Structure and Bonding Question

Exam questions often ask you to explain a property, usually a melting point or electrical conductivity, in terms of structure and bonding. A good answer does three things.1. Name the structure: giant ionic, giant metallic, giant covalent or simple molecular.2. Name the bonds or forces that must be overcome, and say whether they are strong or weak.3. Link this to energy: strong forces need a lot of energy to overcome, so the melting point is high.Worked example: explain why diamond has a much higher melting point than iodine.Diamond has a giant covalent structure. To melt it, many strong covalent bonds between carbon atoms must be broken, which needs a lot of energy.Iodine has a simple molecular structure. When it melts, only the weak London forces between I₂ molecules are overcome; the covalent I-I bonds do not break. Much less energy is needed, so the melting point is low.For conductivity, look for mobile charged particles. Metals and graphite have delocalised electrons that can move. Ionic compounds conduct only when molten or in solution, when the ions are free to move. Diamond and simple molecular substances have no mobile charged particles, so they do not conduct.A common mistake is to say that covalent bonds break when a simple molecular substance melts or boils. Only the intermolecular forces are overcome.