Writing chemical formulae
A chemical formula shows the ratio of atoms in a compound. For ionic compounds we combine ions so the total positive charge equals the total negative charge, giving a neutral compound. Common ions include Na+, Mg2+, Al3+, Cl−, O2−, OH−, NO3−, SO42− and CO32−.
Key idea
Balance the charges by 'swapping and dropping': the charge on one ion becomes the subscript of the other. Example: Al3+ and O2− give Al2O3.
Balancing equations
A balanced equation must have the same number of each type of atom on both sides, because atoms are neither created nor destroyed. You may only place numbers in front of formulae (multipliers); never change a correct formula.
Worked example
Balance the combustion of methane. Start: CH4 + O2 → CO2 + H2O.
Balance H: 4 H on the left, so put 2 in front of H2O → 2H2O (4 H). Now count O on the right: 2 (from CO2) + 2 (from 2H2O) = 4 O. Put 2 in front of O2.
Balanced: CH4 + 2O2 → CO2 + 2H2O. Check: C 1=1, H 4=4, O 4=4. ✓
Remember
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
- The diatomic elements are H2, N2, O2, F2, Cl2, Br2, I2.