Recall the typical size (order of magnitude) of atoms and small molecules

The idea

Atoms and small molecules have a typical diameter of about 1 × 10⁻¹⁰ m, that is 0.1 nanometres or 100 picometres. This is an order-of-magnitude figure: real atoms range roughly from 10⁻¹⁰ m to a few times 10⁻¹⁰ m, and small molecules such as water are of similar size, around 10⁻¹⁰ m to 10⁻⁹ m across. The key skill is recognising the correct power of ten. A value near 10⁻¹⁰ m is right for an atom; 10⁻¹⁵ m is the scale of a nucleus, and 10⁻³ m is a millimetre, far too large. In a multiple-choice question, compare each option's exponent with 10⁻¹⁰ m and reject options that are orders of magnitude too big or too small. Converting between metres and nanometres helps: 1 nm = 1 × 10⁻⁹ m, so 0.1 nm = 1 × 10⁻¹⁰ m.

What earns the marks

• Recalls that the typical diameter of an atom is of the order of 1 × 10⁻¹⁰ m.
• Recognises that small molecules are of a similar order of magnitude to atoms, around 10⁻¹⁰ m.
• Rejects values of the order of 10⁻¹⁵ m, which describe the nucleus rather than the whole atom.
• Converts correctly between metres and nanometres, using 1 nm = 1 × 10⁻⁹ m.

Common mistakes and how to put them right

• Chooses 10⁻¹⁵ m as the size of an atom.

  Put it right: State that 10⁻¹⁰ m is the order of magnitude for an atom and 10⁻¹⁵ m is the nuclear scale.

• Confuses nanometres with metres.

  Put it right: Convert using 1 nm = 1 × 10⁻⁹ m so 0.1 nm equals 1 × 10⁻¹⁰ m.

• Treats a small molecule as far larger than an atom.

  Put it right: State that small molecules are of a similar order of magnitude, around 10⁻¹⁰ m.

• Selects a millimetre-scale value.

  Put it right: Reject options near 10⁻³ m as far too large for an atom.

Exam tips

• Focus on the power of ten: the correct order of magnitude for an atom is 10⁻¹⁰ m.
• Use the conversion 1 nm = 1 × 10⁻⁹ m to check any option given in nanometres.
• Reject options near 10⁻¹⁵ m, which belong to the nucleus, and options near 10⁻³ m, which are millimetre scale.
• Remember that small molecules are about the same order of magnitude as atoms.