1 - PRINCIPLES OF CHEMISTRY 

Covalent bonding - 

• Shared pair of electrons 
• Both atoms have full outer shells 
• Each atom has to have enough covalent bonds to fill up outer shell 
• Strong attraction between the shared electrons and the nuclei of the atoms

eg: 
Hydrogen and chlorine = hydrogen chloride (HCl)
Ammonia (NH3)
Nitrogen (N2)
Water (H2O) 
Methane (CH4) 
Ethane (C2H6) 
Ethene (C2H4)

Simple molecular substances -

• Atoms within molecule held together by very strong covalent bonds 
• Forces of attraction between molecules are very weak - intermolecular forces
• Melting and boiling points are very low 
• Most are gases or liquids are room temperature 
• 'Mushy' physical state - liquid/gas/easily melted solid 

Giant covalent structures - 

• No charged ions 
• Atoms bonded to eachother by strong covalent bonds 
• Lots of energy to break the large amount of bonds 
• Very high melting and boiling points 
• Don't conduct electricity - no delocalised electrons 
• eg: diamond and graphite (carbon atoms) 

Diamond - 

• Each carbon atom forms 4 covalent bonds 
• Very rigid giant covalent structure 
• Hardest natural substance 

Graphite - 

• Each carbon atom forms 3 covalent bonds 
• Creates layers 
• Slide over each other 
• Good lubricant 
• Free electrons - only non-metal that is a good conductor