Size and mass of atoms: Atoms are very small, having a radius of about 0.1 nm (1 x 10⁻¹⁰ m).

The idea

Atoms are the basic building blocks of matter, but they are far too small to see directly. Their radius is about 0.1 nm, which is 1 x 10⁻¹⁰ m. To appreciate this scale, imagine lining up atoms across a 1 mm gap: about 5 million atoms would fit. A nanometre is one billionth of a metre, so 0.1 nm is one ten-billionth of a metre. This tiny size explains why atoms were not observed until powerful microscopes were developed. When comparing atomic sizes, remember that the value is a typical radius, not a fixed diameter, and different elements have slightly different atomic radii. In calculations, convert nanometres to metres by multiplying by 1 x 10⁻⁹, so 0.1 nm becomes 1 x 10⁻¹⁰ m.

What earns the marks

• States that the radius of an atom is about 0.1 nm.
• Converts 0.1 nm to 1 x 10⁻¹⁰ m correctly.
• Recognises that atoms are extremely small and cannot be seen with the naked eye.
• Uses the scale of 1 nm = 1 x 10⁻⁹ m to convert between units.
• Describes the approximate size of an atom relative to everyday objects, such as about 5 million atoms across 1 mm.

Common mistakes and how to put them right

• Writing 0.1 nm as 1 x 10⁻⁹ m instead of 1 x 10⁻¹⁰ m.

  Put it right: 0.1 nm = 0.1 x 10⁻⁹ m = 1 x 10⁻¹⁰ m.

• Confusing radius with diameter.

  Put it right: The stated value is a radius; the diameter would be about 0.2 nm.

• Using the word 'atom' to mean the nucleus.

  Put it right: The nucleus is much smaller and is found at the centre of the atom.

Exam tips

• Learn the conversion 1 nm = 1 x 10⁻⁹ m and practise converting 0.1 nm into metres.
• When asked to compare sizes, quote the atomic radius as about 0.1 nm or 1 x 10⁻¹⁰ m.
• Use standard form correctly in calculations and show your working clearly.