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.
You can also write isotopes like this:
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.
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.
Lithium is a metal that has two stable isotopes: lithium-6 and lithium-7.
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.