Atomic Structure and the Periodic Table
Isotopes and Relative Atomic Mass
AQA GCSE Chemistry
Mass number and the symbol
- Mass number is the number of protons plus neutrons. Never write the mass of the protons and neutrons.
- In a symbol, the mass number is at the top left and the atomic number is at the bottom left.
Protons, neutrons and electrons
| Particle | Number in an atom |
|---|---|
| Protons | the atomic number |
| Neutrons | the mass number minus the atomic number |
| Electrons | the same as the number of protons |
In an ion, the protons and neutrons are counted the same way. A positive ion has fewer electrons than protons, and a negative ion has more, by the size of its charge.
Isotopes
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Never write different relative atomic mass.
- Isotopes of an element have different mass numbers because they have different numbers of neutrons.
Relative atomic mass
relative atomic mass =
(mass number × percentage) for each isotope, added together100
Method: relative atomic mass from percentage abundances
- For each isotope, multiply its mass number by its percentage abundance.
- Add these together for every isotope.
- Divide the total by 100, and write down the value in full.
- Round it to the number of decimal places or significant figures asked for.
- Relative atomic mass lies closer to the mass number of the more abundant isotope.
- A relative atomic mass exactly halfway between the mass numbers of two isotopes means there are the same number of atoms of each isotope.
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