# 3.2 Isotopes

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## What is an isotope?

The chemical properties of an element depend on the number of protons and electrons inside the atom. So if a neutron or two is added or removed from the nucleus, then the chemical properties will not change. This means that such an atom would remain in the same place in the Periodic Table. For example, no matter how many neutrons we add or subtract from a nucleus with 6 protons, that element will always be called carbon and have the element symbol $\mathrm{C}$ (see the Table of Elements). Atoms which have the same number of protons, but a different number of neutrons, are called isotopes .

Isotope

The isotope of a particular element is made up of atoms which have the same number of protons as the atoms in the original element, but a different number of neutrons.

The different isotopes of an element have the same atomic number $Z$ but different mass numbers $A$ because they have a different number of neutrons $N$ . The chemical properties of the different isotopes of an element are the same, but they might vary in how stable their nucleus is. Note that we can also write elements as $\mathrm{X - A}$ where the X is the element symbol and the A is the atomic mass of that element. For example, $\mathrm{C-}12$ has an atomic mass of 12 and $\mathrm{Cl-}35$ has an atomic mass of 35 u, while $\mathrm{Cl-}37$ has an atomic mass of 37 u.

## Interesting fact

In Greek, “same place” reads as $\stackrel{}{\iota }\sigma o\varsigma$ $\tau \stackrel{}{o}\pi o\varsigma$ (isos topos). This is why atoms which have the same number of protons, but different numbers of neutrons, are called isotopes . They are in the same place on the Periodic Table!

It is important to realise that the atomic mass of isotopes of the same element will be different because they have a different number of nucleons. Chlorine, for example, has two common isotopes which are chlorine-35 and chlorine-37. Chlorine-35 has an atomic mass of 35 u, while chlorine-37 has an atomic mass of 37 u. In the world around us, both of these isotopes occur naturally. It doesn't make sense to say that the element chlorine has an atomic mass of 35 u, or that it has an atomic mass of 37 u. Neither of these are absolutely true since the mass varies depending on the form in which the element occurs. We need to look at how much more common one is than the other in order to calculate the relative atomic mass for the element chlorine. This is the number that you find on the Periodic Table.

## Interesting fact

The relative atomic mass of some elements depends on where on Earth the element is found. This is because the isotopes can be found in varying ratios depending on certain factors such as geological composition, etc. The International Union of Pure and Applied Chemistry (IUPAC) has decided to give the relative atomic mass of some elements as a range to better represent the varying isotope ratios on the Earth. For the calculations that you will do at high school, it is enough to simply use one number without worrying about these ranges.

The element chlorine has two isotopes, chlorine-35 and chlorine-37. The abundance of these isotopes when they occur naturally is 75% chlorine-35 and 25% chlorine-37. Calculate the average relative atomic mass for chlorine.

1. Contribution of $\mathrm{Cl-}35=\left(\frac{75}{100}×35\right)=26,25\phantom{\rule{2pt}{0ex}}\mathrm{u}$

2. Contribution of $\mathrm{Cl-}37=\left(\frac{25}{100}×37\right)=9,25\phantom{\rule{2pt}{0ex}}\mathrm{u}$

3. $\mathrm{Relative atomic mass of chlorine}=26,25\phantom{\rule{2pt}{0ex}}\mathrm{u}+9,25\phantom{\rule{2pt}{0ex}}\mathrm{u}=35,5\phantom{\rule{2pt}{0ex}}\mathrm{u}$

If you look on the periodic table, the average relative atomic mass for chlorine is $35,5\phantom{\rule{2pt}{0ex}}\mathrm{u}$ . You will notice that for many elements, the relative atomic mass that is shown is not a whole number. You should now understand that this number is the average relative atomic mass for those elements that have naturally occurring isotopes.

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