the idea
First decide which kind of compound it is, then apply that kind's rules.
1. Does the formula start with H and is it dissolved in water?
Yes → acid rules. HCl(aq) is hydrochloric acid. Start further down for anything else.
No → go to question 2.
2. Does it start with a metal (or NH₄⁺)?
Yes → ionic. Name the cation, then the anion. No prefixes, ever. NaCl is sodium chloride, not "monosodium monochloride."
No, two nonmetals → molecular. Use prefixes: CO₂ is carbon dioxide.
3. Ionic: can the metal have more than one charge?
Fixed charge (Group 1, Group 2, Al³⁺, Zn²⁺, Ag⁺): just the name. CaCl₂ is calcium chloride.
Variable (most transition metals, Sn, Pb): add a Roman numeral for the charge. FeCl₃ is iron(III) chloride.
Plant version: you already read these names on the chemical shelf. Caustic is sodium hydroxide (ionic). Chlorine sanitizer is sodium hypochlorite (ionic, with a polyatomic anion). The acid wash is nitric or phosphoric acid (acid rules). The refrigerant is ammonia (molecular, with a common name). Each one is a different rulebook.
try it
Build an ionic compound
Pick a cation and an anion. The charges have to cancel to zero. That's the whole reason subscripts exist.
Cation (+)
Anion (−)
memorize these
Polyatomic ions you'll actually see
The pattern: -ate has one more oxygen than -ite. Per-…-ate adds another, and hypo-…-ite takes one away. Adding H⁺ to the front lowers the charge by 1.
| Ion | Name | Where you've seen it |
|---|---|---|
| NH₄⁺ | ammonium | The only common positive polyatomic ion |
| OH⁻ | hydroxide | Caustic (NaOH) and KOH cleaners |
| NO₃⁻ / NO₂⁻ | nitrate / nitrite | Nitric acid wash · cured meats |
| SO₄²⁻ / SO₃²⁻ | sulfate / sulfite | Sulfites are a labeled allergen |
| PO₄³⁻ | phosphate | Calcium phosphate holds casein micelles together |
| HPO₄²⁻ / H₂PO₄⁻ | hydrogen / dihydrogen phosphate | Buffering salts in processed cheese and creamers |
| CO₃²⁻ / HCO₃⁻ | carbonate / bicarbonate | Soda ash · baking soda |
| C₂H₃O₂⁻ | acetate | Vinegar is acetic acid |
| ClO⁻ · ClO₂⁻ · ClO₃⁻ · ClO₄⁻ | hypochlorite · chlorite · chlorate · perchlorate | Bleach sanitizer is hypochlorite |
| MnO₄⁻ | permanganate | Purple oxidizer, classic titrant |
| CN⁻ | cyanide | — |
the other two rulebooks
Molecular compounds and acids
Molecular (two nonmetals)
Prefixes: mono-1, di-2, tri-3, tetra-4, penta-5, hexa-6, hepta-7, octa-8, nona-9, deca-10.• Never put mono- on the first element: CO is carbon monoxide, not monocarbon.
• Drop the a/o before "oxide": pentoxide, monoxide.
• Second element ends in -ide.
ClO₂ = chlorine dioxide (a sanitizer)
N₂O = dinitrogen monoxide (whipped cream propellant)
Acids (H + anion, in water)
-ide → hydro…ic acidCl⁻ chloride → HCl, hydrochloric acid
-ate → …ic acid
NO₃⁻ nitrate → HNO₃, nitric acid
-ite → …ous acid
ClO⁻ hypochlorite → HClO, hypochlorous acid
Mnemonic: "I ate something icky," "a bite of something delicious."
The sanitizer connection: sodium hypochlorite (NaOCl) in water forms hypochlorous acid (HOCl), the actual killer. Its pKa is about 7.5, so HOCl dominates below pH 7.5 and the much weaker OCl⁻ ion takes over above it. That's the naming rules and acid-base chemistry explaining why chlorine sanitizer loses power as the pH climbs.
drill
Name ↔ formula
exam traps
Common mistakes
- Using prefixes on ionic compounds. MgCl₂ is magnesium chloride, not magnesium dichloride.
- Forgetting parentheses: three hydroxides on aluminum is Al(OH)₃, not AlOH₃.
- Reading the Roman numeral as a subscript. Iron(III) oxide is Fe₂O₃. The III is the charge on each Fe, not the number of Fe atoms.
- Not reducing. Ca²⁺ + O²⁻ makes CaO, not Ca₂O₂. Ionic formulas use the lowest whole-number ratio.
- Calling HCl gas "hydrochloric acid." The acid name only applies in water: HCl(aq).