Chemical Composition of the Body
21
The amino group can bind a hydrogen ion to form an ionized
amino group (R—NH
3
+
):
R—NH
2
+ H
+
12
R—NH
3
+
The ionization of each of these groups can be reversed, as indi-
cated by the double arrows; the ionized carboxyl group can
combine with a hydrogen ion to form an un-ionized carboxyl
group, and the ionized amino group can lose a hydrogen ion
and become an un-ionized amino group.
Free Radicals
The electrons that revolve around the nucleus of an atom
occupy regions known as orbitals, each of which can be occu-
pied by one or more pairs of electrons, depending on the dis-
tance of the orbital from the nucleus. An atom is most stable
when each orbital is occupied by its full complement of elec-
trons. An atom containing a single (unpaired) electron in its
outermost orbital is known as a
free radical,
as are molecules
Hydrogen atoms and most mineral and trace element
atoms readily form ions.
Table 2–3
lists the ionic forms of some
of these elements that are found in the body. Ions that have a
net positive charge are called
cations,
while those that have a
net negative charge are called
anions.
Because of their ability to
conduct electricity when dissolved in water, the ionic forms of
mineral elements are collectively referred to as
electrolytes.
The process of ion formation, known as ionization, can
occur in single atoms or in atoms that are covalently linked
in molecules. Within molecules, two commonly encoun-
tered groups of atoms that undergo ionization are the
car-
boxyl group
(—COOH) and the
amino group
(—NH
2
).
The shorthand formula for only a portion of a molecule can
be written as R—COOH or R—NH
2
, where R signifi es the
remaining portion of the molecule. The carboxyl group ion-
izes when the oxygen linked to the hydrogen captures the
hydrogen’s only electron to form a carboxyl ion (R—COO
),
releasing a hydrogen ion (H
+
):
R—COOH
12
R—COO
+ H
+
Neutrons
Protons
Electrons
Carbon
Hydrogen
Methane
(four covalent bonds)
6
0
6
1
6
1
+
+
+
+
+
+
+
+
+
+
+
+
H
H
H
H
C
Figure 2–1
Each of the four hydrogen atoms in a molecule of methane (CH
4
) forms a covalent bond with the carbon atom by sharing its one electron with
one of the electrons in carbon. Each shared pair of electrons—one electron from the carbon and one from a hydrogen atom—forms a covalent
bond. The sizes of protons, neutrons, and electrons are not to scale.
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