52
UNIT ONE
elements are known and at least 26 more have been created
in the laboratory. Among these elements are such common
materials as iron, copper, silver, gold, aluminum, carbon,
hydrogen, and oxygen. Some elements exist in a pure form,
but these and other elements are more commonly parts of
chemical combinations called
compounds
(kom
powndz).
Elements the body requires in large amounts—such as car-
bon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus—are
termed
bulk elements.
These elements make up more than
95% (by weight) of the human body
(table 2.2)
. Elements
required in small amounts are called
trace elements.
Many
trace elements are important parts of enzymes, proteins that
regulate the rates of chemical reactions in living organisms.
Some elements toxic in large amounts, such as arsenic, may
be vital in very small amounts, and these are called
ultra-
trace elements.
Elements are composed of particles called
atoms
(at
omz),
the smallest complete units of the elements. The atoms that
make up each element are chemically identical but they dif-
fer from the atoms that make up other elements. Atoms vary
in size, weight, and the ways they interact with other atoms.
Some atoms can combine with atoms like themselves or with
other atoms by forming attractions called
chemical bonds,
while other atoms cannot form such bonds.
Atomic Structure
An atom consists of a central portion called the
nucleus
(nu
kle-us) and one or more
electrons
(e-lek
tronz) that
constantly move around the nucleus. The nucleus contains
one or more relatively large particles,
protons
(pro
tonz) and
usually
neutrons
(nu
tronz). Protons and neutrons are about
equal in weight, but they are otherwise different
(f g. 2.1)
.
Electrons, so small that they have almost no weight,
carry a single, negative electrical charge (e
). Each proton
carries a single, positive electrical charge (p
+
). Neutrons are
uncharged and thus are electrically neutral (n
0
).
The nucleus contains protons, so this part of an atom
is always positively charged. However, the number of elec-
trons outside the nucleus equals the number of protons, so
a complete atom is said to have no net charge and is thus
electrically neutral.
The atoms of different elements have different numbers
of protons. The number of protons in the atoms of a particular
element is called its
atomic number.
Hydrogen, for example,
whose atoms have one proton, has atomic number 1; carbon,
whose atoms have six protons, has atomic number 6.
The weight of an atom of an element is primarily due to
the protons and neutrons in its nucleus, because the elec-
trons are so light. For this reason, the number of protons plus
the number of neutrons in each of an element’s atoms essen-
tially equals the
atomic weight
of that atom. The atomic
weight of a hydrogen atom, which has only one proton and
no neutrons, is approximately 1. The atomic weight of a
carbon atom, with six protons and six neutrons, is approxi-
mately 12
(table 2.3)
.
Isotopes
All the atoms of a particular element have the same atomic
number because they have the same number of protons and
electrons. However, the atoms of an element vary in the
number of neutrons in their nuclei; thus, they vary in atomic
TABLE
2.2
|
Major Elements in the Human
Body (By Weight)
Major Elements
Symbol
Approximate Percentage
of the Human Body
Oxygen
O
65.0
Carbon
C
18.5
Hydrogen
H
9.5
Nitrogen
N
3.2
Calcium
Ca
1.5
Phosphorus
P
1.0
99.9%
Potassium
K
0.4
Sulfur
S
0.3
Chlorine
Cl
0.2
Sodium
Na
0.2
Magnesium
Mg
0.1
Trace Elements
Cobalt
Co
Copper
Cu
Fluorine
F
Iodine
I
less than 0.1%
Iron
Fe
Manganese
Mn
Zinc
Zn
0
0
+
+
0
+
0
Electron
(e
)
Lithium (Li)
Proton
(p
+
)
Neutron
(n
0
)
Nucleus
FIGURE 2.1
An atom consists of subatomic particles. In an atom of
the element lithium, three electrons encircle a nucleus that consists of
three protons and four neutrons.
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