AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Size and mass of atoms: Almost all of the mass of an atom is in the nucleus.
The mass of an atom is concentrated in its nucleus, which contains protons and neutrons.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 6
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The mass of an atom is concentrated in its nucleus, which contains protons and neutrons. Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. Therefore, the total mass of an atom is essentially the sum of the masses of its protons and neutrons. For example, a carbon-12 atom has 6 protons and 6 neutrons, giving a mass number of 12; the mass of its electrons is so small that it is ignored in calculations. This concept is crucial for understanding mass number, isotopes, and relative atomic mass. It also explains why the nucleus, despite being tiny compared to the atom, contains nearly all the mass.
What earns the marks
- State that protons and neutrons are located in the nucleus and have a relative mass of 1 each.
- Explain that electrons have negligible mass compared to protons and neutrons.
- Calculate the mass number of an atom by adding the number of protons and neutrons.
- Recognise that the mass of an atom is concentrated in the nucleus.
- Use the concept to explain why isotopes have different masses but the same chemical properties.
Common mistakes and how to put them right
Thinking that electrons contribute significantly to atomic mass.
Put it right by recalling that electron mass is about 1/1836 of a proton, so it is negligible.
Confusing mass number with relative atomic mass.
Put it right: Mass number is the sum of protons and neutrons in a specific atom, while relative atomic mass is an average.
Forgetting that neutrons are in the nucleus and contribute to mass.
Put it right by remembering that neutrons have a relative mass of 1.
Exam tips
- Remember the relative masses: proton = 1, neutron = 1, electron ≈ 0 (or 1/1836).
- When calculating mass number, simply add the number of protons and neutrons; do not include electrons.
- Use the phrase 'almost all of the mass is in the nucleus' to answer questions about mass distribution.
Try the lesson
The start of the Size and mass of atoms: Almost all of the mass of an atom is in the nucleus. lesson, made from checked teaching notes for this specification point.
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Where is almost all of the mass of an atom found?
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The mass of an atom is concentrated in its nucleus, which contains protons and neutrons. Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. Therefore, the total mass of an atom is essentially the sum of the masses of its protons and neutrons. For example, a carbon-12 atom has 6 protons and 6 neutrons, giving a mass number of 12; the mass of its electrons is so small that it is ignored in calculations. This concept is crucial for understanding mass number, isotopes, and relative atomic mass. It also explains why the nucleus, despite being tiny compared to the atom, contains nearly all the mass. A strong answer earns marks for these points. State that protons and neutrons are located in the nucleus and have a relative mass of 1 each. Explain that electrons have negligible mass compared to protons and neutrons. Calculate the mass number of an atom by adding the number of protons and neutrons. Recognise that the mass of an atom is concentrated in the nucleus. A common slip: Thinking that electrons contribute significantly to atomic mass. Instead, recalling that electron mass is about 1/1836 of a proton, so it is negligible. Exam tip: Remember the relative masses: proton = 1, neutron = 1, electron ≈ 0 (or 1/1836).
Key words
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Relative mass of subatomic particles
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Nucleus as the mass centre
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See every definition
- Relative mass of subatomic particles
- Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible.
- Nucleus as the mass centre
- Almost all of the mass of an atom is in the nucleus, which contains protons and neutrons.
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