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
SI derived unit Until 1995, the SI classified the radian and the steradian as "supplementary units", but this designation was abandoned and the units were grouped as derived units. SI derived unit SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation. The SI has special names for 22 of these derived units (for example, hertz, the SI unit of measurement of frequency), but the