determines which element you’re examining. For example, all atoms of carbon have six
protons, all atoms of oxygen have eight protons, and all atoms of gold have 79 protons. The
number of neutrons, however, is variable. An atom of an element with a different number
of neutrons is anisotopeof that element. For example, the isotope carbon-12 contains 6
neutrons in its nucleus, while the isotope carbon-13 has 7 neutrons.
Some isotopes areradioactive, which means they are unstable and likely to decay. This
means the atom will spontaneously change from an unstable form to a stable form. There
are two forms of nuclear decay that are relevant in how geologists can date rocks (Table
(11.1):
Table 11.1: Types of Radioactive DecayParticle Composition Effect on Nucleus
Alpha 2 protons, 2 neutrons The nucleus contains two
fewer protons and two fewer
neutrons.
Beta 1 electron One neutron decays to form
a proton and an electron,
which is emitted.(Source:Kurt Rosenkrantz,License:CC-BY-SA)
If an element decays by losing an alpha particle, it will lose 2 protons and 2 neutrons. If an
atom decays by losing a beta particle, it loses just one electron.
So what does this have to do with the age of Earth? Radioactive decay eventually results in
the formation of stabledaughter products. Radioactive materials decay at known rates.
As time passes, the proportion of radioactive isotopes will decrease and the proportion of
daughter isotopes will increase. A rock with a relatively high proportion of radioactive
isotopes is probably very young, while a rock with a high proportion of daughter products
is probably very old.
Scientists measure the rate of radioactive decay with a unit calledhalf-life. The half-life of
a radioactive substance is the amount of time, on average, it takes for half of the atoms to
decay. For example, imagine a radioactive substance with a half-life of one year. When a
rock is formed, it contains a certain number of radioactive atoms. After one year (one half-
life), half of the radioactive atoms have decayed to form stable daughter products, and 50%
of the radioactive atoms remain. After another year (two half-lives), half of the remaining
radioactive atoms have decayed, and 25% of the radioactive atoms remain. After the third
year (three half-lives), 12.5% of the radioactive atoms remain. After four years (four half-
lives), 6.25% of the radioactive atoms remain, and after 5 years (five half-lives), only 3.125%
of the radioactive atoms remain.
If you find a rock whose radioactive material has a half life of one year and measure 3.125%