Ions
High-Yield Summary
- An ion forms when proton count ≠ electron count. Cation (positive) loses electrons; anion (negative) gains electrons.
- Binary compound: molecular if both elements are nonmetals; ionic if one metal + one nonmetal.
- Type I ionic compounds (fixed-charge metal): cation name + anion root + '-ide.' Type II (variable-charge metal): cation charge shown with a Roman numeral, or the older -ic (higher charge)/-ous (lower charge) system.
- Binary covalent (Type III) compounds use numerical prefixes (mono- through deca-); 'mono-' is never used on the first element; water and ammonia keep common names.
- Acids: anion without O → hydro-...-ic (HCl → hydrochloric acid); anion ending -ate → -ic acid; anion ending -ite → -ous acid.
Type I vs. Type II Ionic Naming
| Type I (fixed charge) | Type II (variable charge) |
|---|---|
| Cation name = element name (e.g., Na = sodium) | Cation needs a Roman numeral: FeCl₂ = iron(II) chloride, FeCl₃ = iron(III) chloride |
| Group 1A, 2A, Al³⁺, Ag⁺, Zn²⁺ — never need a Roman numeral | Older system: higher charge = '-ic' (ferric, cupric), lower charge = '-ous' (ferrous, cuprous) |
| Anion = root + '-ide' (Cl⁻ = chloride) | Same anion-naming rule applies once cation charge is specified |
Key Terms
- Cation
- Positively charged ion; forms when an atom loses electrons.
- Anion
- Negatively charged ion; forms when an atom gains electrons.
- Polyatomic ion
- A charged group of multiple atoms that behaves as one unit (e.g., NH₄⁺, SO₄²⁻) — names must be memorized.
- Strong electrolyte
- Dissociates completely into ions in water (e.g., NaCl, KI, HCl).
- Weak electrolyte
- Ionizes/hydrolyzes only incompletely in water.
High-Yield Polyatomic Ions
| Ion | Formula |
|---|---|
| Ammonium | NH₄⁺ |
| Hydroxide | OH⁻ |
| Nitrite / Nitrate | NO₂⁻ / NO₃⁻ |
| Sulfite / Sulfate | SO₃²⁻ / SO₄²⁻ |
| Carbonate / Bicarbonate | CO₃²⁻ / HCO₃⁻ |
| Phosphate / Hydrogen phosphate / Dihydrogen phosphate | PO₄³⁻ / HPO₄²⁻ / H₂PO₄⁻ |
| Acetate | C₂H₃O₂⁻ |
| Permanganate / Chromate / Dichromate | MnO₄⁻ / CrO₄²⁻ / Cr₂O₇²⁻ |
Oxyanion Series & Acid Naming
- 2-member oxyanion series: fewer O = '-ite,' more O = '-ate' (sulfite SO₃²⁻ vs. sulfate SO₄²⁻).
- 4-member series: hypo-...-ite (fewest O) → -ite → -ate → per-...-ate (most O) — e.g., hypochlorite → chlorite → chlorate → perchlorate.
- Anion without oxygen → hydro- + root + -ic acid: HCl → hydrochloric acid; HCN → hydrocyanic acid.
- Anion ending -ate → acid ends -ic: H₂SO₄ (sulfate) → sulfuric acid. Anion ending -ite → acid ends -ous: H₂SO₃ (sulfite) → sulfurous acid.
Common MCAT Trap
- 'Mono-' is never used on the first element in binary covalent naming — CO is 'carbon monoxide,' never 'monocarbon monoxide.'
- Silver, zinc, and aluminum form only ONE charge each (Ag⁺, Zn²⁺, Al³⁺) — don't add a Roman numeral even though they're transition/post-transition metals.
- Water and ammonia always keep their common names (never 'dihydrogen monoxide' or 'nitrogen trihydride') on the MCAT.
- Nonpolar covalent compounds are the WEAKEST electrolytes (no ions form) — don't confuse 'covalent' broadly with 'weak electrolyte'; polar covalent compounds like HCl can be strong electrolytes.
Quick Recall
Why does FeCl₃ need a Roman numeral in its name but AlCl₃ doesn't?
What is the systematic name for H₂SO₃, and why does it end in '-ous' rather than '-ic'?
What distinguishes a strong electrolyte from a weak electrolyte?