In chemistry we often talk about how 'weak' or 'strong' bonds are. The stronger the bonds are between two atoms, the more (kinetic) energy is required to break them apart. This means that more heat is needed to break them apart. This is the reason why ionic compunds like sodium chloride, giant covalent molecules like diamond and giant metallic structures such as iron have very high melting points. If you remember, there are some types of bonding that do NOT give high melting points. These are for small, covelent molecules. Although each atom in a molecule is covalently bonded like diamond, individual molecules do not have strong bonds between them. This is unlike diamond, where every carbon atom in the structure has 4 strong covalent bonds with 4 other carbon atoms. An example of a small covalent molecule is water, which is liquid at room temperature.