Ionic compounds form when metals and non-metals react. Electrons are transferred from the metal atom to the non-metal atom to create two oppositely charged ions. The postive metal and negative non-metal ions are attracted to each other and form giant lattice structures.
The ions are held in place by strong electrostatic attraction between the oppositely charge ions, known as an ionic bond. As a result, ionic compounds
tend to have regular crystal shapes
have high melting and boiling points
do not conduct electricity when solid
As the ions are charged, ionic compounds tend to dissolve in polar solvents, such as water. In addition when molten, or in solution, they conduct electricity as the ions are free to move.
Covalent compounds - simples
Covalent compounds form when non-metals react with non-metals. They form strong covalent bonds between atoms by sharing pairs of electrons between atoms. They can be classified in two broad groups
Tend to be made of only a few atoms bonded together. They tend to have low melting and boiling points as the weak forces which exist between the molecules - intermolecular forces - only require a small amount of energy to break them. In liquid or gas state, it is the molecules which are free to move around. The atoms in each molecule are still strongly bonded to each other by covalent bonds.
Good examples of simple covalent molecules include water, carbon dioxide, the elements nitrogen, oxygen sulphur and the group 7 elements from fluorine to iodine.
Most organic compounds are also simple covalent molecules - for example ethanol, propene and petrol.
Metal atoms form giant lattices where the outer electrons are shared, or at least free to move, around the whole structure. This leaves the full inner electron shells and the nucleus as postive ions which are held together by the free, or delocalised, electrons. This explains why most metals are strong with high melting points. The electrons allow the metal atoms / ions to slip or move around when the metal is bent or pressed into new shapes whilst still holding them together.
As the free electrons are also able to move around the structure, they can carry energy through the structure explaining why metals are such good conductors of heat and electricity.
Covalent compounds - Giants
Giant covalent structures
Theses are made from billions of individual atoms all connected together through strong covalent bonds in a giant lattice structure. As a result, they tend to have high melting and boiling points as they require a large amount of energy to break the bonds holding the atoms in fixed positions. In liquid or gaseous form, it would be the individual atoms whcih would be free to move around.
Good examples of these types of compound include silicon ioxide - the main component in sand - and the carbon structures diamond and graphite. In diamond, each carbon is bonded to 4 others making diamonds the hardest known natural substance. In graphite, each carbon is bonded to 3 others in giant sheets. This explains the unusual properties of graphite;
the non-bonding electron in each carbon is free to move through the structure so graphite conducts electricity
the sheets are only held together by weak forces so the layers can slip over each other making graphite useuful as a lubricant