Comparing structures and properties

Last time

Before we start: last time, in States of matter and changes of state, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The detective question

A scientist is given an unknown white solid. She tests it, and she can say what kind of bonding it has, without ever looking at the atoms. How? Each type of structure has its own pattern of properties. Learn the four patterns, and you can work backwards from the data.

The four types of structure

Ionic compounds are giant lattices of positive and negative ions, held together by ionic bonds. Simple molecular substances are small molecules, with covalent bonds inside and weak forces between the molecules. Giant covalent structures are huge networks of atoms, all joined by covalent bonds. Metals are lattices of positive ions in a sea of delocalised electrons.

Properties compared

Here is the comparison to learn. Ionic: high melting point, conducts only when molten or dissolved. Simple molecular: low melting point, never conducts. Giant covalent: very high melting point, does not conduct, except graphite. Metallic: high melting point, and conducts as a solid.

Explaining with forces

Ionic compounds melt only when the strong ionic bonds in the lattice are overcome, so the melting point is high. Simple molecular substances melt when the weak forces between molecules are overcome. The covalent bonds stay. Giant covalent structures melt only when many strong covalent bonds are broken, so the melting point is very high. Metals melt when the strong metallic bonds are overcome, so the melting point is high.

The elements tell you the bonding

A metal and a non-metal bond ionically. That means a giant ionic lattice. Two non-metals bond covalently. Usually that means small molecules, like water or methane. A few non-metal elements form giant covalent structures: carbon as diamond and graphite, and silicon(IV) oxide. A metal element on its own is a metallic lattice.

Working out the structure

Question one: does it conduct when it is solid? If yes, it is a metal, or graphite. Question two: is the melting point low? If yes, it is simple molecular. Question three: does it conduct when molten or dissolved? If yes, it is ionic. If it does not conduct in any state, and has a very high melting point, it is giant covalent.