Radioactivity is the process where an unstable atomic nucleus spontaneously loses energy by emitting radiation to achieve a more stable arrangement. This instability is particularly common in very heavy elements, where the balance of forces inside the nucleus is strained. Alpha decay represents one of the most fundamental ways these unstable nuclei shed excess mass and energy to transform into a different, more balanced atomic structure. This specific mode of decay involves the ejection of a distinct, relatively heavy particle, fundamentally altering the identity of the original atom.
The Identity of the Alpha Particle
The particle emitted during this process is known as an alpha particle, which is structurally identical to the nucleus of a helium-4 atom. It is a tightly bound cluster composed of two protons and two neutrons. Because it contains two protons and lacks orbiting electrons, the alpha particle carries a net positive charge of two elementary units (+2e). This composition gives the alpha particle a comparatively large mass. Due to its size and charge, the alpha particle interacts strongly with matter, making it highly ionizing but giving it a low penetrating ability, and it is typically stopped by just a few centimeters of air or a simple sheet of paper.
Calculating the Nuclear Change
The expulsion of the alpha particle fundamentally changes the composition of the parent nucleus, resulting in a new element known as the daughter nucleus. This transformation is governed by the conservation laws of mass number and electric charge. The mass number (\(A\)) decreases by four, and the atomic number (\(Z\)) is reduced by two, because the alpha particle carries away two protons. This loss of two protons causes a nuclear transmutation, changing the atom from one element to another. For example, the unstable isotope Uranium-238 (\(^{238}_{92}U\)) undergoes alpha decay to become Thorium-234 (\(^{234}_{90}Th\)).
The Mechanism of Emission
Alpha decay is observed in very heavy nuclei, which are unstable because the strong nuclear force cannot fully overcome the electrostatic repulsion between the numerous protons. This interplay of forces creates a potential energy barrier, often called the Coulomb barrier, that the alpha particle must overcome to escape. Classically, the particle’s kinetic energy is not high enough to surmount this barrier, meaning the decay should be impossible; the typical energy of an emitted alpha particle is around 5 MeV, while the height of the Coulomb barrier is often three to five times greater. The escape is explained by the principles of quantum mechanics, specifically a phenomenon called quantum tunneling. Quantum tunneling describes the non-zero probability that a particle can pass through a barrier even when it lacks the necessary energy, which explains the wide range of half-lives observed in alpha-emitting isotopes.