Writing a formula
Compounds are formed by atoms or ions joining through bonds. So to write the formula of a compound you need to know the combining powers — the valencies — of what is in it. The formula is written so those combining powers balance.
Start with three simple ones
Hydrogen has valency 1; oxygen has valency 2. So two hydrogen atoms can combine with one oxygen atom → H2O
Nitrogen has valency 3. So three hydrogen atoms can combine with one nitrogen atom → NH3
Carbon has valency 4. So four hydrogen atoms can combine with one carbon atom → CH4
The crossing-over method
There is a quick way to balance the combining powers of two elements: write the two symbols side by side and swap their valencies, writing each below and to the right of the other symbol.
| Compound | Symbols | Valencies | Chemical formula |
|---|---|---|---|
| Sodium chloride | Na · Cl | 1 · 1 | NaCl |
| Calcium chloride | Ca · Cl | 2 · 1 | CaCl₂ |
| Sodium oxide | Na · O | 1 · 2 | Na₂O |
| Calcium oxide | Ca · O | 2 · 2 | CaO |
| Magnesium nitride | Mg · N | 2 · 3 | Mg₃N₂ |
Watch calcium oxide
Crossing over gives Ca2O2. But both valencies are 2, so they already balance — and the formula is written in its simplest ratio: CaO. Sodium chloride is the same case with 1 and 1.