Ionization energy The sciences of physics and chemistry use different measures of ionization energy. In physics, the unit is the amount of energy required to remove a single electron from a single atom or molecule, expressed as electronvolts. In chemistry, the unit is the amount of energy required for all of the atoms in a mole of substance to lose one electron each: molar ionization energy or enthalpy, expressed as kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Comparison of "E" of elements in the periodic table reveals two periodic trends: The latter trend results from the outer electron shell being
Barn (unit) of a nucleus with an electric field gradient. While the barn is not an SI unit, the SI standards body acknowledges its existence due to its continued use in particle physics. The etymology of the unit barn is whimsical: during Manhattan Project research on the atomic bomb during World War II, American physicists at Purdue University needed a secretive unit to describe the approximate cross sectional area presented by the typical nucleus (10 m) and decided on "barn". This was particularly applicable because they considered this a large target for particle accelerators that needed to have direct strikes on nuclei