Radioactive
The term radioactive is applied to unstable isotopes which spontaneous shed energy, and subatomic particles.
The first three kinds of radioactivity to be discovered were alpha decay, beta decay and gamma decay.
In beta decay one of the neutrons in an atom's nucleus emits an electron. This change adds one positively charged proton, changing the atom from one element on the periodic table, to an adjacent element. For example while the most common isotope of Carbon, Carbon-12, with six protons and six neutrons, is stable, Carbon-14, with two additional neutrons, is not. Carbon-14's half-life is 5730 years. This means any Carbon-14 atom has a fifty percent chance of emitting an electron, and turning into an atom of Nitrogen, with seven protons and seven neutrons.
Archeologists exploit the useful half-life of Carbon-14 to determine the age of once living samples. Living organisms incorporate Carbon into their cells at the ratio found in regular air and dissolved in regular water. That ratio has been stable, throughout history, as new Carbon-14 comes into existence due to the nuclear decay of heavier elements. However, no new carbon is incorporated into the once living structures of bones, shells, and charcoal. So, by measuring the ratio of Carbon-12 to Carbon-14 in their samples they can calculate how long ago that material was living.
Alpha decay occurs when four nucleons tunnel out of the nucleus at once -- two protons and two neutrons. All the Helium found on Planet Earth is the result of alpha decay.