
Two diagrams representing the isotopes of Lithium: Lithium-6 and Lithium-7. On the left, the Lithium-6 isotope is depicted with the atomic symbol above the nucleus, which consists of three red protons and three blue neutrons, surrounded by an electron path with two electrons. Below the diagram, the text states: "Atomic Weight = 6, Protons = 3, Neutrons = 3." On the right, the Lithium-7 isotope is similarly illustrated with the atomic symbol above the nucleus, which contains three red protons and four blue neutrons, also encircled by an electron path with two electrons. The text below this diagram reads: "Atomic Weight = 7, Protons = 3, Neutrons = 4." Both diagrams visually represent the composition of each lithium isotope's nucleus and its surrounding electrons, with a label providing the atomic weight, and counts of protons and neutrons.

Isotopes of Oxygen

A third example is oxygen. Although three stable isotopes of oxygen exist, as shown below, almost all oxygen in nature is oxygen-16.

IsotopeSymbolProtonsElectronsNeutrons
Oxygen-16{}_{8}^{16}\mathrm{O}8816 − 8 = 8
Oxygen-17{}_{8}^{17}\mathrm{O}8817 − 8 = 9
Oxygen-18{}_{8}^{18}\mathrm{O}8818 − 8 = 10
Three diagrams representing the isotopes of Oxygen: Oxygen-16, Oxygen-17, and Oxygen-18. On the left, the isotope Oxygen-16 is depicted, with eight red protons and eight blue neutrons in the nucleus, and eight electrons in two orbits around the nucleus. The accompanying text states: "Atomic Weight = 16, Protons = 8, Neutrons = 8." In the centre, the isotope Oxygen-17 is shown, with eight red protons and nine blue neutrons in the nucleus, and eight electrons in orbits. The text reads: "Atomic Weight = 17, Protons = 8, Neutrons = 9." On the right, the isotope Oxygen-18 is illustrated, with eight red protons and ten blue neutrons in the nucleus, and eight electrons in orbits. Below this diagram, the text states: "Atomic Weight = 18, Protons = 8, Neutrons = 10." Each diagram provides a visual representation of the respective oxygen isotope's nuclear composition, including the number of protons, neutrons, and electrons, with labels indicating the atomic weight, and the number of protons and neutrons.
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