Question

Which best describes ionization energy?
energy needed to add an electron to a neutral atom in the gas phase
energy needed to add an electron to a neutral atom in the liquid phase
energy needed to remove an electron from an atom or ion in the gas phase
energy needed to remove an electron from an atom or ion in the liquid phase

Answer 1

energy needed to remove an electron from an atom or ion in the gas phase

(Usually defined for the first electron removed from a neutral atom in the gas phase; successive ionization energies refer to removing additional electrons.)