Introduction
Welcome back! In our last lesson, you learned how to name simple binary compounds made from a metal with a fixed charge and a non-metal. Today, we continue that work and sharpen your skills with a few trickier cases: metals that always keep the same charge but are easy to mix up with transition metals, and non-metal anions with slightly different endings. By the end of this lesson, you should be able to confidently name and write formulas for any binary ionic compound where the metal's charge never changes.
Quick recap: what is a "fixed charge" metal?
A fixed-charge metal is one that always forms the same ion, no matter which compound it's in. These include:
- Group 1 metals → always 1+ (Na⁺, K⁺, Li⁺)
- Group 2 metals → always 2+ (Mg²⁺, Ca²⁺, Ba²⁺)
- A few other metals by convention: Al³⁺, Zn²⁺, Ag⁺
Because the charge never varies, we do not need to write a Roman numeral in the name (Talbot, p.130). For example, barium always forms Ba²⁺, so "barium chloride" is unambiguous — there's no need to say "barium(II) chloride."
This is different from transition metals (like iron, cobalt, copper), which can form more than one stable ion. For those, we must include the oxidation state as a Roman numeral, e.g. cobalt(II) chloride for CoCl₂ (Talbot, p.130).
Naming binary compounds: the pattern
The general naming rule for a binary ionic compound is:
[name of metal] + [name of non-metal with -ide ending]
Steps:
- Write the metal name unchanged.
- Take the non-metal's name, keep the root, and change the ending to -ide.
- If the metal is fixed-charge, do not add a Roman numeral.
Worked Examples
Example 1 — Writing a name from a formula Formula: MgBr₂
- Magnesium is Group 2 → always Mg²⁺ (fixed charge)
- Bromine → bromide
- Name: magnesium bromide
Example 2 — Writing a formula from a name Name: aluminium oxide
- Aluminium is fixed-charge: Al³⁺
- Oxide ion: O²⁻
- Balance charges: need 2 Al³⁺ (total 6+) and 3 O²⁻ (total 6−)
- Formula: Al₂O₃
Example 3 — Spotting the difference Compare CaCl₂ and FeCl₂.
- Calcium is Group 2 → fixed charge 2+ → name is simply calcium chloride
- Iron is a transition metal → can be 2+ or 3+ → must specify → iron(II) chloride
This comparison shows why checking the periodic table position matters before naming!
Example 4 — Zinc, a "fixed-charge" exception Formula: ZnS
- Although zinc sits among the transition metals on the periodic table, it only ever forms Zn²⁺ in compounds, so no Roman numeral is needed.
- Name: zinc sulfide
Key takeaways
- Fixed-charge metals (Group 1, Group 2, Al³⁺, Zn²⁺, Ag⁺) never need a Roman numeral in their names.
- Transition metals usually need a Roman numeral because they can form more than one ion charge.
- To name a binary compound: metal name + non-metal root + -ide.
- Always balance the total positive and negative charge when writing a formula from a name.
- IUPAC nomenclature exists to make chemical names unambiguous and universally understood (Talbot, p.172) — a small set of rules lets chemists worldwide "speak the same language."