What happens to radioactice substances when they decay?

Answer 1

The atom that decays leaves emits some radiation, and leaves behind another type of atom (or atoms), with less mass (the mass of the original atom, minus the mass that left the atom).

There are several common types of radioactive decay. In alpha decay, an alpha particle is emitted. This is two protons and two neutrons bound together. In beta decay an electron or positron is emitted. Excited nuclei may shed their excess energy by emitting a neutral gamma ray. Some nuclei even emit neutrons (the fission products in a nuclear reactor are especially prone to this). And sometimes the nucleus may grab an orbiting electron in "K-capture".

Some of these events alter the nucleus's charge thus changing it into a different chemical element. In addition, the proton-to-neutron balance may be changed enough to make the new nucleus unstable so it decays again. In most cases the decay leaves the nucleus in an excited state which may destabilise it resulting in further decay too. It is not uncommon for a material to decay through several generations at hugely different rates.

But eventually the nucleus settles down as a different chemical element, the orbiting electrons re-shuffle and, well, that's it. The substance therefore changes into another one. Since the processes are very specific for every isotope, the composition of an old rock, for example, can give us complete information about what was in it a thousand, a million, a billion years ago.