How Big Is an Atom? GCSE Chemistry Explained
An atom has a radius of about 0.1 nm. Learn how that compares with the nucleus, convert nanometres to metres, and avoid a common GCSE exam mistake.
This article covers 4.1.1.54.1.1.54.1.1.5 of the AQA GCSE Chemistry 8462 specification. The size and mass of atoms are studied at both Foundation and Higher Tier.
An atom has a radius of about 0.1 nm0.1\,\text{nm}0.1nm, or 1×10−10 m1 \times 10^{-10}\,\text{m}1×10−10m. That is roughly one ten-billionth of a metre from the centre of the atom to its outer edge. The exact size varies between elements, so this is an approximate figure.
Radius or diameter?
The radius is the distance from the centre to the edge. The diameter is the distance all the way across, so it is twice the radius. For an atom with a radius of 0.1 nm0.1\,\text{nm}0.1nm, the diameter is about 0.2 nm0.2\,\text{nm}0.2nm.
A nanometre, written nm\text{nm}nm, is one billionth of a metre. To convert the typical atomic radius into metres:
0.1 nm=0.1×10−9 m=1×10−10 m.0.1\,\text{nm}=0.1\times 10^{-9}\,\text{m}=1\times 10^{-10}\,\text{m}.0.1nm=0.1×10−9m=1×10−10m.The power of ten is important: 1 nm=10−9 m1\,\text{nm}=10^{-9}\,\text{m}1nm=10−9m, and multiplying by 0.10.10.1 makes the value ten times smaller. Writing the conversion in stages makes it easier to check your answer and your units.
How big is the nucleus?
The nucleus is much smaller than the whole atom. Its radius is less than one ten-thousandth of the atom’s radius, at about 1×10−14 m1\times 10^{-14}\,\text{m}1×10−14m. Yet almost all the atom’s mass is concentrated in the nucleus. Electrons occupy the space around it, making up the rest of the atom’s overall size.
For a sense of scale, imagine enlarging an atom until its radius is 100 m100\,\text{m}100m. On the same scale, its nucleus would have a radius of less than 1 cm1\,\text{cm}1cm. This helps explain why an atom is often described as mostly empty space. The nucleus is tiny, but it contains the protons and neutrons, which account for nearly all the mass.
Exam mistake to avoid: do not give 0.1 nm0.1\,\text{nm}0.1nm as the diameter, or confuse the size of the nucleus with the size of the whole atom. Check whether a question asks for a radius or a diameter.
