Nuclear Physics
Atoms have most of their mass in a central nucleus that is approximately only 1/100,000th the diameter of the atom itself. The nucleus carries a strong positive charge; this charge attracts negatively charged but much lighter electrons. It is the electrons that participate in chemical bonds, so that the charge of the nucleus determines the chemical properties of the atom.
Nuclei are made up of two kinds of particles – protons and neutrons. Together they are called nucleons. Both are nearly the same mass; the atomic weight, usually designated A, is the total number of nucleons in the nucleus. Of the two, neutrons have no electric charge but protons have one unit of positive charge. The atomic number, usually represented as Z, is the number of protons in the nucleus. All nuclei with the same atomic number will have nearly the same chemistry and so are grouped together as the same kind of element. Those nuclei with the same Z but different A are different isotopes of the same element.
Nuclei are bound together with far more energy than electrons are bound to the nucleus or than the chemical bonds the electrons form. When nuclei react, they can give off prodigious amounts of energy in the process. The three most common types of nuclear reactions are
- When a nucleus re-arranges itself or changes its type to a more stable configuration, throwing off particles in the process. This is called radioactive decay
- When a heavy nucleus splits into two smaller nuclei and a few additional particles. This is called nuclear fission.
- When two light nuclei join together into one larger nucleus, throwing off energetic particles in the process. This is called nuclear fusion.
These processes, their effect, and the radiation they give off are described in the nuclear radiation page.
In order to better study nuclei, scientists often collide them together at high speed and see how they react. They accelerate the nuclei using machines called particle accelerators. These machines are described in the particle accelerators page.