Writing the formula
Aluminium has valency 3. Chlorine has valency 1. Swapped, they give AlCl₃.
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The rule
A symbol is an element's short name, such as Al. A count is a small number, such as the 3 in AlCl₃. To cut down is to divide both counts by the same number.
Swap the two valencies for counts. Cut the counts down by their shared factor. This gives the simplest formula. Write the symbols, and leave out a count of one.
Crossing the numbers is the same as swapping them.
- Aluminium (valency 3) and chlorine (valency 1), side by side.
- Cross the numbers: 3 becomes chlorine's count, 1 becomes aluminium's.
- Counts 1 and 3 share no factor. Write AlCl₃.
Swap, cut down, write
| Elements | Valencies | Swapped | Cut down | Formula |
|---|---|---|---|---|
| Aluminium, chlorine | 3, 1 | 1, 3 | 1, 3 | AlCl₃ |
| Potassium, oxygen | 1, 2 | 2, 1 | 2, 1 | K₂O |
| Calcium, oxygen | 2, 2 | 2, 2 | 1, 1 | CaO |
| Nitrogen, hydrogen | 3, 1 | 1, 3 | 1, 3 | NH₃ |
Calcium and oxygen swap to 2 and 2. Both share the factor 2. Divide each by 2 to get 1 and 1. A count of 1 is not written. So the formula is CaO.
Check the answer. One aluminium carries 1 × 3 = 3 hooks. Three chlorine carry 3 × 1 = 3 hooks. Both totals are 3. So every hook is used, and AlCl₃ is right.