Size and mass of atoms: The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10⁻¹⁴ m).

The idea

The nucleus sits at the centre of an atom and is much smaller than the atom itself. Its radius is less than 1/10 000 of the atomic radius, roughly 1 x 10⁻¹⁴ m. If an atom were scaled up to the size of a sports stadium, the nucleus would be about the size of a pea in the middle. This means most of an atom is empty space, with electrons occupying the outer regions. To compare, divide the atomic radius by the nuclear radius: 1 x 10⁻¹⁰ m divided by 1 x 10⁻¹⁴ m gives 10 000, so the atom is about 10 000 times wider than its nucleus. The nucleus contains protons and neutrons, and although it is tiny, it contains nearly all the mass of the atom.

What earns the marks

• States that the radius of a nucleus is less than 1/10 000 of the radius of the atom.
• Gives the approximate nuclear radius as 1 x 10⁻¹⁴ m.
• Calculates the ratio of atomic radius to nuclear radius as about 10 000.
• Explains that most of the atom is empty space because the nucleus is so small.
• Recognises that the nucleus contains most of the atom's mass despite its small size.

Common mistakes and how to put them right

• Writing the nuclear radius as 1 x 10⁻¹⁰ m instead of 1 x 10⁻¹⁴ m.

  Put it right: The atomic radius is about 1 x 10⁻¹⁰ m and the nuclear radius is about 1 x 10⁻¹⁴ m.

• Thinking the nucleus occupies most of the atom's volume.

  Put it right: The nucleus is tiny; most of the atom is empty space.

• Confusing the ratio 1/10 000 with 1/1000.

  Put it right: The nuclear radius is less than one ten-thousandth of the atomic radius.

Exam tips

• Memorise the approximate values: atomic radius about 1 x 10⁻¹⁰ m and nuclear radius about 1 x 10⁻¹⁴ m.
• When comparing sizes, divide the atomic radius by the nuclear radius to show the atom is about 10 000 times wider.
• Use the idea of empty space to explain why atoms are mostly empty.