Organization of the Body
as a cause of lung cancer. (Radon results naturally from decay
of uranium in the ground.) Gamma emission has the greatest
penetrating power. Radium-226, cobalt-60, and certain other
radioisotopes that decay by gamma emission are used to destroy
localized cancers.
Contrary to what some believe, ionizing radiation does not
damage organic molecules directly. Instead, it knocks elec-
trons out of other atoms and sends them flying, like bowl-
ing balls smashing through pins all along their path. It is the
electron energy and the unstable molecules leF behind that
do the damage.
Check Your Understanding
What two elements besides H and N make up the bulk of
living matter?
An element has a mass of 207 and has 125 neutrons in its
nucleus. How many protons and electrons does it have and
where are they located?
How do the terms atomic mass and atomic weight differ?
For answers, see Appendix H.
How Matter Is Combined:
Molecules and Mixtures
Define molecule, and distinguish between a compound and
a mixture.
Compare solutions, colloids, and suspensions.
Molecules and Compounds
Most atoms do not exist in the free state, but instead are chemi-
cally combined with other atoms. Such a combination of two
or more atoms held together by chemical bonds is called a
If two or more atoms of the
element combine, the resulting
substance is called a
molecule of that element
. When two hydrogen
atoms bond, the product is a molecule of hydrogen gas and is writ-
ten as H
. Similarly, when two oxygen atoms combine, a molecule
of oxygen gas (O
) is formed. Sulfur atoms commonly combine to
form sulfur molecules containing eight sulfur atoms (S
When two or more
kinds of atoms bind, they form
molecules of a
. Two hydrogen atoms combine with
one oxygen atom to form the compound water (H
O). ±our hy-
drogen atoms combine with one carbon atom to form the com-
pound methane (CH
). Notice again that molecules of methane
and water are compounds, but molecules of hydrogen gas are
not, because compounds always contain atoms of at least two
different elements.
Compounds are chemically pure, and all of their molecules
are identical. So, just as an atom is the smallest particle of an
element that still has the properties of the element, a molecule
is the smallest particle of a compound that still has the specific
Carbon has several isotopes. ²e most abundant of these are
C, and
C. Each of the carbon isotopes has six protons
(otherwise it would not be carbon), but
C has six neutrons,
C has seven, and
C has eight. Isotopes can also be written
with the mass number following the symbol: C-14, for example.
Atomic Weight
You might think that atomic weight should be the same as
atomic mass, and this would be so if atomic weight referred to
the weight of a single atom. However,
atomic weight
is an aver-
age of the relative weights (mass numbers) of
the isotopes of
an element, taking into account their relative abundance in na-
ture. As a rule, the atomic weight of an element is approximately
equal to the mass number of its most abundant isotope. ±or
example, the atomic weight of hydrogen is 1.008, which reveals
that its lightest isotope (
H) is present in much greater amounts
in our world than its
H or
H forms.
²e heavier isotopes of many elements are unstable, and their
atoms decompose spontaneously into more stable forms. ²is
process of atomic decay is called
, and isotopes that
exhibit this behavior are called
tōps). ²e disintegration of a radioactive nucleus may be com-
pared to a tiny explosion. It occurs when subatomic
(packets of 2p
negative particles), or
(electromagnetic energy)
are ejected from the atomic nucleus.
Why does this happen? ²e answer is complex, but for our
purposes, the important point to know is that the dense nuclear
particles are composed of even smaller particles called
that associate in one way to form protons and in another way
to form neutrons. Apparently, the “glue” that holds these nu-
clear particles together is weaker in the heavier isotopes. When
radioisotopes disintegrate, the element may transform to a dif-
ferent element.
Because we can detect radioactivity with scanners, and ra-
dioactive isotopes share the same chemistry as their more stable
isotopes, radioisotopes are valuable tools for biological research
and medicine. Most radioisotopes used in the clinical setting are
used for diagnosis, that is, to localize and illuminate damaged or
cancerous tissues. ±or example, iodine-131 is used to determine
the size and activity of the thyroid gland and to detect thyroid
cancer. ²e sophisticated PET scans described in
A Closer Look
in Chapter 1 use radioisotopes to probe the workings of mol-
ecules deep within our bodies. All radioisotopes, regardless of
the purpose for which they are used, damage living tissue, and
all radioisotopes gradually lose their radioactive behavior. ²e
time required for a radioisotope to lose one-half of its activity is
called its
. ²e half-lives of radioisotopes vary dramati-
cally from hours to thousands of years.
Alpha emission is easily blocked outside the body but if ab-
sorbed causes considerable damage. ±or this reason, inhaled al-
pha particles from decaying radon are second only to smoking
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