Define ionization enthalpy, its units, and the principle of its measurement

Ionization enthalpy is the energy needed to remove one electron from a gaseous atom or ion; its unit is kilojoules per mole (kJ·mol⁻¹); it is measured by determining the energy change between the gaseous atom and the resulting ion plus electron, often via photoelectron spectroscopy.

Ionization enthalpy (also called ionization energy in thermodynamic units) is the minimum energy required to detach the most loosely bound electron from one mole of isolated gaseous atoms to form one mole of gaseous cations. For successive ionizations, the second, third, and higher ionization enthalpies refer to removing additional electrons from the resulting ions.

Units

• SI unit: kJ·mol⁻¹
• Legacy or atomic unit in some contexts: eV per atom (convert using 1 eV·atom⁻¹ ≈ 96.485 kJ·mol⁻¹)

Principle of Measurement

• Photoelectron Spectroscopy (PES): Monochromatic photons eject electrons from gaseous atoms; the kinetic energy of emitted electrons is measured to obtain binding energy. The threshold gives the first ionization enthalpy.
• Thermochemical cycles: Use Hess’s law with measured heats of formation, sublimation, and electron affinity to back-calculate ionization enthalpy.
• Spectroscopic limits: For simple atoms, Rydberg-series limits help estimate ionization energies.