TheIBAxolotl
Forming ions
Atoms gain or lose electrons to reach a stable arrangement, usually the electron configuration of a noble gas.
- Metals (low ionisation energy) lose electrons → cations (positive). Group 1 → 1+, group 2 → 2+, aluminium → 3+.
- Non-metals (high electron affinity) gain electrons → anions (negative). Group 17 → 1−, group 16 → 2−, group 15 → 3−.
- Transition metals can form more than one ion (Fe\(^{2+}\) and Fe\(^{3+}\)), shown with Roman numerals: iron(III) oxide.
Naming: cation first, then anion; monatomic anions end in "-ide" (chloride, oxide, nitride).
Polyatomic ions to know
| Ion | Formula | Ion | Formula |
|---|---|---|---|
| ammonium | NH\(_4^+\) | hydroxide | OH\(^-\) |
| nitrate | NO\(_3^-\) | hydrogencarbonate | HCO\(_3^-\) |
| carbonate | CO\(_3^{2-}\) | sulfate | SO\(_4^{2-}\) |
| phosphate | PO\(_4^{3-}\) |
The ionic bond and formulas
An ionic bond is the electrostatic attraction between oppositely charged ions. The formula is the ratio of ions that makes the total charge zero.
Worked example
Deduce the formula of aluminium sulfate.
- Ions: Al\(^{3+}\) and SO\(_4^{2-}\)
- Lowest common multiple of 3 and 2 is 6: two Al\(^{3+}\) give +6, three SO\(_4^{2-}\) give −6
- Formula: Al\(_2\)(SO\(_4\))\(_3\) (brackets around a polyatomic ion when more than one is needed)
Other examples: magnesium nitride is Mg\(_3\)N\(_2\); calcium nitrate is Ca(NO\(_3\))\(_2\); ammonium sulfate is (NH\(_4\))\(_2\)SO\(_4\).
Ionic character is greatest when the electronegativity difference between the elements is large. Bonding is better seen as a continuum than as strict ionic/covalent categories.
Lattice structure
Ionic compounds do not form molecules. They form a giant ionic lattice: a repeating 3-D arrangement where each ion is surrounded by ions of the opposite charge. In NaCl, each Na\(^+\) is surrounded by six Cl\(^-\) and vice versa. The formula only gives the ratio of ions.
Lattice enthalpy
Lattice enthalpy is the enthalpy change to separate one mole of an ionic solid into its gaseous ions. It is endothermic (positive) and measures lattice strength.
Lattice enthalpy is larger when:
- Ionic charges are higher (stronger attraction): MgO ≫ NaCl.
- Ionic radii are smaller (ions closer together): LiF > KBr.
Charge usually has the bigger effect. Rough comparison: NaCl about +790 kJ mol\(^{-1}\) versus MgO about +3800 kJ mol\(^{-1}\).
Properties of ionic compounds
| Property | Explanation |
|---|---|
| High melting and boiling points | many strong electrostatic attractions in the lattice must be overcome |
| Conduct when molten or dissolved, not when solid | ions must be free to move; in the solid they are fixed |
| Hard but brittle | a shift brings like charges next to each other and they repel, cracking the lattice |
| Often soluble in water | polar water molecules surround and stabilise separated ions |
| Insoluble in non-polar solvents | no strong attraction between ions and solvent |
Common mistakes
- Saying ionic solids conduct because of moving electrons: it is moving ions in melts and solutions.
- Writing "NaCl molecules".
- Forgetting brackets: Ca(OH)\(_2\), not CaOH\(_2\).
- Explaining high melting points as "strong bonds" without saying they are electrostatic attractions between oppositely charged ions throughout a lattice.
- Ignoring charge when comparing lattice enthalpies and looking only at size.
Exam tips
- To compare melting points, compare lattice enthalpies: consider charge first, then size.
- Use the phrase "electrostatic attraction between oppositely charged ions" in definitions.
- State what happens physically in brittleness: layers shift, like charges align, repulsion.
- In conductivity answers, specify the state and name the mobile charge carriers.