Radioactive decay occurs when the nucleus of an atom changes by emitting or absorbing matter or by emitting energy. One explanation for why decay occurs says that there is an optimum ratio of neutrons to protons for a nucleus to be stable. Other reasons for decay is the nucleus is too big or has too much internal energy. These nuclei emit matter or energy to reduce their excess. During alpha decay the nucleus of the atom expels 2 protons and 2 neutrons.
Alpha decay reduces the size of the nucleus.
An atom of a new element is formed which has a mass number reduced by 4 and an atomic number reduced by 2. In beta decay a neutron breaks apart to form a proton and an electron. The proton remains in the nucleus and the electron is expelled as a beta particle. Beta decay decreases the neutron to proton ratio. An atom of a new element is formed which has an atomic number increased by 1.