How Do Polar Molecules Form Hydrogen Bonds?

Polar molecules that include a hydrogen atom can form electrostatic bonds called hydrogen bonds. The hydrogen atom is unique in that it is made up of a single electron around a single proton. When the electron is attracted to the other atoms in the molecule, the positive charge of the exposed proton results in molecular polarization.

This mechanism allows such molecules to form strong hydrogen bonds over and above the covalent and ionic bonds that are the basis of most compounds. Hydrogen bonds can give compounds special properties and can make materials more stable than compounds that can't form hydrogen bonds.

In covalent bonds, atoms share electrons to form a stable compound. In nonpolar covalent bonds, the electrons are shared equally. For example, in a nonpolar peptide bond, electrons are shared equally between the carbon atom of the carbon-oxygen carbonyl group and the nitrogen atom of the nitrogen-hydrogen amide group.

For polar molecules, the electrons shared in a covalent bond tend to gather on one side of the molecule while the other side becomes positively charged. The electrons migrate because one of the atoms has a greater affinity for electrons than the other atoms in the covalent bond. For example, while the peptide bond itself is non-polar, the structure of the associated protein is due to hydrogen bonds between the oxygen atom of the carbonyl group and the hydrogen atom of the amide group.

Typical covalent bond configurations pair atoms that have several electrons in their outer shell with those that need the same number of electrons to complete their outer shell. The atoms share the extra electrons from the former atom, and each atom has a complete outer electron shell some of the time.

Often the atom that needs extra electrons to complete its outer shell attracts the electrons more strongly than the atom providing the extra electrons. In this case, the electrons are not shared evenly, and they spend more time with the receiving atom. As a result, the receiving atom tends to have a negative charge while the donor atom is positively charged. Such molecules are polarized.

Molecules that include a covalently bonded hydrogen atom are often polarized because the single electron of the hydrogen atom is comparatively loosely held. It easily migrates to the other atom of the covalent bond, leaving the single positively charged proton of the hydrogen atom on one side.

When the hydrogen atom loses its electron, it can form a strong electrostatic bond because, unlike other atoms, it no longer has any electrons shielding the positive charge. The proton is attracted to the electrons of the other molecules, and the resulting bond is called a hydrogen bond.

The molecules of water, with chemical formula H₂O, are polarized and form strong hydrogen bonds. The single oxygen atom forms covalent bonds with the two hydrogen atoms but does not share the electrons equally. The two hydrogen electrons spend most of their time with the oxygen atom, which becomes negatively charged. The two hydrogen atoms become positively charged protons and form hydrogen bonds with the electrons from the oxygen atoms of other water molecules.

Because water forms these extra bonds between its molecules, it has several unusual properties. Water has exceptionally strong surface tension, has an unusually high boiling point and requires a lot of energy to change from liquid water into steam. Such properties are typical of materials for which polarized molecules form hydrogen bonds.