Atomic Foundations of Matter · Lesson 9 of 14
Naming Ionic Compounds
“Ionic compounds are extended ion lattices, and their names identify cations and anions.”
• Explain why ionic compounds form repeating crystal structures. • Define a crystal lattice in the context of ionic compounds. • Apply cation-first naming for ionic compounds. • Distinguish monoatomic from polyatomic ions. • Recall the names, formulae and valencies of the common ions listed in the chapter.
A formula such as NaCl does not mean that a crystal of salt is made from separate NaCl molecules sitting side by side. Ionic compounds generally form extended three-dimensional arrangements in which many oppositely charged ions repeat in a regular pattern.
Ionic Crystal Structure
In sodium chloride, each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions in the three-dimensional crystal structure described in the chapter. A simplified crystal lattice represents this repeating arrangement using points or spheres.
A repeating representation of the arrangement of ions in an ionic crystal.
Naming Ionic Compounds
The name of the cation is written first, followed by the name of the anion. Simple anion names generally end in -ide. Metals generally form cations and non-metals generally form anions in the examples considered here.
| Compound | Cation name | Anion name | Compound name |
|---|---|---|---|
| NaCl | Sodium | Chloride | Sodium chloride |
| CaO | Calcium | Oxide | Calcium oxide |
| MgS | Magnesium | Sulfide | Magnesium sulfide |
Monoatomic Ions
An ion formed from a single atom carrying a net electrical charge.
| Name of ion | Formula | Valency |
|---|---|---|
| Sodium | Na⁺ | 1 |
| Lithium | Li⁺ | 1 |
| Potassium | K⁺ | 1 |
| Silver | Ag⁺ | 1 |
| Calcium | Ca²⁺ | 2 |
| Barium | Ba²⁺ | 2 |
| Iron (Ferrous) | Fe²⁺ | 2 |
| Iron (Ferric) | Fe³⁺ | 3 |
| Copper (Cuprous) | Cu⁺ | 1 |
| Copper (Cupric) | Cu²⁺ | 2 |
| Magnesium | Mg²⁺ | 2 |
| Zinc | Zn²⁺ | 2 |
| Aluminium | Al³⁺ | 3 |
| Fluoride | F⁻ | 1 |
| Chloride | Cl⁻ | 1 |
| Bromide | Br⁻ | 1 |
| Iodide | I⁻ | 1 |
| Oxide | O²⁻ | 2 |
| Sulfide | S²⁻ | 2 |
Polyatomic Ions
A charged group formed from atoms of two or more elements that behaves as one ion in a compound.
| Name of ion | Formula | Valency |
|---|---|---|
| Hydroxide | OH⁻ | 1 |
| Nitrate | NO₃⁻ | 1 |
| Hydrogencarbonate | HCO₃⁻ | 1 |
| Carbonate | CO₃²⁻ | 2 |
| Sulfate | SO₄²⁻ | 2 |
| Ammonium | NH₄⁺ | 1 |
Polyatomic-ion names generally do not end in -ide. When writing a formula later, the atoms inside a polyatomic ion must be kept together as a single group.
If atoms could be seen directly, scientists could compare arrangements at an extremely small scale. The chapter uses this question to encourage thinking about both the usefulness and difficulty of observing atomic structures.
Quiz
Which description best matches Crystal Lattice?
Which description best matches Monoatomic Ion?
Which term matches this description: A repeating representation of the arrangement of ions in an ionic crystal.
Which term matches this description: An ion formed from a single atom carrying a net electrical charge.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why an ionic crystal should not be pictured as separate NaCl molecules.
- Name the cation and anion in calcium oxide.
- Distinguish a monoatomic ion from a polyatomic ion.
- State the formulae and valencies of hydroxide, sulfate and ammonium ions.
- Compare Fe²⁺ with Fe³⁺ and Cu⁺ with Cu²⁺.
Key Takeaways
• Ionic compounds consist of oppositely charged ions arranged in a repeating crystal lattice. • The positive ion is generally named first and the negative ion second. • Simple negative ions commonly use names ending in -ide. • Polyatomic ions are charged groups of atoms that behave as single units.