Hydrogen bonding is the strongest form of intermolecular bonding. An intermolecular bond is a bond that exists between two molecules within a physical state of a compound. Following are the conditions needed for molecules to form hydrogen bond: A molecule containing fluorine, oxygen or nitrogen with lone pair of electrons on atoms afore mentioned. Another molecule containing hydrogen bonded to fluorine, oxygen or nitrogen. Water molecules have hydrogen bonding and this results in some anomalous properties of ice as compared to water: Ice is less dense than water, due to three-dimensional hydrogen bonded network of molecules. The physical properties like melting point of ice is higher than corresponding molecules of same group, for example Hydrogen Sulfide. This is because these molecules only have Vander Waal Forces of Attraction while Ice has hydrogen bonding in it. Greater amount of energy is required to weaken and finally, overcome these strong intermolecular forces. Hence, it has higher melting point and greater latent heat of vaporization.