A polar bond between hydrogen and a very electronegative element, such as O, N, or F, allows a unique secondary bonding between the partially positive hydrogen and atoms with a partial negative charge. The attraction between hydrogen and these negative species is called the hydrogen bond, which is much weaker than the primary polar bond. Hydrogen bonding exists between water molecules because the electronegativity difference between hydrogen and oxygen is 1.4, indicating a polar bond of about 36% ionic character. Refer to Figure 1.
Figure 1. Polar bonds in the water molecule.
The polarity of the hydrogen‐oxygen bond is responsible for much of the force of attraction between water molecules in liquid water. This strong force of attraction is responsible for the unusually high melting and boiling points of water. (See Figure 2.)
Figure 2. Hydrogen bonds between water molecules in liquid water.