For 10 points, name this quantity that is the energy required to remove an electron from a gas of atoms or ions.
The Franck-Condon principle suggests that wavefunction overlap decreases this quantity for electrons with the same geometric configuration as the molecule from which the electron is being excited. This quantity can be calculated by integrating over an electron cloud determined using a function created by Slater determinants. A related quantity is governed by Moseley's law and applies to surface-adsorbed atoms rather than free ones. Koopman's theorem states that its value is equal to the negative orbital energy of the HOMO, and like electron binding energy, this can be given in quantities of electron volts, or more commonly, kJ/mol of gas. This quantity is lowest for the alkali metals, and it decreases down a period due to electron shielding. For 10 points, name this quantity that is the energy required to remove an electron from a gas of atoms or ions.
The Franck-Condon principle suggests that wavefunction overlap decreases this quantity for electrons with the same geometric configuration as the molecule from which the electron is being excited.
ionization energy (accept first ionization energy)