Mass Number and Isotopes

We have seen that the number or protons in an atom or ion is called its atomic number.

Atoms and ions also have another number called their mass number, which is their total number of protons and neutrons.

For example, if an atom has 6 protons and 7 neutrons, then its mass number is 13 (which is the sum of 6 and 7).

Mass number is also known as nucleon number. The word nucleon simply means proton or neutron. On other words, if an ion has 3 protons and 3 neutrons then we could say that it has 6 nucleons.

Isotopes are atoms or ions with the same atomic number but different mass numbers

Because atoms and ions are grouped into elements based only on their number of protons (in other words, their atomic number), it is possible for atoms and ions of the same element to have different numbers of neutrons.

For example, the atoms shown below are all carbon atoms because they all have 6 protons. However, they have different number of neutrons. The first one has 6 neutrons, the second has 7 neutrons, and the third has 8 neutrons.

These are all carbon atoms because they all have 6 protons. However, they have different numbers of neutrons.

This means that they have different mass numbers. The first has a mass number of 12 (which is 6 + 6), the second has a mass number of 13 (which is 6 + 7), and the third has a mass number of 14 (which is 8 + 8).

Atoms or ions of the same element that have different number of neutrons are called isotopes of that element. So the atoms in the image above would be described as isotopes of carbon.

There are two different ways of defining isotopes, but they both mean the same thing:

Isotopes are atom/ions that have the same number of protons, but different numbers of neutrons.

Isotopes are atom/ions that have the same atomic number, but different mass numbers.

Isotopes are named using their mass number

The different isotopes of an element are distinguished from each other by including their mass number in their name.

For example, the carbon isotope with a mass number of 12 is called carbon-12, the carbon isotope with a mass number of 13 is called carbon-13, and the carbon isotope with a mass number of 14 is called carbon-14.

The three isotopes of carbon.

Different isotopes of the same element usually have the same chemical properties

When it comes to chemical reactions, different isotopes of the same element behave in identical ways. For example, all of the isotopes of carbon take part in the same chemical reactions. If a reaction works using carbon-12 then it will work in exactly the same way using carbon-13 or carbon-14.

This is because the only difference between different isotopes is the number of neutrons, and the number of neutrons doesn't have much affect on chemical reactions. Chemical reactions are all about charge, and since neutrons are neutral, changing the number of neutrons doesn't make much difference.

Different isotopes of an element do have different masses and technically this can affect chemical reactions. However for the vast majority of chemical reactions, this effect is so small that it is negligible, meaning that the different isotopes essentially all behave in the same way.

This is why atoms and ions are grouped into elements based only on their number of protons, even if they have different numbers of neutrons. It wouldn't be useful to think of carbon-12, carbon-13 and carbon-14 as different elements when their chemical properties are basically the same. It is also why we don't usually refer to specific isotopes when we describe chemical reactions.

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