When an alpha particle is emitted from a nucleus, two protons and two neutrons are lost from the nucleus. This results in the atomic number (number of protons) decreasing by two and the mass number (total number of protons and neutrons) decreasing by four. The element therefore changes into an element with two fewer protons. For example, if an alpha particle is emitted from a nucleus of uranium-238, the resulting nucleus would be thorium-234.