Isotopes (AQA GCSE Physics): Revision Note

Although the number of protons in a particular element is always the same, the number of neutrons can be different

Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons

In the diagram below are three isotopes of Hydrogen:

Isotopes occur naturally, but some are more rare than others

For example, about 2 in every 10,000 Hydrogen atoms is Deuterium

• Tritium is even more rare (about 1 in every billion billion hydrogen atoms)

The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass

• This is because neutrons have no charge but do have mass

In the periodic table, the mass number of chlorine is often given as 35.5

The mass number of chlorine is given as 35.5 because it has 2 isotopes, one with a mass number of 35 and the other with a mass number of 37

Chlorine-35 is about three times more abundant than chlorine-37, so the given mass number of chlorine is closer to 35 than 37

The number of electrons and protons in different isotopes remains the same

Some isotopes are unstable as they have an imbalance of protons and neutrons