Writing formulae from charges
The formula of an ionic compound tells you the ratio of ions in it. Because a compound is neutral overall, the positive and negative charges must cancel exactly. So the first job is to know the charges (the valency) of the ions involved.
For simple ions the charge follows the group number. Group 1 metals form 1+ ions, Group 2 form 2+, Group 3 form 3+, Group 6 non-metals form 2-, and Group 7 form 1-. Some common ones to memorise:
| Positive ions | Charge | Negative ions | Charge |
|---|---|---|---|
| Sodium, Na+ | 1+ | Chloride, Cl- | 1- |
| Magnesium, Mg2+ | 2+ | Oxide, O2- | 2- |
| Aluminium, Al3+ | 3+ | Hydroxide, OH- | 1- |
| Ammonium, NH4+ | 1+ | Sulfate, SO42- | 2- |
- Valency
- The combining power of an atom or ion, equal to the size of the charge it carries or the number of bonds it forms.
The swap-and-drop method
To build the formula, balance the charges so they cancel. A quick trick: write each ion with its charge, then swap the size of the charges to become the numbers below the line, and cancel down if you can.
Write the formula of aluminium oxide, made from Al3+ and O2-.
Common slip
When a formula needs more than one of a group ion, wrap it in brackets, so calcium hydroxide is Ca(OH)2, not CaOH2. The small 2 must multiply the whole hydroxide group.
Balancing equations
In a chemical reaction, atoms are only rearranged, never created or destroyed (the law of conservation of mass). So a balanced equation must have the same number of each type of atom on both sides. You balance by putting big numbers (coefficients) in front of formulae. You must never change a formula's small subscript numbers to balance it.
Balance the equation for hydrogen burning in oxygen: H2 + O2 to H2O.
A useful order
Balance metals first, then non-metals, and leave hydrogen and oxygen until last, since they often appear in several compounds. Recount every element after each change: fixing one can unbalance another.
State symbols
A small letter in brackets after each formula shows its physical state in the reaction. They are written in italics and placed as subscripts.
| Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | NaCl(s) |
| (l) | Liquid | H2O(l) |
| (g) | Gas | CO2(g) |
| (aq) | Aqueous (dissolved in water) | HCl(aq) |
Take care with (l) and (aq): a pure liquid like molten water is (l), while a substance dissolved in water, such as hydrochloric acid, is (aq). For example: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l).
Where this is assessed
A fully correct, balanced symbol equation with state symbols is prized under Criterion C (communicating in scientific language). Losing marks here is nearly always from changing a subscript instead of adding a coefficient, or forgetting a state symbol.
Check yourself
1. Write the formula of magnesium chloride, from Mg2+ and Cl-. +
Magnesium is 2+ and chloride is 1-, so you need two chloride ions to balance one magnesium. Swapping the charges down gives MgCl2. Check: \(1 \times (2+) = 2+\) and \(2 \times (1-) = 2-\), which cancel.
2. Balance: Na + Cl2 to NaCl. +
The right side has 1 chlorine but the left has 2, so put a 2 before NaCl: Na + Cl2 to 2NaCl. Now sodium is 2 on the right and 1 on the left, so put a 2 before Na. Final: 2Na + Cl2 → 2NaCl (2 Na and 2 Cl each side).
3. What does the (aq) mean in HCl(aq), and how does it differ from (l)? +
(aq) means aqueous, that is, the substance is dissolved in water. It is different from (l), which means a pure substance in the liquid state on its own. So HCl(aq) is hydrochloric acid dissolved in water, while H2O(l) is pure liquid water.
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