When alpha particles are emitted from an element's nucleus, the element's atomic number and mass number undergo specific changes.
An alpha particle is composed of two protons and two neutrons, which is equivalent to a helium nucleus. When an alpha particle is emitted, the atomic number of the element decreases by 2, as it loses two protons. Similarly, the mass number decreases by 4, as the emission of an alpha particle involves the loss of two protons and two neutrons.
This emission of an alpha particle is a form of radioactive decay called alpha decay. As a result of alpha decay, the original element is transformed into a new element with an atomic number two units lower and a mass number four units lower than the original. The new element is located two positions to the left on the periodic table compared to the original element.