An ion is a molecule of atom that carries a net positive or negative electric charge, that is it is not electrically neutral. The word ion is Greek, meaning the ones that move. A neutral atom has equal numbers of protons and electrons, as does a neutral molecule. If an atom or molecule has a deficit or excess of electrons then it becomes electrically charged. Positive ions, cations, are formed when neutral molecules or atoms lose valence electrons and negative ions, anions, are those that have gained electrons. The process by which atoms or molecules become electrically charged is called ionization and can be induced by radiation, such as x rays or light with sufficient energy to rip electrons off of the valence shell, or by fast-moving particles, like those emitted by radioactive materials.
Typical levels in the atmosphere in the year 2000 are 3,000 cations and 4,000 anions per cubic centimeter. Indoors there are very few ions and there are more cations than anions. This is because negative ions exist for a short time relatively speaking and are destroyed by air pollution. Hot electrical discharges such as sparking, electric motors, and furnaces generate an excess of positive ions. Ions produced in air or water can carry electric currents. Although air is a pretty good insulator, when ions are present an electric charges can be carried along. This phenomenon was used in 1900 to detect radioactive emissions and measure their intensity.
There are several atoms that commonly lose an electron, like sodium, and become cations, Na+ , in an ionic solid such as sodium chloride, NaCl. Sodium chloride is composed of cations, Na+ , and anions, Cl- . Although chloride ions have captured the outer electrons of the sodium atoms, the crystal as a whole has equal numbers of protons and electrons and so is neutral. When the crystal is dissolved in water the ions are separated from each other and surrounded by the polar water molecules that are oriented about the ions in a solvation sphere. The water molecules have a positive end and orient it towards the Cl- ions and a negative end that is directed towards the Na+ ions. Ions can also consist of multiple atoms, such as sulfate ions (SO- ) and ammonium ions (NH- ), and exist in a similar way to that described for sodium chloride.
Radioactivity was first discovered in 1895 when heavy elements such as uranium were observed to emit rays, which could ionize air. Ernest Rutherford noted that these rays seemed to be composed of electrically charged components or opposite signs, positive alpha rays and negative beta rays. He was awarded the Nobel Prize for this work in 1908. Later these rays were identified as electrons, beta rays, and completely ionized helium nuclei He+2 , alpha particles. The Sun gets its energy by a process in which protons are combined to eventually form helium nuclei deep in the Sun's core. Since the helium nucleus is an unusually stable combination of particles, energy is released during this process. Today, ions are regularly produced and used to study the structure of matter and to produce a variety of new particles.
This is the complete article, containing 524 words
(approx. 2 pages at 300 words per page).
