Right now, in our atmosphere, there are countless numbers of nitrogen atoms
floating high above the Earth's surface. These atoms are, in a sense,
Cosmic radiation, in the form of neutrons, zips through the atmosphere at a
high rate of speed. Occasionally, and purely by chance, some of these neutrons
collide with the nuclei of some of the nitrogen atoms.
The nucleus of each nitrogen atom contains seven protons and seven neutrons.
That is, it does until a collision happens. The incoming neutron hitting the
nucleus causes a proton to shoot out of the nucleus, just as a cue ball on a
pool table, hitting one of two balls that are touching, might cause the ball
that it hits to stay in place and the other ball to shoot off.
The nitrogen atom now has six protons and eight neutrons. This means two
One, with this arrangement of protons and neutrons, it's unstable.
In other words, it's radioactive. And two, it's no longer nitrogen. The
reason is that the number of protons an atom contains determines what that atom
is. Since it now contains six protons, it's carbon. Carbon usually has six
neutrons as well—in this form it's called carbon-12 (6+6=12). The unstable,
eight-neutron version, however, is carbon-14 (6+8=14).
All radioactive atoms will eventually decay, or change, in some way. When a
carbon-14 atom decays, one of its eight neutrons turns into a proton, emitting
an electron (with a charge of -1) in the process. The atom is now stable. And
with seven protons and seven neutrons, it is again nitrogen-14.