To understand how energy can be released from the nucleus of an atom, it is important to become familiar with the concept of binding energy. Throughout the study of astronomy we encounter systems held together by forces — atoms, molecules, people, planets, solar systems, and galaxies. The binding energy of a composite system is the energy required to take it apart. For example, objects are bound to the Earth by gravity. To liberate anything from the gravity of the Earth we must give it a velocity of 11 km/s. This speed corresponds to a kinetic energy sufficient to counter the gravitational binding energy. In the context of nuclear fission, the binding energy of atoms is the one of the most important quantities. The electron in a hydrogen atom is bound to the proton by the electrical force. The binding energy of the hydrogen atom is a tiny 2.2 × 10-18 Joules. So any photon with a frequency higher than 3.3 × 1015 Hz (or equivalently a wavelength smaller than about 100 nm) carries enough energy to liberate an electron if it hits an atom.
Atomic nuclei are bound together with the strong nuclear force, and individual nucleons The word for both protons and neutrons) are held together by the weak force. Energy can be released from the atomic nucleus by the spontaneous decay of radioactive elements. This process is called nuclear fission. These heavy elements occur naturally in the Earth's crust, but in very small concentrations. In nuclear fission, a nucleus splits into two or more pieces and the fragments have a combined mass that is less than the mass of the original nucleus. The remaining mass is released as energy according to the equation E = mc2. When a single uranium-235 nucleus decays, 3.2 × 10-11 Joules of mass-energy are released. This may not sound like much, but that is the nuclear binding energy released by a single atom. A fistful of uranium-235 could power a small city if all the atoms decayed at once! We can compare this to chemical energy by considering the burning of coal. The molecular binding energy released when a carbon atom combines with an oxygen atom to form a CO2 molecule is 6.4 × 10-19 Joules. This is 50 million times less energy than we get from a radioactive decay of a single uranium atom.
br/>Energy release from radioactive elements within the Earth contributes to the heating of the planet. This is why early estimates of the age of the Earth were wrong. They assumed that the Earth was a cooling ball with no internal energy source. When radioactivity is taken into account, the estimated age of the Earth increases greatly. In fact, radioactive decay can be used as a clock. The age of the Earth has been estimatedquite accurately using this technique at 4.54 billion years.
Humans have learned to harness nuclear fission. Heavy nuclei can be split by a collision with a neutron. Since some radioactive elements also release neutrons as a decay product (including some isotopes of plutonium and uranium), the possibility exists that a nucleus will decay, releasing a neutron, which then triggers another decay, which releases another neutron, and so on. This is called a chain reaction. All that is required is a high enough concentration of the radioactive material so that each decay triggers at least one more decay. We have learned to mine and process uranium to the required level of purity to allow sustained reactions. (There is even evidence for a natural fission reactor in Gabon in Africa; a high underground concentration of uranium may have created a chain reaction there 1.7 billion years ago!) If a chain reaction happens in a controlled way, the energy can be harnessed, as in a nuclear reactor. If it happens in a catastrophic and uncontrolled way, the result is a bomb of terrible destructive power. In either case, energy is released from the mass of nuclear particles as they fission into smaller pieces.