26
UNIT 1
Organization of the Body
2
the
electron cloud
). However, the planetary model is simpler to
depict, so we will use that model in most illustrations of atomic
structure in this text.
Hydrogen, with just one proton and one electron, is the sim-
plest atom. You can visualize the spatial relationships in the
hydrogen atom by imagining it as a sphere enlarged until its
diameter equals the length of a football field. In that case, the
nucleus could be represented by a lead ball the size of a gum-
drop in the exact center of the sphere. Its lone electron could be
pictured as a fly buzzing about unpredictably within the sphere.
Tough not completely accurate, this mental image demon-
strates that most of the volume of an atom is empty space, and
nearly all of its mass is concentrated in the central nucleus.
Te
planetary model
of the atom, illustrated in Figure 2.1a,
is a simplified (and now outdated) model of atomic structure.
As you can see, it depicts electrons moving around the nucleus
in fixed, generally circular orbits. But we can never determine
the exact location of electrons at a particular time because
they jump around following unknown trajectories. So, instead
of speaking of specific orbits, chemists talk about
orbitals
regions around the nucleus in which a given electron or electron
pair is likely to be found most of the time. Tis more modern
model of atomic structure, called the
orbital model
, is more
useful for predicting the chemical behavior of atoms. As illus-
trated in Figure 2.1b, the orbital model depicts
probable
regions
of greatest electron density by denser shading (this haze is called
Table 2.1
Common Elements Composing the Human Body*
ELEMENT
ATOMIC
SYMBOL
APPROX. %
BODY MASS
FUNCTIONS
Major (96.1%)
Oxygen
O
65.0
A major component of both organic (carbon-containing) and inorganic (non-
carbon-containing) molecules. As a gas, it is needed for the production of
cellular energy (ATP).
Carbon
C
18.5
A primary component of all organic molecules, which include carbohydrates,
lipids (fats), proteins, and nucleic acids.
Hydrogen
H
9.5
A component of all organic molecules. As an ion (proton), it influences the pH
of body fluids.
Nitrogen
N
3.2
A component of proteins and nucleic acids (genetic material).
Lesser (3.9%)
Calcium
Ca
1.5
Found as a salt in bones and teeth. Its ionic (Ca
2
1
) form is required for muscle
contraction, conduction of nerve impulses, and blood clotting.
Phosphorus
P
1.0
Part of calcium phosphate salts in bones and teeth. Also present in nucleic
acids, and part of ATP.
Potassium
K
0.4
Its ion (K
1
) is the major positive ion (cation) in cells. Necessary for conduction
of nerve impulses and muscle contraction.
Sulfur
S
0.3
Component of proteins, particularly muscle proteins.
Sodium
Na
0.2
As an ion (Na
1
), sodium is the major positive ion found in extracellular fluids
(fluids outside of cells). Important for water balance, conduction of nerve
impulses, and muscle contraction.
Chlorine
Cl
0.2
Its ion (chloride, Cl
2
) is the most abundant negative ion (anion) in extracellular
fluids.
Magnesium
Mg
0.1
Present in bone. Also an important cofactor in a number of metabolic
reactions.
Iodine
I
0.1
Needed to make functional thyroid hormones.
Iron
Fe
0.1
Component of hemoglobin (which transports oxygen within red blood cells)
and some enzymes.
Trace (less than 0.01%)
Chromium (Cr); cobalt (Co); copper (Cu); fluorine (F); manganese (Mn); molybdenum (Mo); selenium (Se); silicon (Si); tin (Sn); vanadium (V);
zinc (Zn)
These elements are referred to as
trace elements
because they are required in very minute amounts; many are found as part of enzymes or are
required for enzyme activation.
*A listing of the elements by ascending order of atomic number appears in the periodic table, Appendix E.
Percentage of “wet” body mass; includes water.
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