How does the Lewis model for covalent bonding account for the relatively low melting and boiling points of molecular compounds (compared to ionic compounds)?
Solution 18E The ionic bonds that we can find in ionic compounds are non directional because the coulombic force acts in the same way in the three dimensions, thus allowing the formation of ordered structures of ions called lattices, in which positive ions alternate to negative ions and vice-versa. This means that electrostatic interaction doesn’t act only between a couple of ions having opposite charges, but even between ions that are more distant in the same lattice, despite of the direction we consider. Since the coulombic force acts in all directions, the ions in lattice are binded with a higher energy, hence the melting points for ionic compound will be higher, because the heat ( a form of energy) we need to destroy these interactions need to be high. Covalent bond between two atoms is highly directional, thus the interactions between molecules will be weak if we consider a 3D structure, so it won’t be necessary a high amount of energy to break them. This is why covalent compounds have higher values of melting and boiling points respect to covalent compounds.