The Atomic Nucleus and Radioactivity The nucleus is composed of particles called nucleons, which are protons and neutrons. The number of nucleons is called the mass number.The nucleus is composed of particles called nucleons, which are protons and neutrons. The number of nucleons is called the mass number. The number of protons in an atom determines what the element is. The number of protons is called the atomic number.The number of protons in an atom determines what the element is. The number of protons is called the atomic number.

Nucleons are bound together by an attractive force called the strong force and sometimes called the nuclear force. The nuclear force is extremely strong but is only acts over a very strong distance. The more protons in a n atom the more neutrons are needed to hold the atom together and it becomes less stable. All elements heavier than Bismuth atomic number 83 will spontaneously decay.

Decay There are three basic spontaneous decays. Alpha decay – an emission of the alpha particle α. Beta decay – an emission of the beta particle β. Gamma emission – an emission of a gamma particle