Giant covalent structures: atoms bond into one endless network

1. A giant covalent structure is a huge, repeating lattice of atoms joined together by strong covalent bonds.
2. The bonding carries on unbroken through the whole structure, so there are no separate small molecules.
   1. Instead, the entire sample behaves as one single giant molecule of bonded atoms.
3. Because the strong bonds lock every atom into a fixed position, giant covalent substances are solids at room temperature.

Very high melting points: melting means breaking strong bonds

1. Melting a giant covalent substance means breaking a huge number of strong covalent bonds right through the lattice.
2. Overcoming so many strong bonds needs a very large amount of energy, so these substances have very high melting points.
3. Boiling also breaks strong covalent bonds, so their boiling points are very high too.
4. Most giant covalent substances do not conduct electricity because they have no charged particles free to move.
   1. Graphite is the exception, because it has delocalised electrons that are free to carry charge.

Diamond, graphite and silica: three giant covalent examples

1. Diamond
   1. Diamond is made only of carbon atoms, $C$, in a three-dimensional giant covalent lattice.
   2. Each carbon atom forms four covalent bonds to four other carbon atoms.
   3. These four strong bonds in every direction make diamond very hard.
2. Graphite
   1. Graphite is also made only of carbon atoms, $C$, arranged in flat layers of hexagonal rings.
   2. Each carbon atom forms only three covalent bonds to three other carbon atoms within its layer.
   3. The bonds within a layer are strong, but the layers themselves are held together only weakly and can slide over one another.
3. Silicon dioxide
   1. Silicon dioxide, ${SiO}_2$, also called silica, is a repeating three-dimensional network of silicon and oxygen atoms.
   2. Each silicon atom, $Si$, is covalently bonded to four oxygen atoms, and each oxygen atom, $O$, is bonded to two silicon atoms.

Reading structure diagrams: spot the repeating network

1. Step 1: Look for a pattern of atoms and bonds that repeats across the diagram.
2. Step 2: Check the atoms are all joined together rather than drawn as separate small groups.
3. Step 3: Use the type of atoms and the number of bonds around each atom to identify the substance.
4. A diagram usually shows only a small section of the structure, but the repeating pattern carries on beyond the part shown.
5. Diamond and graphite both contain only carbon, $C$, so you must use their different bonding to tell them apart.
   1. Four bonds per carbon means diamond, while three bonds per carbon in layers means graphite.
6. A repeating network containing both silicon and oxygen atoms represents silicon dioxide, ${SiO}_2$.