The nucleus of an atom is a bit of a paradox. Positively charged protons are packed together in a small dense region of the atom. All these positive particles repel each other. This confers a degree of instability to the nucleus. Not surprisingly, some nuclei fall apart. The emissions of these unstable nuclei are called radioactivity. This Powerpoint presentation explores the instability of the nucleus and describes the major types of radiation emitted by unstable nuclei.
Free worksheets on this topic are available on my website at http://www.evanschemistrycorner.com.