Structure and Bonding

Ionic Substances

• Mainly metal compounds and ammonium compounds
• Form giant ionic lattice structures with alternating positive and negative ions
• Usually have high melting and boiling points because a lot of energy is needed to break the strong ionic bonds
• Are often soluble in water because the water can form ion-dipole forces with the dissolved ions to compensate for the loss of ionic bonds
• Conduct electricity when molten or in aqueous solution because the ions are then mobile. However in the solid state there are no mobile electrons and the ions are locked together so they are not mobile and the material does not conduct electricity

Covalent Substances

• Mainly non-metallic elements and compounds involving only non-metals
• These can have a giant molecular structure e.g.Diamond or Graphite or a simple molecular substance

Simple Molecular Structures

• Each molecule is made up of a defined number of atoms, covalently bonded together
• These molecules are then attracted together by intermolecular forces

Substances with simple molecular structures typically have;

• Relatively low melting and boiling points. This is because only the intermolecular forces have to be broken in these proceses and these are relatively weak. The covalent bonds are strong byut they remain intact even in the gas phase so this does not affect the melting and boiling points
• Low electrical conductivity because they have no mobile electrons or ions
• Usually low solubiliity in water unless either; they react with water to form ions orthey form hydrogen bonds with water

Giant Molecular Substances

• A giant molecule is one in which the covalent bonding exists throughout the whole crystal, usually in three dimensions. As a result of this;
• Substances with giant covalent lattices tend to have very high melting and boiling points. This is because the covalent bonding exists throughout the whole crystal so covalent bonds have to be broken to melt the substance

You need to know about two specific cases of giant molecular structures;

Diamond

• Contains a three dimensional lattice of carbon atoms, each covalently bonded to and tetrahedrally surrounded by four others
• The melting point is very high (over 4000 degrees centigrade) because strong C-C covalent bonds have to be broken for diamond to melt
• Diamond is also very hard because of the giant lattice holding the atoms together and the strong covalent bonds
• Diamond is a non-conductor of electricity because it has no mobile ions or electrons

Graphite

• Another form of carbon and also contains a giant molecular structure but in this case it is a layer structure
• The covalent bonding only extends in two dimensions, giving layers which are held together by van der Waals forces
• Each carbon is bonded to 3 others in the sheet is a trigonal planar geometry
• The layers are held together by van der Waals forces. These are relatively weak so the layers can slide over each other. Hence graphite feels slippery and is used as a lubricant
• Graphite has a very high melting point, because strong covalent bonds have to be broken for graphite to melt
• One electron per carbon is delocalised, meaning it can move freely within a layer. Hence graphite conducts electricity

Use: Diamond tipped drill bits

Property: Very hard and high melting point due to giant lattice

Use: Graphite re-entry shields for missile nose cones

Property: High melting point due to giant covalent structure

Use: Graphite Pencils + Graphite Lubricants e.g. on rail tracks

Property: Layers able to slide over each other 

Use: Graphite electrodes in arc lamps

Property: Mobile electrons allow graphite to conduct electricity. High melting point

• Metallic elements (and allows comprising more than one metallic element) have giant metallic structures. The metal atoms form a lattice structure, held together by metallic bonding
• In metallic bonding, the outer orbitals of the metal atoms overlap to some extens, so the metal can be thought of as consisting of a lattice of positive ions surrounded by a 'sea' of delacalised electrons
• The sea only consists of the outer shell electrons
• Sodium only has one delocalised electron per atom

'Metallic bonding is the attraction between a lattice of positive ions and a sea of delocalised electrons'

• Metals are very good conductors of electricity because the delocalised electrons are mobile. Generally the more delocalised electrons there are per atom, the higher the conductivity
• Metals generally have relatively high melting points. This is becuase a lot of energy is needed to break the strong metallic bonding. Generally, the more delocalised electrons there are per atom, the stronger the metallic bonding
• Other physical properties of metals include; conduction of heat, malleable, ductile, shiny, sonorous