1) Explain why almost all naturally-occurring isotopes have a greater number of neutrons than protons.
2) When two small nuclei are fused together, as happens inside the Sun or a fusion reactor, energy is released. Explain the origin of this energy.
3) The half-life of a radioactive substance has been defined as "the time it takes for half of a given number of radioactive nuclei to decay". Given this definition, why won't a radioactive substance decay completely away within two half-lives? Explain.