Section 1 g) Summary

Covalent bonding is the bonding of two non-metal atoms. It involves a shared pair of electrons. The attraction between the nucleus of each of the atoms and the electrons of the other atom creates a very strong covalent bond.

Simple covalent structures usually have low melting and boiling points, and are often fluid at room temperature, because the forces between the molecules are weak and don't require much energy to break. They have weak intermolecular forces.
Examples include: water, oxygen, carbon dioxide, hydrogen chloride, etc.

Giant covalent structures have very high melting and boiling points, because they have so many strong covalent bonds holding the atoms together. It requires a lot of energy to break that many covalent bonds, so they have high melting points.
Examples include: diamond (carbon), graphite (carbon), silicon dioxide, etc.

Dot and cross diagrams can be used to represent covalent bonding. Here are the most important ones for this topic:

Methane

Water

Hydrogen Chloride

Chlorine

Hydrogen

Ammonia

Oxygen

Nitrogen

Carbon Dioxide

Ethane

Ethene

Section 1 g) Key Words

Allotrope: The different molecular forms in which an atom can exist.

Covalent bond: A bond involving the attraction of a pair of electrons and a pair of nuclei from two non-metal atoms

Diamond: An allotrope of carbon with a tetrahedral structure. Used in jewellery and in power tools, as it is the hardest material naturally occurring on Earth and can cut through virtually any solid.

Intermolecular forces: The weak forces of attraction between covalent molecules

Graphite: An allotrope of carbon in a layered structure that easily slides. Used in pencils and as a lubricant, as it transfers easily and slides easily.

Tetrahedral: An atom directly bonded with four other atoms: it has four covalent bonds. Structure of diamond.

Section 1 g) Specification

1.38 describe the formation of a covalent bond by the sharing of a pair of
electrons between two atoms

A covalent bond is formed between two non-metals and involves the atoms sharing one or more pairs of electrons

1.39 understand covalent bonding as a strong attraction between the bonding
pair of electrons and the nuclei of the atoms involved in the bond

In a covalent bond, the nucleus of each atom is attracted to the other atom's outer shell electron(s).

1.40 explain, using dot and cross diagrams, the formation of covalent compounds
by electron sharing for the following substances:
i hydrogen
ii chlorine
iii hydrogen chloride
iv water
v methane
vi ammonia
vii oxygen
viii nitrogen
ix carbon dioxide
x ethane
xi ethene

1.41 understand that substances with simple molecular structures are gases or
liquids, or solids with low melting points

Simple covalent compounds have strong bonds, but very weak intermolecular forces. These don't require much energy to break, leading to low melting and boiling points. As a result, most simple covalent compounds are gas or liquid at room temperature.

1.42 explain why substances with simple molecular structures have low melting
and boiling points in terms of the relatively weak forces between the
molecules

There aren't very strong forces of attraction between covalent molecules, they have weak intermolecular forces that are easily broken; they don't require much energy to break. This means that they have low melting and boiling points.

1.43 explain the high melting and boiling points of substances with giant covalent
structures in terms of the breaking of many strong covalent bonds

In giant covalent structures, each atom is bonded to another with by strong covalent bonds. These are not easily broken, they require a lot of energy to break. Additionally, in a giant structure there are so many bonds to break that it requires even more. This results in very high melting and boiling points.

1.44 draw diagrams representing the positions of the atoms in diamond
and graphite

1.45 explain how the uses of diamond and graphite depend on their
structures, limited to graphite as a lubricant and diamond in cutting.

In diamond, the carbon atoms are arranged in a tetrahedronal structure, each atom with four strong covalent bonds. This makes it very hard, with a very high melting and boiling point. It has lots of very strong bonds. Its giant structure means that it forms in crystals that reflect light and let it pass through. It is so hard that its use in industrial saws etc. is ideal, it can cut through almost anything.

Graphite is arranged in layers held together by weak intermolecular forces. This means that when force is applied, the layers slide over each other easily, making graphite a good lubricant.

Both diamond and graphite are allotropes of carbon, but buckminsterfullerene is another one, it's ball shaped, made of 60 carbon atoms, and used for lubrication, drug administration, and in catalysts.