Formulae

Section 1 of 10Chemical symbols and molecular formulae

Chemical symbol

• Chemical symbol an element is represented by the symbol from the Periodic Table, such as Li, Cr, Al, Ca, Au and S

Diatomic elements

• Diatomic elements seven elements exist as molecules of two atoms: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂

Molecular formula

• Molecular formula shows the type of atoms by their symbols and the number of atoms by the subscript after each symbol
• No subscript means one atom.
• A subscript after a bracket multiplies every atom inside the bracket.

Deducing a formula

• Deducing a formula count the atoms of each element in a model or diagram of the molecule
• In the diagram of water the two hydrogen atoms are each bonded to the oxygen atom, not joined in a row.

Substance	Molecular formula	Atoms present
Hydrogen	H2	2 H
Chlorine	Cl2	2 Cl
Water	H2O	2 H, 1 O
Methane	CH4	1 C, 4 H
Ammonia	NH3	1 N, 3 H
Sulfuric acid	H2SO4	2 H, 1 S, 4 O

Section 2 of 10Empirical formulae (Extended)

Empirical formula

Empirical formula the simplest whole-number ratio of the atoms of each element in one molecule or formula unit

Organic compounds

• Organic compounds the empirical formula is often different from the molecular formula
• Ethanoic acid is C₂H₄O₂ but its empirical formula is CH₂O.
• C₂H₄ has the empirical formula CH₂.

Ionic compounds

• Ionic compounds the chemical formula is always the empirical formula, such as NaCl

Section 3 of 10Formulae of ionic compounds (Extended)

Ionic compounds

• Ionic compounds form when metals and non-metals react, so they are not simple molecules

No overall charge

• No overall charge the positive charge of the ions cancels the negative charge

Ions to know

• H⁺; Group 1 ions such as Li⁺, Na⁺, K⁺; Group 7 ions F⁻, Cl⁻, Br⁻; Cu²⁺, Fe²⁺, Fe³⁺

Polyatomic ions

• Polyatomic ions carbonate CO₃²⁻, sulfate SO₄²⁻, hydroxide OH⁻, nitrate NO₃⁻, ammonium NH₄⁺

1

Write the ions with their charges.

2

Compare the charges and add ions until the total positive charge equals the total negative charge.

3

Or use swap-and-drop: swap the charge numbers between the ions and drop them as subscripts, ignoring the signs and the number 1.

4

If both ions have the same size of charge, give the simplest whole-number ratio.

5

Put brackets around a polyatomic ion when more than one is needed.

Section 4 of 10Word equations

Reactants

• Reactants the substances on the left of the arrow, like the chemical ingredients

Products

• Products the new substances formed, on the right of the arrow

Arrow

• Arrow means ‘to form’ or ‘produces’
• Conditions or the name of a catalyst can be written above the arrow.

Burning in air

• Burning in air means reacting with oxygen, for example carbon + oxygen → carbon dioxide

Example

• Example sodium hydroxide + hydrochloric acid → sodium chloride + water

Order in a sentence

• Order in a sentence the products may be given before the reactants, so check which are formed

Section 5 of 10Symbol equations and state symbols

Symbol equation

• Symbol equation uses the formulae of the reactants and products, with reactants on the left and products on the right

Diatomic non-metals

• Diatomic non-metals write H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ as molecules

State symbols

• (s) solid, (l) liquid, (g) gas, (aq) aqueous
  • (dissolved in water)

Clues for state

• Clues for state metal compounds and ionic compounds are usually solid unless in solution, non-metal compounds can be solids, liquids or gases,
  • and precipitates count as solids

Ethanol

• Ethanol pure ethanol is (l) but ethanol in water is (aq)

Combustion of sulfur

• S(s) + O₂(g) → SO₂(g)

Section 6 of 10Balancing equations

1Law of conservation of mass

• Law of conservation of mass there must be the same number of atoms of each element on both sides

2Change only coefficients

• Change only coefficients the numbers in front of formulae are changed, never the formulae themselves

1

Write the correct formulae for all reactants and products.

2

Work across from left to right, checking one element after another.

3

Count an unchanged group such as NO₃ as one unit.

4

Balance elements that appear on their own last.

5

Check the number of atoms of each element on both sides.

• Deducing equations (Extended)
  • work out the formulae from the element, the ion charges and the given states before balancing
  • Aluminium burning in chlorine gives 2Al(s) + 3Cl₂(g) → 2AlCl₃(s).

Section 7 of 10Ionic equations (Extended)

1Dissociation

• Dissociation in aqueous solution ionic compounds separate into ions, such as HCl(aq) → H⁺(aq) + Cl⁻(aq)

2Compounds that dissociate

• Compounds that dissociate acids such as HCl and H₂SO₄, Group 1 and 2 hydroxides and soluble salts

1

Write the full balanced symbol equation.

2

Replace the aqueous ionic compounds with their component ions.

3

Remove the ions that appear on both sides (spectator ions).

4

Include state symbols in the ionic equation.

Section 8 of 10More from the specification

More from the specification

Deducing Formulae from Ionic Charges

• Ionic compounds must be electrically neutral;
  • combining magnesium ions (Mg²⁺) with nitrate ions (NO₃⁻) requires two nitrate ions per magnesium ion,
• giving magnesium nitrate the formula Mg(NO₃)₂.

Transition Metal Valency and Roman Numerals

• Roman numerals in chemical names denote oxidation states of transition metals,
  • such as copper(II) indicating Cu²⁺ in copper(II) chloride (CuCl₂) and iron(III) indicating Fe³⁺ in iron(III) chloride (FeCl₃).

Relative Atomic Mass and Molecular Formulae

• The relative formula mass (Mr) of a compound is calculated by summing the relative atomic masses (Ar) of all constituent atoms shown in its chemical formula.

Section 9 of 10Key terms

Term	Meaning
Compound	Two or more elements chemically joined.
Molecular formula	The number and type of atoms in one molecule.
Empirical formula	The simplest whole-number ratio of atoms or ions.
Subscript	The small number after a symbol that gives how many atoms.
Reactant	A substance on the left of the arrow.
Product	A substance formed in a reaction.

Section 10 of 10Exam technique

1

Changing a subscript to balance an equation — change only the coefficients

2

Writing CaOH₂ — use bracketsCa(OH)₂

3

Writing the products before the reactants — reactants go on the left

4

Leaving out state symbols — include (s), (l), (g) or (aq)