KEY POINTS:
- Ionic compounds exist as giant ionic lattices. The strength of the lattice – and the related physical properties (melting points, solubility) – is determined by two factors:
- the size of the ions, and
- the charge on the ions.
- Small ions lead to strong bonds. High charge also results in strong bonds. Obviously strong ionic bonds tend to increase melting points (Al2O3 for example). NaCl has a relatively low melting point for an ionic compound due to weaker ionic bonds.
- The relative weakness of the ionic bonds in common salt means that it is soluble in water. The attractive pull of the polar water molecules is strong enough to break apart the giant ionic lattice. If the bonds are too strong then the compound tends to be insoluble in water.
- Polyatomic ions can be very complex (simple ion charges can be deduced from the periodic table). There’s no easy way to remember them apart from familiarity and practice of using in formulae. The videos may help with this.
THINK OUTSIDE THE BOX:
- What types of structure does glass have? How do we add colours to create stained glass?
- Enthalpy of hydration and lattice enthalpy determine solubility – investigate the mathematics behind this.