Chemistry / Unit 5: Chemical Names and Formulas
5.1 Naming Binary Ionic Compounds
Notes
A binary ionic compound has a metal cation and a nonmetal anion. Name the metal, then the nonmetal with the ending -ide.
- NaCl is sodium chloride; $\text{CaO}$ is calcium oxide; $\text{Al}_2\text{S}_3$ is aluminum sulfide.
- To write a formula, criss-cross the charges and reduce: $\text{Al}^{3+}$ and $\text{O}^{2-}$ give $\text{Al}_2\text{O}_3$.
- Many transition metals form more than one ion, so use a Roman numeral for the charge: $\text{FeCl}_3$ is iron(III) chloride; $\text{FeCl}_2$ is iron(II) chloride.
- To find the Roman numeral, work backward from the anion: in $\text{CuO}$, oxide is 2-, so copper is 2+: copper(II) oxide.
- Common fixed charges: group 1 is 1+, group 2 is 2+, Al 3+, Zn 2+, Ag 1+; N 3-, O 2-, S 2-, halogens 1-.
Practice
What is the name of $\text{MgBr}_2$?
Magnesium has only one charge (2+), so no prefix or Roman numeral is needed.
What is the formula for aluminum oxide?
$2(+3) + 3(-2) = 0$.
What is the name of $\text{Fe}_2\text{O}_3$?
Three oxide ions give $-6$, so the two Fe ions share $+6$: each is $3+$.
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