RADIOACTIVITY AND HALF-LIVES
Atomic nuclei, as you know, are made of protons and neutrons. In some atoms,
the combination of protons and neutrons makes the nucleus unstable. These
atoms will decay—on their own—spontaneously. As they decay, they emit high-
energy radioactive particles. Radioactive particles include alpha (α) particles,
beta (β) particles, and gamma (γ) rays. If you think about it, the process of
radioactive decay makes sense: A radioactive nucleus is trying to become more
stable; greater stability means lower energy, so the radioactive nucleus wants to
lose energy.
As a radioactive atom decays—emitting α or β particles and γ rays—its identity
changes, and it becomes either (1) another isotope of the element it originally
was or (2) another element entirely. Some nuclei are stable, and some are
unstable; the unstable ones have a tendency to break apart, and they are said to
be radioactive.
Why are some nuclei unstable? For this test, you have to know only that the
instability has something to do with the combination of neutrons and protons.
Some combinations of neutrons and protons just don’t get along well, and they
try to solve this problem by undergoing nuclear decay. When you think of
radioactivity, think this: When an unstable nucleus undergoes nuclear decay, it’s
radioactive, and it gives off radioactivity. A Geiger counter is used to detect and
measure radioactive particles.
You should know about four kinds of radioactive decay.
Radioactive Decay Type 1: Alpha Decay
An alpha particle is made up of 2 protons and 2 neutrons. When a nucleus gives
off an alpha particle, its atomic number is reduced by 2 and its mass number is
reduced by 4. Since the atomic number changes, it actually turns into a different
element. After all, the atomic number is the basis of an atom’s identity.
Another thing about alpha particles: Since an alpha particle consists of 2 protons
and 2 neutrons, it’s actually the same thing as a helium-4 nucleus, and it’s often