AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The total number of protons and neutrons in an atom is called its mass…
Full specification wording
Mass number, atomic number and isotopes: The total number of protons and neutrons in an atom is called its mass number.
The mass number, symbol A, counts the particles in the nucleus: protons plus neutrons together.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 4 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The mass number, symbol A, counts the particles in the nucleus: protons plus neutrons together. It is written as the upper number in the nuclide symbol, so ⁵⁶₂₆Fe has a mass number of 56 and an atomic number of 26. Subtracting gives the number of neutrons: 56 - 26 = 30. Electrons are not counted, because their mass is tiny compared with a proton or a neutron, which is why the mass number is a good guide to the mass of the whole atom. Like the atomic number, it is a count and has no unit. In alpha decay the mass number falls by four; in beta and gamma emission it does not change at all.
What earns the marks
- The mass number being the total number of protons and neutrons.
- Calculating the number of neutrons as mass number minus atomic number.
- Reading the mass number as the upper, larger number in the nuclide symbol.
- Stating that the mass number is a count and so has no unit.
Common mistakes and how to put them right
Including the electrons in the total when working out the mass number.
Subtracting the wrong way round, giving a negative number of neutrons.
Attaching a unit such as grams to the mass number.
Confusing the mass number of one isotope with the relative atomic mass printed in the periodic table.
Exam tips
- Write out A - Z = number of neutrons before you substitute; it prevents the reversed subtraction.
- Check that your neutron count is not negative and not larger than the mass number.
- Nucleon number means the same thing as mass number if a question uses that word.
Try the lesson
The start of the Mass number, atomic number and isotopes: The total number of protons and neutrons in an atom is called its mass number. lesson, made from checked teaching notes for this specification point.
Have a go first
An atom of boron contains 5 protons and 6 neutrons. What is the mass number of this atom?
Listen
The mass number, symbol A, counts the particles in the nucleus: protons plus neutrons together. It is written as the upper number in the nuclide symbol, so ⁵⁶₂₆Fe has a mass number of 56 and an atomic number of 26. Subtracting gives the number of neutrons: 56 - 26 = 30. Electrons are not counted, because their mass is tiny compared with a proton or a neutron, which is why the mass number is a good guide to the mass of the whole atom. Like the atomic number, it is a count and has no unit. In alpha decay the mass number falls by four; in beta and gamma emission it does not change at all. A strong answer earns marks for these points. the mass number being the total number of protons and neutrons. calculating the number of neutrons as mass number minus atomic number. reading the mass number as the upper, larger number in the nuclide symbol. stating that the mass number is a count and so has no unit. Exam tip: Write out A - Z = number of neutrons before you substitute; it prevents the reversed subtraction.
See it
Key words
Say what each word means before you turn the card.
mass number
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nucleon number
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counting neutrons
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See every definition
- mass number
- The total number of protons and neutrons in an atom, written as the upper number in the nuclide symbol.
- nucleon number
- Another word for the mass number, meaning the same thing if a question uses that word.
- counting neutrons
- Working out the number of neutrons by subtracting the atomic number from the mass number.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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