TOPIC D: BONDING AND THE PROPERTIES OF SOLIDS

An alloy is a mixture of metals. Two types are common: 1. An interstitial alloy – additional, smaller atoms of a different element fill the spaces in the metallic lattice. The most common example is steel where carbon atoms fill the spaces between the iron atoms. Alloys of this type are less malleable and ductile than the pure metals since the presence of the smaller atoms make the structure more rigid and less flexible.

Substitutional alloy – where one metals’ atoms are replaced by another metals’ atoms. The metal atoms are usually of a similar radius as in brass where copper atoms are replaced with zinc atoms. Substitutional alloys have similar, reduced malleability and ductility to interstitial alloys, and have densities that typically lay between the densities of the component metals. In both cases the sea of electrons is maintained and the alloys remain good conductors.

Giant covalent network solids (diamond, graphite, silicon dioxide and silicon carbide) The elements in group 14 can make four covalent bonds, and as such allow then to bond together in large, continuous networks. Diamond and graphite are allotropes (different forms of carbon), made entirely of carbon atoms, and covalently bonded together in a continuous network.

The diamond structure is based on the 3-dimensional tetrahedral unit. All the carbon atoms are bonded to one another with very strong covalent bonds This makes a diamond strong and hard with a high melting point and boiling point.

Graphite had a 2-dimensional layered structure where each carbon atom is bonded to three others in a plane. It will conduct electricity only in one plane. (There is no conduction in the vertical plane.) Since each carbon atom is bonded to three others, this leaves one of the outer electrons free. These electrons are delocalized. It can be used as a lubricant LDF’s hold the planes together vertically, so they can slide over one another.

Silicon and semi-conductors Pure semi-conductors are generally poor conductors of electricity, If ‘doped’ (a deliberate introduction of an impurity) the conductivity increases. When the doping is carried out with an element that has an extra valence electron compared to silicon (a group 15 element like phosphorus) An n-type conductor is produced (n meaning ‘negative’ because of the extra electron).

Conversely, when the doping is carried out with an element that has one less extra valence electron compared to silicon, (a group 13 element like boron) A p-type conductor is produced (p meaning ‘positive’ because of the one less electron). Disrupting the valence shells of the silicon atoms in this manner effectively allows electrons to flow, and the previous insulator becomes a good conductor.

Molecular solids (iodine) Molecular solids are made from non-metals. Sometimes these might be diatomic molecules like iodine, or much larger molecules like polymers. The intermolecular forces are weak, have low melting points and do not conduct electricity.

Iodine is held in a similar lattice structure to NaCl, only the I 2 molecules are held by weak dispersion forces. Solid iodine has a low melting point and a non conductor It typically sublimes at room temperature.