Hydrogen bonds form because of the attraction between a slightly positive hydrogen atom of one molecule and the slightly negative atom of another molecule. Differences in electronegativity between the hydrogen atom and the other atom or atoms of the molecule lead to these partial positive and partial negative charges.
Hydrogen forms covalent bonds with nonmetal atoms; this means that the two atoms share electrons with one another. Hydrogen does not have a strong attraction for electrons, so its electronegativity is quite low. The nonmetal usually has a greater attraction or affinity for electrons; therefore, the nonmetallic atom is more electronegative than the hydrogen atom. Shared electrons spend more time on the side of the nonmetallic atom and less time with hydrogen. This unequal sharing of electrons leads to a partial positive charge on the hydrogen portion of the molecule and a partial negative charge on the other atom. The partial positive region of one molecule attracts the partial positive area of another molecule within the compound, and a weak bond called a hydrogen bond forms.
Hydrogen bonding contributes to a number of physical characteristics of compounds such as higher boiling points and greater viscosity. Although a relatively weak force that amounts only to about 5 percent of the strength of a covalent bond, hydrogen bonds become strong in numbers. Hydrogen bonding keeps liquids in liquid form over a wider temperature range than expected, meaning that liquids containing hydrogen bonds freeze at lower temperatures and boil at higher temperatures than other liquids. The effects of hydrogen bonding is seen in properties of water such as the cohesion that causes water to form spheres, adhesion where water holds onto other substances such as soil and surface tension that allows insects to walk on water's surface.