Properties of Simple Molecular Compounds (Cambridge O Level Chemistry)

• Small molecules are compounds made up of molecules that contain just a few atoms covalently bonded together
• They have low melting and boiling points so covalent compounds are usually liquids or gases at room temperature
• As the molecules increase in size, the melting and boiling points generally increase
• Small molecules have poor electrical conductivity

Explaining melting / boiling points

• Small molecules have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules
• They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules
• These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature
• As the molecules increase in size the intermolecular forces also increase as there are more electrons available
• This causes the melting and boiling points to increase

The bonds between hydrogen and oxygen in water are COVALENT, and the attractions between the molecules are INTERMOLECULAR FORCES which are about one tenth as strong as covalent bonds

Explaining electrical conductivity

Molecular compounds are poor conductors of electricity as there are no free ions or electrons to carry the charge.

• Most covalent compounds do not conduct at all in the solid state and are thus insulators
• Common insulators include the plastic coating around household electrical wiring, rubber and wood

The plastic coating around electrical wires is made from covalent molecules that do not allow a flow of charge