ARCHAEOLOGICAL CHEMISTRY—EXPLORING HISTORY WITH CHEMISTRY - Chemistry: Notable Research and Discoveries - page 170

radiocarbondating
All atoms of the same element have the same number of
protons in the nucleus—the atomic number—but some at-
oms have a different number of neutrons, giving rise to dif-
ferent isotopes. Although isotopes of a given element have
chemical properties that are very nearly identical, the num-
ber of neutrons in the nucleus can cause instability, leading
to the emission of particles and a decay, known as radioac-
tive decay, of the nucleus. In this process, the composition
of the nucleus changes, resulting in a different isotope or
an isotope of a different element. For example, carbon 14
(which has 14 particles in the nucleus—six protons and eight
neutrons) decays into nitrogen 14 (seven protons and seven
neutrons) when a neutron in carbon 14 converts into a pro-
ton and an electron (which is emitted).
Carbon is a critical element in biology since the element
plays a vital role in the formation of long molecular chains
such as DNA and protein, which are essential for all living or-
ganisms on the planet. Almost all of the carbon compounds
on Earth and in the atmosphere consists of stable carbon
isotopes, mostly carbon 12 and a little carbon 13, which
do not decay. But high-energy protons called cosmic rays
constantly bombard the planet; although astronomers are
not sure of the source of these speedy particles, they exert
considerable effects, including collisions with nitrogen 14 at-
oms in the upper atmosphere that produce unstable carbon
14. (Most of the reactions occur at high altitudes since only
a few cosmic rays reach the ground.) The decay of carbon
14 occurs spontaneously, at random, and is unaffected by
environmental conditions. But even though the decay is ran-
dom, the probability—the likelihood that decay will occur—is
constant, and a group of atoms decays at a specifi c and
measurable rate. Half of a given sample of carbon 14 at-
oms will decay in 5,730 years—this period is called the half-
life —and half of this remainder will decay in another 5,730