Naming chemical compounds is like giving everyone a unique address – if we all follow the same rules, any chemist anywhere in the world can read a formula and know exactly what substance it refers to. This system of rules is called nomenclature, and it's maintained by IUPAC, the international body responsible for keeping chemical names consistent across the globe (Talbot, p.172). Today we start with the simplest case: binary compounds (compounds made of just two elements) where the metal has a fixed charge.
What is a "binary compound"?
A binary compound contains only two different elements. When one of those elements is a metal and the other is a non-metal, the compound is usually ionic – made of a metal cation (positive ion) and a non-metal anion (negative ion).
Metals with a fixed charge
Some metals always form ions with the same charge, no matter what compound they're in. These are mainly:
- Group 1 metals (alkali metals) → always form a 1+ ion (e.g., Na⁺, K⁺)
- Group 2 metals (alkaline earth metals) → always form a 2+ ion (e.g., Mg²⁺, Ca²⁺)
- Aluminium (group 13) → always forms a 3+ ion (Al³⁺)
- A few others you'll meet later, like Zn²⁺ and Ag⁺, also behave this way even though they are transition/d-block metals
Because the charge is always the same, we don't need to state it in the name (Talbot, p.130). This is different from transition metals like cobalt or iron, which can form more than one possible ion – those need special treatment (that's next lesson!).
The naming rule
For a binary ionic compound with a fixed-charge metal:
[name of metal] + [non-metal stem] + "-ide"
The non-metal's name is shortened to its stem and the ending -ide is added.
| Non-metal | Ion formed | Name ending |
|---|---|---|
| Chlorine | Cl⁻ | chloride |
| Oxygen | O²⁻ | oxide |
| Nitrogen | N³⁻ | nitride |
| Sulfur | S²⁻ | sulfide |
| Fluorine | F⁻ | fluoride |
Worked Examples
Example 1: NaCl Sodium is group 1 → always 1+. Chlorine becomes "chloride." → sodium chloride
Example 2: MgO Magnesium is group 2 → always 2+. Oxygen becomes "oxide." → magnesium oxide
Example 3: AlN Aluminium is fixed at 3+, nitrogen becomes "nitride." → aluminium nitride
Example 4: BaCl₂ Even though there are two chloride ions, barium (group 2) is always 2+, so we don't need to show this in the name. → barium chloride (not "barium(II) chloride" – that would be unnecessary, as shown in Talbot, p.130)
A quick check: writing formulas from names
The naming process also works in reverse. If you're told "calcium fluoride," you know:
- Ca²⁺ (fixed charge, group 2)
- F⁻ (fluoride)
To balance charges, you need two F⁻ for every one Ca²⁺ → CaF₂
Key takeaways
- A binary compound contains exactly two elements.
- Metals in groups 1, 2, and 13 (Al), plus a few special cases like Zn and Ag, have a fixed charge, so their oxidation state is never written in the name.
- The naming pattern is metal name + non-metal stem + "-ide".
- Always balance the charges when working out the formula from a name (e.g., Ca²⁺ and F⁻ combine as CaF₂).
- Next lesson, we'll tackle metals with variable charges (like transition metals), where we do need to include the oxidation state in the name.