Geology Reference
In-Depth Information
Each atom has a nucleus with both protons and neutrons and is surrounded by elec-
trons. The number of protons and neutrons in the nucleus determines each element's
atomic mass number.
The number of protons is the
atomic number.
Some elements, such as carbon, may have atoms with different numbers of neutrons in
their nucleus. This deviation changes the atomic mass number but not the atomic num-
ber. Atoms that contain the same number of protons but different numbers of neutrons
are called
isotopes.
Isotope names are written with the element and the atomic mass number. For example,
carbon-13 and carbon-14 isotopes exist, as does normal, nonradioactive carbon — car-
bon-12 — which has an atomic mass number of 12. (You may sometimes see these iso-
topes written as
12
C,
13
C, and
14
C). Each type of carbon has the same number of protons
(6) but different numbers of neutrons (6 neutrons in carbon-12, 7 neutrons in carbon-13,
and 8 neutrons in carbon-14).
Some isotopes are unstable, or
radioactive,
which means changes automatic-
ally occur within the nucleus that transform the isotope into a completely differ-
ent element by changing its atomic number. As I explain next, this radioactive de-
cay of certain isotopes is a useful tool for absolute dating.
If the number of neutrons in an atom of an element changes, the atomic mass
number changes, which creates an isotope. If the number of protons in an atom
changes, both the atomic number and the atomic mass number change, and you
have a completely different element.
Three ways to decay
Radioactive elements transform in one of three ways:
Alpha decay:
Alpha decay
describes the change to an atom when two protons and
two neutrons leave the nucleus. The result is an atom of a different element that
has a different atomic mass number. For example, when the isotope of uranium
(which has 92 protons in its nucleus and 146 neutrons) with an atomic mass num-
ber of 238 (uranium-238) experiences alpha decay, the result is an atom of thori-
um-234. Thorium-234 is an isotope with an atomic mass number 234 and an atomic
number of 90. Alpha decay is illustrated in Figure 16-4.
