Molecules of HF, H2O, and NH3 are
all able to form hydrogen bonds. What property of a molecule gives
it the ability to form a hydrogen bond?
A hydrogen bond is a dipole–dipole
interaction between a covalently bonded hydrogen atom and a lone pair of electrons
on an adjacent molecule. Generally, this lone pair is from
an electronegative element. Because atoms like fluorine,
oxygen, and nitrogen are highly electronegative, when covalently bonded to hydrogen,
a dipole is formed. The electronegative atoms tend to
pull bonded pairs of electrons towards themselves, causing a partial separation of
positive and negative charge, which forms the dipole.
The opposite partial charges on
adjacent molecules are attracted to each other. The molecule with the partially
positive hydrogen attracted to the adjacent molecule is the hydrogen bond donor. The molecule with a lone pair on an
electronegative atom attracting an adjacent hydrogen atom is called the hydrogen
bond acceptor. This attraction is known as the
hydrogen bond. Hydrogen fluoride, water, and
ammonia all form hydrogen bonds.
A hydrogen bond tends to be
stronger if there is more pronounced charge separation in the molecules. For example, in hydrogen fluoride,
the separation of charge is very high. Molecules with a hydrogen atom
covalently bonded to fluorine tend to generate particularly intense hydrogen bonds
with adjacent molecules. Molecules like these tend to form
strong dipoles. This is due to the very high
electronegativity value from the Pauling scale of fluorine, especially compared to
that of hydrogen. Molecules of hydrogen fluoride
contain covalent bonds with large differences in electronegativities and very strong
Water molecules also engage in
strong hydrogen bonds, with hydrogen and oxygen also having a large difference in
Ammonia molecules exhibit similar
abilities to hydrogen bond.
These three molecules are all able
to form hydrogen bonds from the difference in electronegativity of the atoms within
them. Therefore, the property of a
molecule that gives it the ability to form a hydrogen bond is a large difference in
electronegativity between hydrogen and the atom it is bonded to.