The concept of radioactive decay was first discovered in 1896 by Henri Becquerel (1852-1908), as he was working the element uranium. After observing the results of several experiments, Becquerel concluded that the uranium was emitting a type of ray. Later, Becquerel demonstrated that the radiation emitted by uranium was similar to X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.
Following this discovery, in 1899 the scientist Ernest Rutherford (1871-1937) discovered alpha ( ), beta ( ), and gamma ( ) radiation. A substance, by emitting any of these particles, changes itself by a process called transmutation. An alpha particle, represented by the notation He2+, consists of two protons and two neutrons, meaning that it must come from the nucleus of an atom. The loss of these charges causes the nucleus to transmute: the mass number of the decaying nucleus is reduced by 4 while the atomic number is reduced by two. For example, in the decay of gold:
Gold, which has an atomic number of 79, becomes an element with an atomic number of 77, and changes from an atomic weight of 185 to an atomic weight of 181. Using a Periodic Table of elements, the element fitting those requirements is Iridium. Beta decay is caused by electrons and antineutrinos being emitted by the nucleus. For this to happen, a neutron within the nucleus must be changed into a proton. Following the Law of Conservation of Charge, another negative charge must be created. Consequently, charge is conserved.
In the beta decay of Helium, a beta particle is emitted and the element becomes Lithium.
In gamma decay a nucleus changes from a higher energy state to a lower energy state through the emission of photons. However, neither mass nor atomic number is changed when the nucleus emits gamma rays. Gamma radiation usually occurs with either beta or alpha decay.
Practice Problems: Fill in the spaces with the appropriate elements or particles.