18
-molecular structure of all matter. Students of anatomy
need to have some -basi c knowledge of this field of study.
This chapter introduces you to some basic principles of
chemistry that will assist in your comprehension of hu-man
anatomy and physiology. To understand the human body, it
is necessary to understand the chemical basis of life. We
will look at the structure of the atom, how at-oms interact
with one another to form compounds, and how those
compounds form the building blocks of life. All nonliving
and living things are made of matter. Matter is composed of
elements, which are primary substances from which all
other things are constructed. Elements cannot be broken
down into simpler substances. There are 92 elements that
occur naturally. Other elements have been created
artificially in the laboratory.
ATOMIC STRUCTURE
Atoms are the smallest particles of an element that main-
tain all the characteristics of that element and enter into
chemical reactions through their electrons. Each atom
consists of a relatively heavy, compact central nucleus
composed of protons and neutrons. Lighter particles
called electrons orbit the nucleus at some distance from
its center.
Electrons are practically weightless, and each one
carries a negative electrical charge (^2 ). Atomic nuclei are
composed of protons and neutrons, except for the
-hydrogen nucleus, which contains only one proton
-(Figure 2-1). Each proton and neutron has one unit of
atomic weight and is about 1800 times heavier than an
electron. Thus, an atom’s weight results almost entirely
from its protons and neutrons. A proton carries a posi-tive
charge (^1 ), whereas a neutron is neutral and has no charge.
Like charges repel—they push away from each
Proton Electron (^)
®^
Learning^
Hydrogen (H)
Cengage^
©^
Figure 2- 1 The hydrogen atom is unique
because its nucleus contains only one proton.
Chapter 2
other. Thus, when you brush your hair on a dry day, like
electrical charges build up on the brush and your hair, so
your hair flies away from the brush. Unlike charges at-tract.
The clinging of clothes taken out of a dryer is due to the
attraction of unlike electrical charges.
ELEMENTS, ISOTOPES, COMPOUNDS
Each element has a distinctive number of protons. An
element- is a substance whose atoms all contain the same
number of protons and the same number of electrons.
Because the number of protons equals the number of
electrons, an atom is electrically neutral. The theory that
suggested that all matter consists of atoms was proposed in
1808 by John Dalton (1766–1844). He stated that atoms-
were responsible for the combinations of elements found in
compounds. The atomic theory developed from his
proposal. The atomic theory proposed that:
■■ All matter is made up of tiny particles called atoms.
■■ All atoms of a given element are similar to one another
but different from the atoms of other elements.^
■■ Atoms of two or more elements combine to form com-
pounds.^
■■ A chemical reaction involves the rearrangement,
separation,- or combination of atoms.^
■■ Atoms are never created or destroyed during a chemi-cal
reaction.
In the atoms of some elements, the number of neu-
trons varies. Carbon is the element found in all living
matter. Life on earth is based on the carbon atom. In fact, a
whole branch of chemistry called organic chemistry studies
the nature of the carbon atom and its chemical reactions.
Different atoms of carbon may have different numbers of
neutrons. Atoms of carbon may have one of three different
atomic weights—12, 13, or 14—-depending on the number
of neutrons. These different kinds of at-oms of the same
element are called isotopes, and are designated as C^12 ,
C^13 , and C^14. Each of these isotopes contains six protons
and six electrons, but C^12 has six neutrons, C^13 has seven
neutrons, and C^14 has eight neu-trons. C^14 is mildly
radioactive and is used to estimate the age of fossilized
human remains. C^14 has a half-life of 5730 years; thus, it
takes that amount of time to lose one half of its
radioactivity and so it does not cause any damage to
biological tissues. A radioactive isotope of iodine- is used
to treat disorders of the thyroid gland. The atomic
number is the number of protons or the num-ber of
electrons. By the late 1800s, scientists discovered
similarities in the behavior of the known elements. It was