# How is the atomic mass changed by alpha decay?

May 18, 2015

Alpha decay involves the emission of an alpha particle by a radioactive isotope of an element. An alpha particle is a helium-4 nucleus, ""_2^4He, which contains two protons and two neutrons. When the alpha particle is emitted, the atomic number of the atom decreases by two, since atomic number is the number of protons, which causes the transmutation of the original element into a different element. The original atom's mass number decreases by four, since the mass number is the sum of protons and neutrons in the nucleus.