Size and mass of atoms — AQA GCSE Chemistry
Test yourself on Size and mass of atoms with AQA GCSE practice questions.
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Size and mass of atoms explained
The mass of an atom is concentrated in its nucleus, which contains protons and neutrons.
Read the full explanation
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.
Your focus
- Identify the location of protons, neutrons, and electrons in an atom.
- State the relative masses of protons, neutrons, and electrons.
- Calculate the mass number of an atom given the number of protons and neutrons.
Size and mass of atoms exam tips
Marking 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.
- Use the concept to explain why isotopes have different masses but the same chemical properties.
Examiner 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.
Common Mistakes
- Thinking that electrons contribute significantly to atomic mass; correct by recalling that electron mass is about 1/1836 of a proton, so it is negligible.
- Confusing mass number with relative atomic mass; 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; correct by remembering that neutrons have a relative mass of 1.