Question 1
Write the formula from Mg²⁺ and OH⁻.
Need a hint for Question 1?
One hydroxide ion has charge −1.
Check Question 1
Mg(OH)₂.
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
Try each question before opening its hint or answer. Use paper for calculations and diagrams.
Question 2
Write the formulae of potassium oxide and iron(III) chloride.
Need a hint for Question 2?
Balance K⁺ with O²⁻, and Fe³⁺ with Cl⁻.
Check Question 2
K₂O and FeCl₃.
Question 3
Write the formula of ammonium sulfate using NH₄⁺ and SO₄²⁻.
Need a hint for Question 3?
Treat each polyatomic ion as one unit and use brackets when needed.
Check Question 3
(NH₄)₂SO₄. Two ammonium ions balance one sulfate ion.
Use the matching chapter worksheet. Attempt independently, compare your reasoning with your partner, then bring unresolved questions to your teacher.