HYDROGEN BONDING
Hydrogen bonds are weak electrostatic attractions between atoms displaying partial positive and partial negative charges. The partial positive charges are produced when hydrogen atoms are associated with another atom through a polar covalent bond. The partial negative charges are associated with an atom (e.g., oxygen) that has a higher affinity for electrons.

The oxygen atom has two pairs of electrons in its outermost energy level that are not in bonding orbitals. These four electrons form a negatively charged "cloud" of electron density on the side of the water molecule opposite from the hydrogen atoms. The oxygen atom is much more electronegative than the hydrogen atom, so, when a bonding orbital forms between the oxygen and hydrogen atoms, the electrons in the bonding orbital are disproportionately distributed toward the oxygen side of the polar covalent bond. This results in a partial negative charge (-) associated with the oxygen atom. The full positive charge of the hydrogen nucleus is poorly covered by the thin electron density and "shows through", resulting in a partial positive charge (+).

The weak electrostatic attraction between the partial negative and partial positive charges is a hydrogen bond.

Hydrogen bonds are critically important in biology because they help explain the solubility of molecules in water, the structure of macromolecules (such as DNA and protein), and the formation of stable lipid bilayer membranes.