Topic 1 · page 1 of 10

Particles and formulae

A formula is a count of atoms. Before you can calculate anything, you need to know what kind of particle a substance is made of and what its formula is counting.

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Five words for particles and substances 1.1

Chemists use these words precisely, and examiners mark them precisely.

Two of these words describe what a substance is (element or compound). The other three describe what it is made of (atoms, molecules or ions). Every substance needs one word from each group.

Four boxes of particles. Argon: separate single atoms. Oxygen: pairs of atoms bonded as O2 molecules. Water: bent molecules of one oxygen and two hydrogen atoms. Sodium chloride: a regular grid of alternating sodium and chloride ions with no separate molecules.
Element or compound, and atoms, molecules or ions. Sodium chloride has no molecules at all.
Substance Element or compound? Made of
argon, Ar element single atoms
oxygen, O₂ element molecules
water, H₂O compound molecules
carbon dioxide, CO₂ compound molecules
sodium chloride, NaCl compound ions in a giant lattice
diamond, C element atoms in a giant covalent lattice
silicon dioxide, SiO₂ compound atoms in a giant covalent lattice

Empirical and molecular formulae 1.1

To get the empirical formula from a molecular formula, divide every subscript by the largest number that goes into all of them.

Substance Molecular formula Empirical formula
ethene C₂H₄ CH₂
glucose C₆H₁₂O₆ CH₂O
hydrogen peroxide H₂O₂ HO
butane C₄H₁₀ C₂H₅
benzene C₆H₆ CH
water H₂O H₂O
sodium chloride none NaCl

The molecular formula is always a whole-number multiple of the empirical formula:

molecular formula=(empirical formula)nn=Mrmass of one empirical unit \text{molecular formula} = (\text{empirical formula})_n \qquad n = \frac{M_r}{\text{mass of one empirical unit}}

You will use this on masses from equations to turn analysis data into a molecular formula.

Check yourself

  1. Classify each as element or compound, and say whether it is made of atoms, molecules or ions: (a) N₂ (b) MgO (c) neon (d) ammonia, NH₃.
  2. Give the empirical formula of: (a) C₂H₆ (b) N₂O₄ (c) C₃H₆O (d) P₄O₁₀.
  3. Explain why magnesium oxide has an empirical formula but no molecular formula.
  4. A compound has empirical formula CH₂ and relative molecular mass 70. Find its molecular formula.
  5. State what is wrong with: "An ion is an atom that has gained electrons."

Answers cover these until you have tried the questions

  1. (a) element, molecules. (b) compound, ions (Mg²⁺ and O²⁻ in a giant lattice). (c) element, single atoms. (d) compound, molecules.
  2. (a) CH₃ (b) NO₂ (c) C₃H₆O: the numbers 3, 6 and 1 share no common factor, so it does not simplify. (d) P₂O₅
  3. Magnesium oxide is a giant ionic lattice. There are no separate molecules to count, only a fixed 1 : 1 ratio of Mg²⁺ to O²⁻ ions.
  4. One CH₂ unit has mass 12.0 + 2(1.0) = 14.0. n = 70 ÷ 14.0 = 5, so the molecular formula is C₅H₁₀.
  5. It is incomplete. An ion can also form when an atom loses electrons, and an ion can be a group of atoms (for example SO₄²⁻ or NH₄⁺).