Chemistry of Life 17What are elements and atoms?- An element is a pure substance. Each kind consists of atoms
having the same number of protons. - Atoms are tiny particles and are the building blocks of all
substances. - Atoms consist of electrons moving around a nucleus of protons
and (except for hydrogen) neutrons.
taKe-home messageisotopes are varying forms of atoms
Atoms of a given element have exactly the same number
of protons, but they may have different numbers of neu-
trons. When an atom of an element has more or fewer
neutrons than the most common number, it is called an
isotope (eye-so-tope). For instance, while a “standard”
carbon atom has six protons and six neutrons, the isotope
called carbon 14 has six protons and eight neutrons. These
two forms of carbon atoms also
can be written as^12 C and^14 C. The
prefix iso- means “same,” and all
isotopes of an element interact
with other atoms in the same
way. Most elements have at least
two isotopes. Cells can use any
isotope of an element for their
metabolic activities, because the
isotopes behave the same as the
standard form of the atom in
chemical reactions.
Have you heard of radio active
isotopes? They are isotopes of
emissions from radioisotopes can reveal the activity of body
cells. as a result, they are useful tools in medicine because
they permit physicians to diagnose disease, or track its
course, without doing surgery.
the technology called pet (short for positron emission
tomography) is a prime example. figure 2.3a shows a pet
scan from a cancer patient. the patient was injected with
a tracer—a molecule in which radioisotopes have been
substituted for some atoms. the cells in a cancerous tumor
are more active than normal body cells, so they take up
the tracer faster. a scanner then detects radioactivity that
becomes concentrated in the tumors.
figure 2.3B shows a
leukemia patient about to
have a scan of her heart prior
to a bone marrow transplant.
a radioactive tracer injected
into her bloodstream will
allow the scanner to make
video images of her heart
pumping blood—helping her
doctor decide if her heart is
healthy enough for surgery.
radioisotopes also are
used to help treat some
cancers. for example,
pet scanning—Using radioisotopes in medicine
Figure 2.3 Animated! Radioisotopes have important medical uses. A PET image showing tumors
(blue) in and near the bowel of a cancer patient. B Patient about to have a MUGA (multi-gated) scan
that uses a radioactive tracer to produce video images of heart function.tumorsA
© Courtesy GE Healthcareelements such as uranium, which emit energy. This unex-
pected chemical behavior is what we call radioactivity.
The nucleus of a radio isotope is unstable, but it
stabilizes itself by emitting energy and certain types of
particles. This process, called radioactive decay, occurs
spontaneously, and it transforms a radioisotope into an
atom of a different element. The decay process happens at
a known rate. For instance, over a predictable time span,
potassium 40 becomes argon 40. Scientists can use radio-
active decay rates to determine the age of very old sub-
stances, such as ancient rocks and fossils. Radioisotopes
also have important uses in medicine, as you can see in
Section 2.2.2.2
implanted “seeds” of radioactive iodine or palladium
may be used to kill cancerous prostate cells. for safety’s
sake, such treatments use only radioisotopes that decay
quickly into a different, more stable element.SCIENCE COMES TO LIFE
Bisotope An atom of an ele-
ment that has a different
number of neutrons than
the most common, standard
number.
radioisotope An isotope
with an unstable nucleus
that becomes stable by
emitting energy and par-
ticles, a process known as
radioactive decay.
tracer Molecule with a
detectable substance such
as a radioisotope attached
to it.
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