A stable electron has all the electrons that it needs, whereas an unstable isotope does not. A stable isotope does not decay and therefore, maintains a constant concentration on Earth. An unstable isotope, also known as a radioactive isotope, decays at a predictable and measurable rate on Earth. An unstable isotope may decay by the ejection of an electron or positron, known as beta decay, or by the ejection of two protons and two neutrons, known as alpha decay.