Half Life Background
Most atoms have a stable nucleus, which means that the nucleus remains unchanged
from the time the atom formed inside a star. Other atoms are radioactive, which means
they are capable of spontaneous decay. During decay, particles and/or energy are
emitted from the nucleus, often resulting in a change in the number of protons inside the
nucleus. If this occurs, the result is a different element, called a daughter product, which
may or may not be stable. (In many cases, the daughter products also are radioactive
and several stages of decay occur before a stable product is formed.)
The three most common types of decay are alpha, beta, and gamma decay. In alpha
decay, an alpha particle (2 protons and 2 neutrons) is emitted from the nucleus. In beta
decay, a beta particle (1 electron) is released as a neutron in the nucleus changes into a
proton. In gamma decay, energy is emitted from the atom in the form of gamma rays. A
Geiger counter detects the radiation that is emitted from radioactive substances—each
particle or gamma ray that strikes the instrument causes an audible “click.” This sound is
reproduced in the Half-life Gizmo.
Although the exact moment an individual
atom will decay cannot be predicted, the
probability it will decay can be determined,
and the decay of a large group of atoms will
fall close to a decay curve such as the one
shown at right. The half-life is the time
required for half of the radioactive atoms to
decay. The decay curve at right shows a halflife of 20 seconds. Notice that every 20
seconds the number of radioactive atoms has
been reduced by half: There are 128 atoms at
0 seconds, 64 atoms at 20 seconds, 32 atoms
at 40 seconds, 16 atoms at 60 seconds, and
so forth. The graph can also be used to
estimate the numbers of radioactive atoms at
other times as well.
The predictable rate of radioactive decay forms the basis of radiometric dating. If the
half-life of a radioactive element is known and the proportions of radioactive and
daughter atoms can be measured, the age of the sample can be determined. Several
radioactive isotopes are useful for radiometric dating: Carbon-14 (half-life = 5,730 years)
is used to date wood, ash, bone, and other organic artifacts up to 50,000 years old.
Potassium-40 (half-life = 1.25 billion years) is used to date igneous rocks and volcanic
ash. Uranium-238 and uranium-235 (half-lives of 4.47 billion and 704 million years) are
also used to date ancient igneous rocks.