Define ionization enthalpy. What are its units? What is the principle of its measurement?

Ionization enthalpy, also known as ionization energy, is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions to produce one mole of gaseous ions with a positive charge. In simpler terms, it's the energy needed to remove an electron from an atom or ion in its gaseous state.

The units of ionization enthalpy are typically kilojoules per mole (kJ/mol) or electron volts (eV).

The principle of measuring ionization enthalpy involves performing experiments where electrons are removed from atoms or ions in the gas phase. One common method is photoelectron spectroscopy, where atoms or molecules are bombarded with photons of known energy, causing the ejection of electrons. By measuring the energy of the photons required to remove electrons, the ionization enthalpy can be determined. Another method involves using electrical discharges or collisions with high-energy particles to remove electrons, with subsequent measurement of the energy changes involved.