 In the same way that atoms can be excited by electrons being shifted to different orbits in higher energy states, the nucleus can have internal excitation energy by rearranging the internal motion of the protons and neutrons in the nucleus. When a nucleus de-excites from a higher lying state to a lower one, the energy lost can be released as a high energy photon. This process is a gamma decay, and the emitted photon is commonly referred to as a gamma ray. We have now looked at the most common decay processes. Let's consider how these decay processes appear on the chart of the nucleites. Remember that we depict all of the nuclei on the chart with their number of protons z on the y-axis and the number of neutrons n on the x-axis. The stable nuclei lie along the diagonal, with this line of stability bending over at the highest mass numbers. Beta decays occur along the lines of constant nucleon number that are at 45 degrees to both axes. Beta minus decays convert neutrons to protons along a chain heeding from the lower half of the chart back towards stability. Beta plus decays convert protons to neutrons and connect along a chain from the upper half of the chart back towards stability. Alpha decays tend to occur for heavy nuclei. This is because heavy nuclei have more protons, and they like to reduce the amount of positive charge in the nucleus to decrease the amount of cooler energy. Chains of alpha and beta decays can be possible depending on the energetics of the nuclei involved. If the nucleus is left in an excited state with internal excitation energy at any stage, it may also emit gamma rays. In general, an unstable nucleus will decay via these modes, emitting alpha, beta and gamma radiation, possibly through a whole chain of decays, before eventually ending up as a stable nucleus. Finally, we usually find that alpha decay is prevalent for heavy nuclei, while beta decay is prevalent for lighter nuclei.