Question

Why is that during alpha emission, the atomic number decreases by two, and the mass number decreases by four? In your answer you should consider what alpha particle is?

Answer 1

An alpha particle is a particle that consists of two protons and two neutrons. When an alpha particle is emitted from an element, the element loses two protons and two neutrons. The atomic number of an element is the number of protons it contains in its atom and atomic mass number is the sum of protons and neutrons present in the atom of element. Since the element, after alpha particle emission loses two protons hence its atomic number decreases by 2 and as it loses two protons, it also loses two neutrons (total 4 sub-atomic particles) and hence its mass number decreases by 4.