Here I'll teach you the basics of radioactivity, which comes in handy for a good number of the elements I might collect. In all simplicity, a radioactive atom(which is just a small particle of an element) is an atom who's core (Protons+Neutrons) is unstable, and, given a certain time, will decay into an atom of a different element.

synthetic (man-made)Isotopes, or atoms with different numbers of Neutrons than normal, are quite often radioactive and depending on what kind of atom they are they will decay either one element forward in the periodic table, or one element back. The decay of these elements is in the form of either Alpha particles, Beta particles, or Gamma rays. Further description below:

Alpha particles are basically Helium cores, being two protons and two neutrons, but with no electrons. Alpha particles are generally the least dangerous, as they can barely penetrate a few inches of air, and are easily blocked by even a thin piece of paper.

Beta particles are free electrons, but in a different form than normal electrical voltage. They can be slightly more hazardous than Alpha particles, and penetrate air easily, only being stopped by a sheet of metal.

Gamma rays are high energy Photons, the same stuff normal light is made out of, but they are invisible and of much higher energy. These tend to bet he most harmful type of radiation, because they can penetrate even semi-thick metal, but are easily blocked by a thick layer of a heavier metal, such as Lead. Because of this Lead jars or boxes are often used to contain radioactive items, and Lead bricks are used to construct radiation-proof walls where needed.

A very basic radioactive molecule is Tritium, which is a name for Hydrogen-3 (Or Hydrogen with 1 proton, 2 neutrons) has an atomic number (number of Protons) of 1, and will decay into Helium-3(2 protons, 1 neutron), which is a unusual, but not radioactive, isotope of Helium. Helium's atomic number is 2. This decay isn't linear, meaning it doesn't decay at a set rate of some number of atoms per year, but binary, meaning that by the time half of the material is decayed, it will decay half as fast. Example 1 gram (or any measurement of weight, pound, ounce, doesn't matter) of Hydrogen-3 will take 12.43 years to decay into .5 grams of Hydrogen-3, and .5 grams of Helium-3(This time for half to decay is called it's "Half life". I will reference this later). However if you wait another 12.43 years, you won't have no Tritium at all, but you'll have half the tritium you had 12.43 years ago, or .25 grams.

Half lives of elements can range from microseconds for synthetic elements (Like Lawrencium) to hundreds of millions of years for very lightly radioactive elements (Like Bismuth, which isn't considered radioactive it's radiation level is so low)Technetium(tek-nee-see-um), atomic number 43, which is a Transition Metal, has no stable Isotopes, meaning every form is radioactive. Some Isotopes are short on Neutrons, example Technetium-96,(42 protons, 54 neutrons) so they decay back from atomic number 43 (Technetium) to atomic number 42 (Molybdenum). But some have too many Neutrons, example Technetium-98, so decay up a number to atomic number 44 (Ruthenium). Notice all Isotopes of technetium decay, but stable elements such as Gold (Atomic number 79), who's common Isotope is Gold-197(79 protons, 117 neutrons), also have isotopes with more or less neutrons that decay up or down one element (Gold-196 decays into Platinum, Gold-198 decays into Mercury)

Some Isotopes of elements are extremely radioactive, so even their decay product (what they decay into) is radioactive, and in turn, decays even further. Some elements have long chains of different elements they decay into, example Radium-222 decays into Radon-222, which decays into Polonium-218, which in turn decays into a stable isotope of Lead.You can also create synthetic elements by artificially adding neutrons to an atom. In theory, you could take something like Mercury, and add enough neutrons for it to slowly decay into Gold, but the price of this process is far greater than the profit from the Gold produced. Please notice that I am not a specialist on radioactivity, I only learn enough to understand how my elements work, and to keep my curiosity at bay. There is a lot I haven't explained here, I suggest if you want to find out more, simply search "radioactivity" on a search engine and you should find lots of pages explaining in much greater detail.