STUDENT SPACE · TOPIC 3
Writing ionic formulae
Use ion charges to work out the simplest whole-number ratio in an ionic formula.
The essentials
- An ionic compound has no overall charge. Total positive charge equals total negative charge.
- Use the simplest whole-number ratio of ions. Al³⁺ and O²⁻ combine in a 2:3 ratio to give Al₂O₃.
- Keep polyatomic ions together. In Ca(NO₃)₂, the subscript 2 applies to the whole nitrate ion.
Key vocabulary
| English | Chinese |
|---|---|
| formula unit | 化学式单位 |
| polyatomic ion | 多原子离子 |
| overall charge | 总电荷 |
Quick practice
Write the formula from Mg²⁺ and OH⁻.
Need a hint?
One hydroxide ion has charge −1.
Check your reasoning after trying
Mg(OH)₂.
Use the matching chapter worksheet. Attempt independently, compare your reasoning with your partner, then bring unresolved questions to your teacher.