|MadSci Network: Physics|
I would start by answering the second part of the question. I don't believe that there is a good safe science fair project using radioactivity for a teenager. This will be clearer after reading the rest of my reply.
As you probably know there are over a 100 elements, from which everything is built. The atom of an element is a composition of a nucleus (protons and neutrons patched together) and electrons orbiting it (The number of electrons in an atom is equal to the number of protons). Different elements will have different numbers of protons, e.g, Hydrogen has 1 proton, Oxygen has 8. There are variations of the elements called isotopes, those are atoms with the same number of proton, but different number of neutrons. Hydrogen for example, has three different isotopes - H (1 proton and no neutron), D (1 p 1 n), T (1 p 2 n).
Some of the Isotopes are not stable (e.g, radium (Ra), 88 p and 138 n), they would like to lose energy by emitting particles and electromagnetic radiation (e.g, light). After that they become a different element (almost alchemy). The particles and energy emitted in the process are called radiation, and those unstable elements (or, isotopes) are called radioactive elements (isotopes).
There are three common types of radiation:
1. Alpha particles - Emission of Helium nuclei (2p 2n).
2. Beta particles - Emission of electrons (or anti-electrons called positrons).
3. Gamma radiation - Emission of very energetic electro-magnetic radiation.
This type of radiation is the most energetic and dangerous to people.
The radiation can cause mutations, cancer, and even death. Most of the first radioactivity scientists died from cancer (e.g, Marie Curie and her husband), so today scientists have many safety regulations when working with radioactive elements. One can't do such experiments at home or at a high-school laboratory.
You may find more information about the elements, their isotopes and radioactive properties at the URL: Radiation
Hope that this answers your question,
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