Symbols, valency, formulae and balanced equations: the shorthand every chemist uses.
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Every element has a symbol of one or two letters (H, C, Na, Fe). A molecule of a compound is shown by a formula, which gives the elements and the number of atoms of each. Valency is the combining capacity of an atom: sodium 1, magnesium 2, aluminium 3; chlorine 1, oxygen 2, nitrogen 3. A radical is a group of atoms that behaves as one unit and carries a charge, such as sulphate SO₄²⁻, nitrate NO₃⁻, hydroxide OH⁻, carbonate CO₃²⁻ and ammonium NH₄⁺. To write a formula use the criss-cross method: write the symbols with their valencies, cross the valencies over as subscripts and reduce to the simplest ratio.
Write the formula of magnesium chloride (Mg valency 2, Cl valency 1).
Cross over: Mg₁Cl₂, written MgCl₂.
Write the formula of aluminium sulphate (Al valency 3, SO₄ valency 2).
Cross over: Al₂(SO₄)₃. The bracket shows that the whole sulphate radical is repeated three times: Al₂(SO₄)₃.
Write the formula of calcium hydroxide (Ca valency 2, OH valency 1).
Ca(OH)₂ — the bracket is needed because OH is a radical taken twice.
A chemical equation shows reactants on the left and products on the right. By the law of conservation of mass, the number of atoms of each element must be equal on both sides, so we balance by writing coefficients in front of formulae (never by changing a formula). The steps: write the correct formulae, count the atoms of each element on both sides, adjust coefficients one element at a time, and recheck. State symbols (s), (l), (g), (aq) show the physical state.
Balance: Fe + O₂ → Fe₂O₃.
4Fe + 3O₂ → 2Fe₂O₃. Check: Fe 4 = 2 × 2 and O 3 × 2 = 6 = 2 × 3. Both sides: 4 Fe and 6 O.
Show that mass is conserved in 2H₂ + O₂ → 2H₂O (H = 1, O = 16).
Left: 2 × 2 + 32 = 36 g. Right: 2 × 18 = 36 g. Equal.
Why can we not balance an equation by changing a subscript, for example writing H₂O₂ for water?
Changing the subscript changes the substance; H₂O₂ is hydrogen peroxide, not water. Only coefficients may be changed.
Relative atomic mass compares an atom with 1/12 of a carbon-12 atom; it has no unit. The relative molecular mass is the sum of the relative atomic masses of all atoms in the formula. Percentage of an element = (mass of the element in one formula ÷ relative molecular mass) × 100. Values used here: H = 1, C = 12, N = 14, O = 16, S = 32, Ca = 40.
Find the relative molecular masses of H₂O, CO₂, CaCO₃ and H₂SO₄.
H₂O: 2 + 16 = 18. CO₂: 12 + 32 = 44. CaCO₃: 40 + 12 + 48 = 100. H₂SO₄: 2 + 32 + 64 = 98.
Find the percentage of oxygen and hydrogen in water.
O: 16 ÷ 18 × 100 = 88.89%. H: 2 ÷ 18 × 100 = 11.11%. Total 100%.
Find the percentage of nitrogen in ammonium nitrate, NH₄NO₃.
Mass = 28 + 4 + 48 = 80. N: 28 ÷ 80 × 100 = 35%.
1. What is a molecule?
2. Define valency.
3. Write the formula of sodium carbonate (Na 1, CO₃ 2).
4. Write the formula of calcium nitrate (Ca 2, NO₃ 1).
5. Write the formula of zinc sulphate (Zn 2, SO₄ 2).
6. What is a radical? Give two examples.
7. Balance: H₂ + N₂ → NH₃.
8. Find the relative molecular mass of Na₂CO₃ (Na = 23, C = 12, O = 16).
9. Find the percentage of calcium in CaCO₃.
10. State the law of conservation of mass.