When it is not molecules
Salt has no single particle you could call "one piece of salt".
Loading lab…
The rule
An ion is a charged atom or group of atoms. Sodium gives away one outer electron. An electron has a negative charge. So sodium becomes a positive ion, Na⁺. Chlorine takes one electron. So chlorine becomes a negative ion, Cl⁻, called chloride. Opposite charges pull together.
Some substances are ions, not molecules. Ions pack in one repeating ratio. Add the ratio's masses for one formula unit. A metal joined to a non-metal usually gives ions.
- Sodium and chloride ions apart. Key: circle is a sodium ion, square is a chloride ion.
- More ions join in. They pack in a repeating pattern, one of each.
- Zoom out. The same pattern goes on and on.
Formula unit mass
One repeat of the ratio of ions is one formula unit. Its mass is the formula unit mass. The negative ions of oxygen and sulfur are called oxide and sulfide.
| Formula | Ions in the ratio | Working | Mass |
|---|---|---|---|
| MgO | 1 magnesium : 1 oxide | 24 + 16 | 40 u |
| CaO | 1 calcium : 1 oxide | 40 + 16 | 56 u |
| NaCl | 1 sodium : 1 chloride | 23 + 35.5 | 58.5 u |
| MgCl₂ | 1 magnesium : 2 chloride | 24 + 2 × 35.5 | 95 u |
| Na₂S | 2 sodium : 1 sulfide | 2 × 23 + 32 | 78 u |
The small number after a bracket multiplies everything inside it. In Ca(OH)₂, the group OH is counted twice. OH is 16 + 1 = 17 u. Two of them are 2 × 17 = 34 u. So 40 + 34 = 74 u.
Salt dissolved in water carries an electric current. The ions of salt can move. Sugar has no ions, so sugar water does not.