STUDENT SPACE · TOPIC 1
Formulae and relative masses
Write ionic formulae and calculate relative molecular or formula masses.
The essentials
- NaCl is a giant ionic lattice: one Na⁺ ion for each Cl⁻ ion.
- A molecular formula gives the number and type of atoms in one molecule.
- Al³⁺ and O²⁻: 2(+3) + 3(−2) = 0 → Al₂O₃.
- Ca²⁺ and NO₃⁻: use brackets → Ca(NO₃)₂.
Key vocabulary
| English | Chinese |
|---|---|
| formula | 化学式 |
| relative atomic mass | 相对原子质量 |
| relative formula mass | 相对式量 |
Quick practice
Why is 24 + 16 + 2(1) incorrect for Mg(OH)₂?
Need a hint?
Use the relationships above and check particle ratios and units.
Check your reasoning after trying
The 2 multiplies O as well as H: use 24 + 2(16 + 1).
Use Lesson 1 in your worksheet for written practice. Your teacher will discuss those answers in class.