bonds, the more strongly the atoms are bound together.
In some molecules, two atoms can form single, double,
or triple covalent bonds (Figure 2.5). Unlike ionic bonds,
covalent bonds do not break apart when the molecule is
dissolved in water. Covalent bonds are the most common
chemical bonds in the body.
Polar molecules and hydrogen bonds
In some
covalent compounds, the electrons are shared unevenly
between the two atoms. One atom attracts electrons more
strongly than the other, so the electrons spend more time
around the stronger atom than around the weaker one.
This results in one end of the molecule having a partial
negative charge (denoted by the symbol S-), while the other
has a partial positive charge (S+). These types of molecules
are called polar molecules, and the bonds between them
are referred to as polar covalent bonds. Polar molecules
usually occur when a weak atom such as hydrogen forms co-
valent bonds with strong atoms such as oxygen or nitrogen.
Water is an example of a polar molecule (Figure 2.5e).
In contrast, other covalent compounds share valence
electrons equally between the atoms. The molecules in these
compounds are non-polar molecules, and the bonds be-
tween them are referred to as non-polar covalent bonds.
For example, hydrogen molecules (H2) and methane mol-
ecules (CH4) are non-polar molecules (Figure 2.5a, d).
You can think of polar molecules as small bar magnets.
The partially positive end of one polar molecule will at-
tract the partially negative end of another, and vice versa.
When a partially positive end (S+) of one polar molecule is
attracted to the partially negative end (S-) of another po-
lar molecule, a weak bond is formed. The partially positive
end usually involves a hydrogen atom, so this type of bond
is called a hydrogen bond. The hydrogen bonds between
water molecules (Figure 2.6) give water some unique
properties, such as a relatively high boiling point and low
freezing point compared to other liquids. In addition, the
hydrogen bonding between water molecules affects their
formation of crystals when water freezes. Because the
crystals take up more space than the number of water
molecules that make it up, water expands when it freezes.
Hydrogen bonds are weak, so they are easily formed
and broken. They play important roles in the shapes of
biologically important molecules such as proteins and de-
oxyribonucleic acid (DNA) (see Figures 2.12 and 2.14).
Free radicals
Most of an atom’s electrons associate in
pairs. However, when an ion or a molecule has an unpaired
electron in its outer shell, it is called a free radical. A
common example of a free radical is
superoxide,
which is
formed by the addition of an electron to an oxygen mol-
ecule. Having an unpaired electron makes a free radical
unstable and destructive to nearby molecules. Free radi-
cals break apart important body molecules by giving an
unpaired electron to another molecule or accepting an
electron from another molecule.
Hydrogen bonds in water • Figure 2.6
a.
W eak hydrogen bonds form between the
partially positive hydrogen atoms of one water
molecule and the partially negative oxygen atoms
of others.
S+
S+
Water
molecule
Q l
5
Crystal of
NaCl
b.
Water’s polarity allows it to dissolve
many substances.
*
Hydrated sodium ion
Hydrated chloride ion
The Building Blocks of Matter Fit Together to Make Ions and Compounds
29
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