Chemical formulas: what a formula tells you

Symbols, formula units and molecules

Stoichiometry uses a balanced equation to work out amounts of reactants and products. It rests on the law of conservation of mass: total mass of reactants equals total mass of products.

Elements that exist as a mass of atoms are written by their symbol alone: Na, Ca, C, Fe. Elements made of molecules show the atoms bonded together: O₂, N₂, H₂, and ozone O₃.

An ionic compound is a crystal of ions, so it is written as a formula unit showing the smallest ion ratio: NaCl (1:1), CaCl₂, KBr.

A covalent compound exists as molecules, so its molecular formula shows every atom in one molecule. H₂O means two hydrogen atoms bonded to one oxygen atom.

Ask first what the substance is made of: single atoms, molecules, or ions. That decides how its formula is written.

Key terms

Stoichiometry

Working out how much reacts and forms.

Chemical formula

Showing which atoms and how many.

Formula unit

Writing ionic compounds.

Molecular formula

Writing covalent compounds and molecular elements.

Short questions with model answers

1. Q1. Read the formula H₂SO₄. (p. 53)

   • Symbols: H, S, O
   • Subscripts: 2, (1), 4

   One molecule has 2 H, 1 S and 4 O atoms: 7 atoms in all.

2. Q2. Why is sodium chloride written NaCl, not as a molecule? (p. 53)

   • NaCl is ionic
   • Ratio 1 : 1

   NaCl is a formula unit: the 1:1 ratio of ions in the crystal.

Common mistakes

• ✗ NaCl is one molecule made of one Na and one Cl atom.

  ✓ NaCl is a formula unit of a crystal of ions in a 1:1 ratio.

• ✗ Oxygen gas is written O, like sodium is written Na.

  ✓ Oxygen exists as O₂ molecules; only elements made of a mass of atoms use the symbol alone.

• ✗ In H₂O there is no oxygen count, so the number of oxygen atoms is unknown.

  ✓ No subscript means one atom: H₂O has one oxygen.

MCQs

1. 1. Which formula is written by the symbol alone?

   1. (a) O₂
   2. (b) Fe
   3. (c) N₂
   4. (d) H₂O
   Show answer

   (b) p. 53: elements that exist as an aggregate of atoms, such as iron, use the symbol alone.

2. 2. What is the formula of ozone?

   1. (a) O
   2. (b) O₂
   3. (c) O₃
   4. (d) O₄
   Show answer

   (c) p. 53: three oxygen atoms are bonded together in ozone.

3. 3. NaCl is called a formula unit because:

   1. (a) It is a gas
   2. (b) It is one molecule
   3. (c) It has no charge
   4. (d) It shows the smallest ratio of ions in a crystal
   Show answer

   (d) p. 53: ionic compounds are written by formula units showing the minimum ion ratio.

4. 4. How many atoms are in one molecule of NH₃?

   1. (a) 4
   2. (b) 3
   3. (c) 2
   4. (d) 1
   Show answer

   (a) One nitrogen and three hydrogen atoms: 4.

5. 5. Stoichiometry is based on which law?

   1. (a) Law of conservation of mass
   2. (b) Octet rule
   3. (c) Boyle's law
   4. (d) Law of gravity
   Show answer

   (a) p. 52: matter can neither be created nor destroyed, so the masses balance.

Quick revision

• Atomic elements use the symbol (Fe); molecular elements show their atoms (O₂, O₃).
• Ionic compounds use formula units: the smallest ion ratio (NaCl, CaCl₂).
• Covalent compounds use molecular formulas: every atom in one molecule (H₂O).