Covalent bonding
In covalent bonding , atoms still want to achieve the noble gas configuration (The octet rule, full outer shell of electrons). But rather than losing or gaining electrons like in ionic bonding it shares an electron pair. This is called a bonding pair covalent bonding is always between two non-metals. E.g chlorine forms a covalent bond with itself (see word document for diagram).
What holds a covalent bond together?
The attraction between pairs of negative shared electrons and positive nuclei of the atoms, this is called electrostatic attraction between positive and negative. They don't usually conduct electricity.
Simple molecular structures
A fixed number of atoms held together by strong covalent bonds e.g H2, O2, H2O
Bonding between non-metals
Gases, liquids, solids with low melting points
Energy brakes the molecular bonds instead of the strong covalent bonds.
Strong covalent bonds hold the atoms together
Weak inter-molecular forces holding molecules together so they can be broken relatively easily.
the melting point increases as relative molecular mass increases (generally) in simple molecular structures because if there is more mass then there are more bonds to be broken down during melting or boiling.
Giant Covalent Structures
A huge number of atoms arranged in a regular way (regular lattices), the number of atoms is not fixed.
Properties of giant covalent structures
Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.
Variable conductivity - Diamond does not conduct electricity. Graphite contains free electrons, so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.
Graphite
Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.
Diamond
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.
Structure of a buckminsterfullerene molecule - a large ball of 60 atoms
Buckminsterfullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.