Chapter 2
Chemistry Comes Alive
25
2
matter is heated, the kinetic energy of its particles increases and
they begin to move more quickly. Te higher the temperature,
the faster the body’s chemical reactions occur. We will learn
more about this later.
Check Your Understanding
1.
What form of energy is found in the food we eat?
2.
What form of energy is used to transmit messages from one
part of the body to another?
3.
What type of energy is available when we are still? When we
are exercising?
For answers, see Appendix H.
Composition of Matter:
Atoms and Elements
Define chemical element and list the four elements that
form the bulk of body matter.
Define atom. List the subatomic particles, and describe
their relative masses, charges, and positions in the atom.
Define atomic number, atomic mass, atomic weight,
isotope, and radioisotope.
All matter is composed of
elements
, unique substances that
cannot be broken down into simpler substances by ordinary
chemical methods. Among the well-known elements are oxy-
gen, carbon, gold, silver, copper, and iron.
At present, 118 elements are recognized. Of these, 92 occur
in nature. Te rest are made artificially in particle accelerator
devices.
Four elements—carbon, oxygen, hydrogen, and nitrogen—
make up about 96% of body weight, and 20 others are present
in the body, some in trace amounts.
Table 2.1
on p. 26 lists the
elements contributing to body mass and gives their importance.
In Appendix E, an oddly shaped checkerboard called the
peri-
odic table
provides a more complete listing of the known ele-
ments and helps to explain the properties of each element that
make it react as it does with other elements.
Each element is composed of more or less identical particles
or building blocks, called
atoms
. Te smallest atoms are less
than 0.1 nanometer (nm) in diameter, and the largest are only
about five times as large. [1 nm
5
0.0000001 (or 10
2
7
) centi-
meter (cm), or 40 billionths of an inch!]
Every element’s atoms differ from those of all other elements
and give the element its unique physical and chemical proper-
ties.
Physical properties
are those we can detect with our senses
(such as color and texture) or measure (such as boiling point
and freezing point).
Chemical properties
pertain to the way at-
oms interact with other atoms (bonding behavior) and account
for the facts that iron rusts, animals can digest their food, and
so on.
We designate each element by a one- or two-letter chemical
shorthand called an
atomic symbol
, usually the first letter(s)
of the element’s name. For example, C stands for carbon, O for
oxygen, and Ca for calcium. In a few cases, the atomic symbol
is taken from the Latin name for the element. For example, so-
dium is indicated by Na, from the Latin word
natrium
.
Atomic Structure
Te word
atom
comes from the Greek word meaning “indivis-
ible.” However, we now know that atoms are clusters of even
smaller particles called protons, neutrons, and electrons and
that even those subatomic particles can be subdivided with
high-technology tools. Still, the old idea of atomic indivisibility
is useful because an atom loses the unique properties of its ele-
ment when it is split into its subatomic particles.
An atom’s subatomic particles differ in mass, electrical
charge, and position in the atom. An atom has a central
nu-
cleus
containing protons and neutrons tightly bound together.
Te nucleus, in turn, is surrounded by orbiting electrons
(Figure 2.1)
.
Protons
(p
1
) bear a positive electrical charge, and
neutrons
(n
0
) are neutral, so the nucleus is positively charged
overall. Protons and neutrons are heavy particles and have ap-
proximately the same mass, arbitrarily designated as 1
atomic
mass unit
(1 amu). Since all of the heavy subatomic particles are
concentrated in the nucleus, the nucleus is fantastically dense. It
accounts for nearly the entire mass (99.9%) of the atom.
Te tiny
electrons
(e
2
) bear a negative charge equal in
strength to the positive charge of the proton. However, an elec-
tron has only about 1/2000 the mass of a proton, and the mass
of an electron is usually designated as 0 amu.
All atoms are electrically neutral because the number of pro-
tons in an atom is precisely balanced by its number of electrons
(the
1
and
2
charges will then cancel the effect of each other).
For example, hydrogen has one proton and one electron, and
iron has 26 protons and 26 electrons. For any atom, the number
of protons and electrons is always equal.
(a) Planetary model
(b) Orbital model
Helium atom
2 protons (p
+
)
2 neutrons (n
0
)
2 electrons (e
)
Helium atom
2 protons (p
+
)
2 neutrons (n
0
)
2 electrons (e
)
Nucleus
Nucleus
Electron
cloud
Proton
Neutron
Electron
Figure 2.1
Two models of the structure of an atom.
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