Chemical formulas
A chemical formula uses element symbols and subscripts to show what a substance is made of.
- A subscript after a symbol tells how many of that atom there are. No subscript means 1.
- Parentheses group atoms together. A subscript after the parentheses multiplies everything inside.
- A coefficient in front of the formula tells how many whole units there are. It multiplies everything in the formula.
For a molecular substance, the formula describes one molecule. For an ionic compound, it gives the simplest ratio of ions.
Worked example: Ca(NO3)2
| Element | Count |
|---|---|
| Ca | 1 |
| N | 1 × 2 = 2 |
| O | 3 × 2 = 6 |
One formula unit contains 9 atoms.
Worked example: 3 H2O
Each water molecule has 2 hydrogen atoms and 1 oxygen atom. Three molecules have 6 hydrogen atoms and 3 oxygen atoms.
Common mistake
Reading a two-letter symbol as two elements. Co is cobalt, one element. CO is carbon monoxide, a compound of carbon and oxygen. The second letter of a symbol is always lowercase.