Chemical Composition of the Body
19
Atoms
The units of matter that form all chemical substances are
called
atoms.
Each type of atom—carbon, hydrogen, oxygen,
and so on—is called a
chemical element.
A one- or two-letter
symbol is used as a shorthand identifi cation for each element.
Although more than 100 elements exist in the universe, only
24 (
Table 2–1
) are known to be essential for the structure and
function of the human body.
The chemical properties of atoms can be described in
terms of three subatomic particles—
protons, neutrons,
and
electrons.
The protons and neutrons are confi ned to a
very small volume at the center of an atom called the
atomic
nucleus.
The electrons revolve in orbits at various distances
from the nucleus. This miniature solar-system model of an
atom is a gross oversimplifi cation, but it is suffi cient to provide
a conceptual framework for understanding the chemical and
physical interactions of atoms.
Each of the subatomic particles has a different electric
charge. Protons have one unit of positive charge, electrons
have one unit of negative charge, and neutrons are electrically
neutral (
Table 2–2
). Because the protons are located in the
atomic nucleus, the nucleus has a net positive charge equal to
the number of protons it contains. The entire atom has no net
electric charge, however, because the number of negatively
charged electrons orbiting the nucleus equals the number of
positively charged protons in the nucleus.
Atomic Number
Each chemical element contains a specifi c number of protons,
and it is this number, known as the
atomic number,
that
distinguishes one type of atom from another. For example,
hydrogen, the simplest atom, has an atomic number of 1, cor-
responding to its single proton. As another example, calcium
has an atomic number of 20, corresponding to its 20 protons.
Because an atom is electrically neutral, the atomic number is
also equal to the number of electrons in the atom.
Atomic Weight
Atoms have very little mass. A single hydrogen atom, for exam-
ple, has a mass of only 1.67
×
10
–24
g. The
atomic weight
scale
indicates an atom’s mass relative to the mass of other atoms. This
scale is based upon assigning the carbon atom a mass of 12. On
this scale, a hydrogen atom has an atomic weight of approxi-
mately 1, indicating that it has one-twelfth the mass of a car-
bon atom. A magnesium atom, with an atomic weight of 24, has
twice the mass of a carbon atom.
Because the atomic weight scale is a
ratio
of atomic
masses, it has no absolute units. The unit of atomic mass is
known as a dalton. One dalton (d) equals one-twelfth the
mass of a carbon atom. Thus, carbon has an atomic weight of
12, and a carbon atom has an atomic mass of 12 daltons.
Table 2–1
Essential Chemical Elements in the
Body
Element
Symbol
Major Elements: 99.3% of Total Atoms in the Body
Hydrogen
H (63%)
Oxygen
O (26%)
Carbon
C (9%)
Nitrogen
N (1%)
Mineral Elements: 0.7% of Total Atoms in the Body
Calcium
Ca
Phosphorus
P
Potassium
K (Latin
kalium
)
Sulfur
S
Sodium
Na (Latin
natrium
)
Chlorine
Cl
Magnesium
Mg
Trace Elements: Less than 0.01% of Total Atoms in the Body
Iron
Fe (Latin
ferrum
)
Iodine
I
Copper
Cu (Latin
cuprum
)
Zinc
Zn
Manganese
Mn
Cobalt
Co
Chromium
Cr
Selenium
Se
Molybdenum
Mo
Fluorine
F
Tin
Sn (Latin
stannum
)
Silicon
Si
Vanadium
V
Table 2–2
Characteristics of Major Subatomic
Particles
Particle
Mass Relative to
Electron
Electric
Charge
Location in
Atom
Proton
1836
+1
Nucleus
Neutron
1839
0
Nucleus
Electron
1
–1
Orbiting
the nucleus
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