Hydrogen atoms in molecules sometimes form strong connecting bridges between atoms called hydrogen bonds. When hydrogen is bonded to N, O, or F atoms, that hydrogen can form a bond between the atom holding it and a different N, O, or F atom.
Hydrogen fluoride (HF) and water (H2O) form extensive hydrogen bonded networks in their condensed states.
Evidence for hydrogen bonding often is taken from the boiling points of selected model compounds.
The actual boiling point of water is much higher than would be predicted based upon the observed boiling points of other members of the same group in the periodic table. When the hydrogen atom is replaced with a group that precludes the possibility of hydrogen bonding, this prediction strategy is satisfactory.
