NANOTECHNOLOGY—TECHNOLOGY ON A MOLECULAR SCALE - Chemistry: Notable Research and Discoveries - page 42

HydrogenBonds
Hydrogen is the smallest atom, consisting of a single proton
and electron. (Some isotopes of hydrogen contain one or two
neutrons in the nucleus, but the vast majority of hydrogen
atoms have no neutrons.) Covalent bonds form when atoms
share electrons, which stabilize their electron confi gurations.
When a hydrogen atom forms a covalent bond with certain
atoms such as oxygen, fl uorine, and nitrogen, it shares a
pair of electrons with the other atom—one from the hydro-
gen and one from the other atom. But the sharing of the
electron pair is not an equal, 50-50 partnership. Oxygen,
fl uorine, and nitrogen are strongly electronegative, meaning
that these atoms tug the “shared” electrons more to their
side of the bond. Tiny hydrogen cannot resist this pull. The
result is that the negative charge of the electrons tends to
be more toward hydrogen's partner, leaving the positively
charged proton of hydrogen “uncovered.” This process cre-
ates electric fi elds that attract opposite fi elds and repels like
fi elds. As shown in the fi gure at right, the hydrogen end of
the molecule, with its partial positive charge, sticks to oxy-
gen, fl uorine, and nitrogen ends of other molecules.
A hydrogen bond is typically about 20 times weaker than a
covalent bond. But hydrogen bonds are strong enough to draw
molecules together, forming attachments that can be tempo-
rary but have important effects. Water molecules form plenty
of hydrogen bonds, for example, and as a result, the mole-
cules stick together more readily than other molecules. This is
why water remains a liquid even at high temperatures—heat
drives molecules apart, but more heat is needed to break the
hydrogen bonds holding water molecules together. A similar
compound, hydrogen sulfi de (H 2 S), does not tend to form hy-
drogen bonds and has a boiling point of -77°F (-60°C), which
is much lower than the boiling point of water, 212°F (100°C).
Hydrogen sulfi de is a gas well below room temperature.