Ionic and Covalent Compound Properties - C2 AQA

An ionic compound is a giant structure of ions. Ionic compounds are held together by strong electrostatic forces of attraction betwen oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding.

Ionic compounds have regular structures (giant ionic lattices) in which there are srtong electrostatic forces in all directions between oppositely chraged ions. These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds.

When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current. When in a solid state, the ions are in a fixed position and cannot move.

Atoms that share electrons also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.

In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.

In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other because there are no covalent bonds between the layers and so graphite is soft and slippery. In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.

Carbon can also form fulluerenes with different numbers of carbon atoms. Fullerenes can be used for drug delivery into the body, in lubricants, as catalysts, and in nanotubes for reinforcing materials, e.g. in tennis rackets. Fullerene structures are based on hexagonal rings of carbon atoms.

Simple covalent molecules have low melting and boiling points. This is because they have weaker forces of electrostatic attraction and they can break down easily. This means that substances made up of simple molecules are liquids or gases at room temperature. Others- which are solids- are solids with low melting points, e.g. iodine.

The simple molecule structure doesn't contain any delocalised electrons therefore do not conduct electricity