34
UNIT 1
Organization of the Body
2
are electrically balanced and are called
nonpolar molecules
(because they do not have separate
1
and
2
poles of charge).
Such electrical balance is not always the case. When covalent
bonds are formed, the resulting molecule always has a specific
three-dimensional shape, with the bonds formed at definite
angles. A molecule’s shape helps determine what other mol-
ecules or atoms it can interact with. It may also result in unequal
electron pair sharing, creating a
polar molecule
, especially in
nonsymmetrical molecules containing atoms with different
electron-attracting abilities.
In general,
small
atoms with six or seven valence shell elec-
trons, such as oxygen, nitrogen, and chlorine, are electron-
hungry and attract electrons very strongly, a capability called
electronegativity
. On the other hand, most atoms with only
one or two valence shell electrons tend to be
electropositive
. In
other words, their electron-attracting ability is so low that they
usually lose
their
valence shell electrons to other atoms. Potas-
sium and sodium, each with one valence shell electron, are good
examples of electropositive atoms.
Carbon dioxide and water illustrate how molecular shape
and the relative electron-attracting abilities of atoms determine
whether a covalently bonded molecule is nonpolar or polar. In
carbon dioxide (CO
2
), carbon shares four electron pairs with two
oxygen atoms (two pairs are shared with each oxygen). Oxygen
is very electronegative and so attracts the shared electrons much
more strongly than does carbon. However, because the carbon
dioxide molecule is linear and symmetrical
(Figure 2.8a)
, the
electron-pulling ability of one oxygen atom offsets that of the
other, like a standoff between equally strong teams in a game of
tug-of-war. As a result, the shared electrons orbit the entire mol-
ecule and carbon dioxide is a nonpolar compound.
In contrast, a water molecule (H
2
O) is bent, or V shaped (Fig-
ure 2.8b). Te two electropositive hydrogen atoms are located
at the same end of the molecule, and the very electronegative
oxygen is at the opposite end. Tis arrangement allows oxygen
to pull the shared electrons toward itself and away from the two
hydrogen atoms. In this case, the electron pairs are
not
shared
equally, but spend more time in the vicinity of oxygen. Because
electrons are negatively charged, the oxygen end of the molecule
is slightly more negative (the charge is indicated with a delta
and minus as δ
2
) and the hydrogen end slightly more positive
(indicated by δ
1
). Because water has two poles of charge, it is a
polar molecule
, or
dipole
(di
9
pōl).
Polar molecules orient themselves toward other dipoles or
toward charged particles (such as ions and some proteins), and
they play essential roles in chemical reactions in body cells. Te
polarity of water is particularly significant, as you will see later
in this chapter.
Different molecules exhibit different degrees of polarity, and
we can see a gradual change from ionic to nonpolar covalent
bonding as summarized in
Figure 2.9
. Ionic bonds (complete
electron transfer) and nonpolar covalent bonds (equal electron
sharing) are the extremes of a continuum, with various degrees
of unequal electron sharing in between.
Hydrogen Bonds
Unlike the stronger ionic and covalent bonds, hydrogen bonds
are more like attractions than true bonds. Hydrogen bonds
form when a hydrogen atom, already covalently linked to one
electronegative atom (usually nitrogen or oxygen), is attracted
by another electron-hungry atom, so that a “bridge” forms be-
tween them.
Hydrogen bonding is common between dipoles such as water
molecules, because the slightly negative oxygen atoms of one mol-
ecule attract the slightly positive hydrogen atoms of other mol-
ecules
(Figure 2.10a)
. Hydrogen bonding is responsible for the
tendency of water molecules to cling together and form films, re-
ferred to as
surface tension
. Tis tendency helps explain why water
O
C
O
O
H
H
(a) Carbon dioxide (CO
2
) molecules are
linear and symmetrical. They are nonpolar.
(b) V-shaped water (H
2
O) molecules have two
poles of charge—a slightly more negative
oxygen end (
δ
-
) and a slightly more positive
hydrogen end (
δ
+
).
δ
-
δ
+
δ
+
Figure 2.8
Carbon dioxide and water molecules have differ-
ent shapes, as illustrated by molecular models.
Ionic bond
Complete
transfer of
electrons
Separate ions
(charged
particles)
form
Na
+
Sodium chloride
Water
Carbon dioxide
Polar covalent
bond
Unequal sharing
of electrons
Slight negative
charge (
δ
) at
one end of
molecule, slight
positive charge (
δ
+
)
at other end
Nonpolar
covalent bond
Equal sharing of
electrons
Charge balanced
among atoms
Cl
H
H
O
O
O
C
δ
+
δ
+
δ
Figure 2.9
Ionic, polar covalent, and nonpolar covalent
bonds compared along a continuum.
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