Naming Compounds

Ionic: metal + non-metal

The metal keeps its name; the non-metal takes the ending -ide. No prefixes.

• $\mathrm{NaCl}$ — sodium chloride
• $\mathrm{MgO}$ — magnesium oxide

Molecular: non-metal + non-metal

Prefixes on both, except mono- is dropped on the first.

Prefix	Number
mono-	1
di-	2
tri-	3
tetra-	4
penta-	5

Decide the type first

Prefixes on an ionic compound is the most common error on this page. Check whether the first element is a metal before you write anything.

Name these

1. $\mathrm{KBr}$

Answer

Potassium bromide. Metal + non-metal, so no prefixes.

2. $\mathrm{Al}{}_2\mathrm{O}{}_3$

Answer

Aluminium oxide. The subscripts balance the charges ($\mathrm{Al}{}^{3+}$ with $\mathrm{O}{}^{2-}$) — they are not named.

3. $\mathrm{SO}{}_2$

Answer

Sulfur dioxide. Two non-metals, so prefixes are used — and mono- is dropped from the first element.

4. $\mathrm{PCl}{}_5$

Answer

Phosphorus pentachloride.

5. $\mathrm{Li}{}_2\mathrm{S}$

Answer

Lithium sulfide. Not “dilithium” — it is ionic.

Write the formula

6. Calcium fluoride

Answer

$\mathrm{CaF}{}_2$ — one $\mathrm{Ca}{}^{2+}$ needs two $\mathrm{F}{}^{-}$.

7. Sodium oxide

Answer

$\mathrm{Na}{}_2\mathrm{O}$ — two $\mathrm{Na}{}^{+}$ to balance one $\mathrm{O}{}^{2-}$.

8. Carbon tetrachloride

Answer

$\mathrm{CCl}{}_4$

9. Diphosphorus pentoxide

Answer

$\mathrm{P}{}_2\mathrm{O}{}_5$

10. Aluminium chloride

Answer

$\mathrm{AlCl}{}_3$ — $\mathrm{Al}{}^{3+}$ needs three $\mathrm{Cl}{}^{-}$.

Reference: Ions and Ionic Compounds, Molecular Compounds