What makes a substance radioactive?
Bob’s answer is wonderfully detailed and is a good explanation, but I suspect might be too technical for your level. You may also find that sources like wikipedia are a little technical as well. I’d suggest you start by considering the three major modes of decay – alpha, beta, and gamma. A nice illustration can be found here: http://library.thinkquest.org/3471/radia… and other relevant material is here: http://en.wikipedia.org/wiki/Radioactive… – just that section. As for how we know which decay will occur, we often look at the neutron-to-proton ratio. There is a “zone of stability”, shown here: http://library.tedankara.k12.tr/chemistr… within which nuclei are stable (non-radioactive). Nuclei outside the zone a
