Introduction
By now you've met the basic idea of naming ionic compounds: a metal cation plus a non-metal anion, with the anion ending in -ide (like chloride, oxide, sulfide). In this lesson, we go a little deeper into binary compounds (compounds made of only two elements) where the metal has a fixed charge — meaning it always forms the same oxidation state, so we never need to specify it in the name.
Which metals have a fixed charge?
Some groups of the periodic table are very predictable:
- Group 1 metals (Li, Na, K, Rb, Cs) → always form 1+ ions
- Group 2 metals (Mg, Ca, Sr, Ba) → always form 2+ ions
- Aluminium (Group 13) → always forms 3+ ions
- Zinc and silver, although technically transition-ish metals, behave as fixed-charge metals: Zn²⁺ and Ag⁺ always
Because these metals only ever show one charge, we don't add a Roman numeral to their name — unlike transition metals such as cobalt or iron, which can form multiple charges and therefore do need one (e.g. cobalt(II) chloride, Talbot p. 130).
How to name a binary compound
Steps:
- Name the metal cation first, unchanged.
- Name the non-metal anion second, changing its ending to -ide.
- If the metal has a fixed charge, no Roman numeral is needed.
Worked Example 1
Formula: MgCl₂
- Magnesium is Group 2 → always 2+, fixed charge
- Chlorine becomes chloride
- Name: magnesium chloride (no numeral needed)
Worked Example 2
Formula: Al₂O₃
- Aluminium always forms Al³⁺
- Oxygen becomes oxide (O²⁻)
- To balance charges: 2 × (3+) = 6+, and 3 × (2−) = 6−, so the ratio 2:3 works
- Name: aluminium oxide
Worked Example 3
Formula: Zinc + Sulfur → ?
- Zn is fixed-charge: Zn²⁺
- Sulfur becomes sulfide: S²⁻
- Charges balance 1:1
- Formula: ZnS, Name: zinc sulfide
Worked Example 4: going from name to formula
Name: Barium n