Covalent bonds and simple molecules

• A covalent bond is formed when a pair of electrons is shared between the atoms of two non-metal atoms
• The number of covalent bonds formed by an atom is equal to the number of electrons it needs to gain
• After forming the bonds each atom has the same electronic structure as the nearest noble gas
• Covalent bonds are strong and require a lot of energy to break
• Covalent bonding can be represented by dot-and-cross diagrams or straight lines drawn between atoms
• Covalently bonded substances can exist as simple molecules, giant covalent structures or polymers
• Simple molecules are neutral particles and have no charges or free electrons. This means they cannot carry an electric current either in liquid or solid form
• Simple molecular substances have weak intermolecular forces between the molecules. These weak attractive forces need little energy to break them. This means that simple molecular substances have low melting and boiling points and quite often, they are liquids or gases at room temperature
• The intermolecular forces increase as the molecular size increases and this means that the melting and boiling points also increase
• When a simple molecular substance melts or boils, the weak intermolecular forces are broken not the strong covalent bonds