essentials of anatomy and physiology; 5e

24 Some Basic Chemistry
When you hear or see the word chemistry, you may
think of test tubes and Bunsen burners in a laboratory
experiment. However, literally everything in our physical
world is made of chemicals. The paper used for
this book, which was once the wood of a tree, is made
of chemicals. The air we breathe is a mixture of chemicals
in the form of gases. Water, gasoline, and diet
soda are chemicals in liquid form. Our foods are
chemicals, and our bodies are complex arrangements
of thousands of chemicals. Recall from Chapter 1 that
the simplest level of organization of the body is the
chemical level.
This chapter covers some very basic aspects of
chemistry as they are related to living organisms, and
most especially as they are related to our understanding
of the human body. So try to think of chemistry
not as a complicated science, but as the air, water, and
food we need, and every substance that is part of us.
ELEMENTS
All matter, both living and not living, is made of elements,
the simplest chemicals. An element is a substance
made of only one type of atom (therefore, an
atom is the smallest part of an element). There are 92
naturally occurring elements in the world around us.
Examples are hydrogen (H), iron (Fe), oxygen (O),
calcium (Ca), nitrogen (N), and carbon (C). In nature,
an element does not usually exist by itself but rather
combines with the atoms of other elements to form
compounds. Examples of some compounds important
to our study of the human body are water (H 2
O), in
which two atoms of hydrogen combine with one atom
of oxygen; carbon dioxide (CO 2
), in which an atom of
carbon combines with two atoms of oxygen; and glucose
(C 6
H 12
O 6
), in which six carbon atoms and six
oxygen atoms combine with 12 hydrogen atoms.
The elements carbon, hydrogen, oxygen, nitrogen,
phosphorus, and sulfur are found in all living things. If
calcium is included, these seven elements make up
approximately 99% of the human body (weight).
More than 20 different elements are found, in varying
amounts, in the human body. Some of these are
listed in Table 2–1. As you can see, each element has a
standard chemical symbol. This is simply the first (and
sometimes the second) letter of the element’s English
or Latin name. You should know the symbols of the
elements in this table, because they are used in text-
Table 2–1 ELEMENTS IN THE
HUMAN BODY
Percent of
Atomic the Body
Elements Symbol Number* by Weight
Hydrogen H 1 9.5
Carbon C 6 18.5
Nitrogen N 7 3.3
Oxygen O 8 65.0
Fluorine F 9 Trace
Sodium Na 11 0.2
Magnesium Mg 12 0.1
Phosphorus P 15 1.0
Sulfur S 16 0.3
Chlorine Cl 17 0.2
Potassium K 19 0.4
Calcium Ca 20 1.5
Manganese Mn 25 Trace
Iron Fe 26 Trace
Cobalt Co 27 Trace
Copper Cu 29 Trace
Zinc Zn 30 Trace
Iodine I 53 Trace
*
Atomic number is the number of protons in the nucleus of
the atom. It also represents the number of electrons that
orbit the nucleus.
books, articles, hospital lab reports, and so on. Notice
that if a two-letter symbol is used for an element, the
second letter is always lowercase, not a capital. For
example, the symbol for calcium is Ca, not CA. CA is
an abbreviation often used for cancer.
ATOMS
Atoms are the smallest parts of an element that have
the characteristics of that element. An atom consists of
three major subunits or particles: protons, neutrons,
and electrons (Fig. 2–1). A proton has a positive electrical
charge and is found in the nucleus (or center) of
the atom. A neutron is electrically neutral (has no
charge) and is also found in the nucleus. An electron
has a negative electrical charge and is found outside
the nucleus orbiting in what may be called an electron
cloud or shell around the nucleus.
The number of protons in an atom gives it its
atomic number. Protons and neutrons have mass and
weight; they give an atom its atomic weight. In an