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1.1.5 Size and mass of atoms

Atoms are extremely small

Definition

Nucleus

The tiny central part of an atom that contains protons and neutrons and accounts for almost all of the atom’s mass.

  1. Atoms have a radius of about 0.10.10.1 nm, which is 1×10−101\times10^{-10}1×10−10 m.
  2. The nucleus sits at the centre and is much smaller still.
  3. The radius of the nucleus is less than 110000\frac{1}{10000}100001​ of the radius of the atom, about 1×10−141\times10^{-14}1×10−14 m.
  4. This means an atom is mostly empty space, with a tiny nucleus and electrons far out in shells.
  5. If an atom were the size of a sports stadium, the nucleus would be about the size of a pea at the centre.
Key Idea
  • An atom's radius (1×10−101\times10^{-10}1×10−10 m) is about 10000 times larger than its nucleus (1×10−141\times10^{-14}1×10−14 m).
  • Numbers this small are written in standard form to keep them manageable.

Almost all the mass sits in the nucleus

Definition

Relative mass

The mass of a particle compared on a scale where a proton and a neutron each have a relative mass of approximately one.

  1. Masses of subatomic particles are compared on a relative mass scale.
  2. A proton and a neutron each have a relative mass of 111.
  3. An electron has a relative mass of about 11835\frac{1}{1835}18351​, so small it is usually treated as almost zero.
  4. Because protons and neutrons are in the nucleus, almost all the mass of an atom is in the nucleus.
  5. The electrons add almost nothing to the mass, even though they fill most of the atom's volume.
  6. So the mass of an atom is set by its number of protons and neutrons, not its electrons.
Self review
  • What is the approximate radius of an atom in metres?
    • Answer: about 1×10−101\times10^{-10}1×10−10 m (0.10.10.1 nm).
  • How does the size of the nucleus compare with the whole atom?
    • Answer: the nucleus radius is less than 110000\frac{1}{10000}100001​ of the atom's radius.
  • What are the relative masses of a proton, neutron and electron?
    • Answer: 111, 111 and about 11835\frac{1}{1835}18351​ (almost zero).
  • Why is almost all the mass of an atom in the nucleus?
    • Answer: the nucleus holds the protons and neutrons, and electrons have negligible mass.
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Scale comparison showing an atom of radius approximately 1 \times 10^{-10} \, \text{m}, its central nucleus of radius approximately 1 \times 10^{-14} \, \text{m}, electrons in shells, and a magnified nucleus containing protons and neutrons

An atom has a radius of about 0.1 nm0.1 \, \text{nm}0.1nm, which is 1×10−10 m1 \times 10^{-10} \, \text{m}1×10−10m. Its nucleus is the tiny central region containing protons and neutrons.

A nucleus has a radius of about 1×10−14 m1 \times 10^{-14} \, \text{m}1×10−14m, about 110000\frac{1}{10000}100001​ of the atom's radius. This means that most of an atom is empty space, with electrons occupying shells far from the nucleus.

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What is the approximate radius of an atom?

1.1.5 Size and mass of atoms Revision Guide

  1. GCSE
  2. /Chemistry
  3. /1.1.5 Size and mass of atoms

Revision notes for AQA GCSE Chemistry 1.1.5 Size and mass of atoms. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.

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