Chemistry - EDEXCEL IGCSE - Ionic, covalent and metallic compounds

IONIC COMPOUNDS:

Ions are formed through the adding (reduction) or removal (oxidation) of electrons.

Ionic Bond - a strong electrostatic attraction between oppositely charged ions

Properties:

• Structure - a 3D lattice structure held together by ionic bonds.
• Melting and boiling points - Very high (Note - the higher the ionic charge the higher the mp/bp)
• Conductivity - only conducts when molten or dissolved
• Solubility - dissolves in water but not organic substances

Ionic compounds have high melting and boiling points because of the strong lattice structure which means the compounds are held together more strongly. This means more energy is needed to break apart the bonds and so more heat. Furthermore, the electrostatic attraction between ions is very strong.

Ionic compounds do not conduct as a solid as the ions are not free to move and so cannot carry the charge. When molten or dissolved, the ions are now free to move and so the compounds conducts.

COVALENT COMPOUNDS:

Covalent bond: the attraction between positive nuclei for a shared pair of electrons.

                    

Covalently bonded molecules are held together by weak intermolecular forces so are mostly volatile liquids or gases as they have very low melting and boiling points.

Covalent substances also cannot conduct electricity as they have no charged particles and so cannot carry the charge. 

Substances with a giant covalent structure such as diamond high a high melting and boiling point as they are held together by strong covalent bonds which hold them together very strongly.

Diamond = carbon joined to 4 others

Graphite = carbon joined to 3 others

Diamond is hard as it has very many covalent bonds holding it together. This makes it very useful for cutting.

Graphite is built in layers of carbon which can slide over each other making it slippery and, therefore, a good lubricant. It has a relatively high melting and boiling point as it is held together by strong covalent bonds. Furthermore, it is also a good conductor as the fact that only 3 carbons are joined to each other means that there is always 1 free electron to carry the charge.

METALLIC CRYSTALS

Metals are a giant structure of positive ions surrounded by a ‘sea’ of delocalised electrons.

They are malleable, this is because they can ‘slide’ over each other. This is because they have a regular shape/structure and also because the ‘sea’ of electrons means the positive ions are always held together even when they are being moulded.

Metals also conduct as the delocalised electrons are free to move and can, therefore, carry the charge as they are ions.