Radioactivity occurs when an unstable nucleus decays into a more stable form. There are three main types of radiation emitted: alpha particles, beta particles, and gamma rays. Alpha particles have a +2 charge and are made of two protons and two neutrons. Beta particles have a -1 charge and are electrons. Gamma rays have no charge or mass. Radioactive decay can occur through alpha or beta decay, changing the element. Isotopes are identified through mass number and atomic number. Half-life is the time for half of a radioactive sample to decay and can be used to calculate how much of a sample remains over time.