AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The radius of a nucleus is less than 1/10 000 of the radius of an atom.…
Full specification wording
The structure of an atom: The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.
An atom is mostly empty space.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An atom is mostly empty space. Its tiny central nucleus has a radius under one ten-thousandth of the whole atom's radius, so if an atom were drawn 1 m across, the nucleus would be smaller than 0.1 mm. Yet nearly all the mass sits in that nucleus, because protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. For example, a carbon atom has a radius of about 7 × 10⁻¹¹ m, so its nucleus is under 7 × 10⁻¹⁵ m across. This explains why alpha particles mostly pass straight through gold foil but a few bounce back: the atom is empty, but its dense nucleus repels them.
What earns the marks
- State that the nucleus radius is less than 1/10 000 of the atomic radius, so the atom is mostly empty space.
- Explain that protons and neutrons in the nucleus carry almost all the atom's mass, while electrons contribute negligible mass.
- Use the relative masses of protons, neutrons and electrons to justify why mass is concentrated in the nucleus.
- Apply the scale idea to a concrete example, such as comparing an atom 1 m across with a nucleus under 0.1 mm across.
- Link the small, dense, positive nucleus to the observation that most alpha particles pass through foil while a few are deflected.
Common mistakes and how to put them right
Thinking the nucleus occupies a large fraction of the atom.
Put it right by stressing the nucleus radius is under 1/10 000 of the atomic radius, so the atom is mostly empty space.
Believing electrons contribute significant mass.
Put it right by comparing relative masses, where an electron is about 1/1836 of a proton, so nearly all mass is in the nucleus.
Confusing radius with mass.
Put it right by separating the two ideas, since the nucleus is tiny in size but contains almost all the mass.
Exam tips
- Quote the comparison as 'less than 1/10 000' rather than a vague phrase such as 'very small'.
- When asked why most alpha particles pass through, refer to the atom being mostly empty space and the nucleus being tiny.
- Use relative masses of 1 for protons and neutrons and about 1/1836 for electrons to support mass statements.
Try the lesson
The start of the The structure of an atom: The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus. lesson, made from checked teaching notes for this specification point.
Have a go first
Which part of an atom contains most of its mass?
Listen
An atom is mostly empty space. Its tiny central nucleus has a radius under one ten-thousandth of the whole atom's radius, so if an atom were drawn 1 m across, the nucleus would be smaller than 0.1 mm. Yet nearly all the mass sits in that nucleus, because protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible. For example, a carbon atom has a radius of about 7 × 10⁻¹¹ m, so its nucleus is under 7 × 10⁻¹⁵ m across. This explains why alpha particles mostly pass straight through gold foil but a few bounce back: the atom is empty, but its dense nucleus repels them. A strong answer earns marks for these points. State that the nucleus radius is less than 1/10 000 of the atomic radius, so the atom is mostly empty space. Explain that protons and neutrons in the nucleus carry almost all the atom's mass, while electrons contribute negligible mass. Use the relative masses of protons, neutrons and electrons to justify why mass is concentrated in the nucleus. Apply the scale idea to a concrete example, such as comparing an atom 1 m across with a nucleus under 0.1 mm across. A common slip: Thinking the nucleus occupies a large fraction of the atom. Instead, stressing the nucleus radius is under 1/10 000 of the atomic radius, so the atom is mostly empty space. Exam tip: Quote the comparison as 'less than 1/10 000' rather than a vague phrase such as 'very small'.
Key words
Say what each word means before you turn the card.
Scale and size of atomic structure
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Distribution of mass within the atom
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Subatomic particle properties
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Evidence from alpha-particle scattering
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See every definition
- Scale and size of atomic structure
- The radius of a nucleus is less than 1/10 000 of the radius of an atom, so the atom is mostly empty space.
- Distribution of mass within the atom
- Most of the mass of an atom is concentrated in the nucleus, because protons and neutrons each have a relative mass of 1 while electrons have a relative mass of about 1/1836, which is negligible.
- Subatomic particle properties
- Protons and neutrons each have a relative mass of 1, while electrons have a relative mass of about 1/1836, which is negligible.
- Evidence from alpha-particle scattering
- Alpha particles mostly pass straight through gold foil but a few bounce back, because the atom is empty but its dense nucleus repels them.
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