When a nucleus of the unstable isotope polonium-211 decays it does so by alpha decay. After some random amount of time it emits an alpha-particle and becomes lead-207. The alpha-particle is a helium-4 nucleus, with 2 protons and 2 neutrons. Its atomic symbol is 42He. The polonium-211 nucleus is different after it decays because it now has 2 fewer protons and 2 fewer neutrons. The result has to be lead-207 because lead has 2 fewer protons than polonium and with a mass number of 207 it must have 2 fewer neutrons, too. If the nucleus of a 40K atom were to undergo alpha decay it would become 36Cl.
The amount of time it takes for a single atom to decay is random. Unstable nuclei do not all decay at once but do so at random times. On average, though, a group of atoms will have half of their number decay before a certain amount of time has passed. This amount of time is called the half-life. After one half-life, half of the nuclei will be left. After a stretch of time equal to two half-lives, one quarter of the nuclei will remain. At that time, three quarters will have decayed.
Radioactive materials do not stay radioactive forever. Once a nucleus decays to become a stable nucleus it no longer changes.
If you have a larger group of unstable nuclei you will see more decay events per second than in a smaller group. Each atom decays at a random time but in a bigger group there are more atoms so for a short period of time there is a higher probability that a nucleus will decay. A larger sample of a radioactive material shows more radioactivity than a smaller sample.
When a nucleus of the unstable isotope hydrogen-3 decays it does so by beta decay. After some random amount of time it emits a beta-particle and becomes helium-3. The beta-particle is an electron and its atomic symbol is 0–1e–. The hydrogen-3 nucleus is different after it decays because one of the neutrons in that nucleus has been transformed into a proton. Because the number of particles in the nucleus is the same the mass number remains 3 but the atomic number increases by 1 and the nucleus become helium, which has two protons. If a nucleus of 14C were to decay by beta-decay it would become 14N.
The half-life of hydrogen-3 (also called tritium) is about 12 years.