Question

define the term ionization energy

Answer 1

Amount of energy needed to remove an electron from its outermost shell

Answer 2

What photon336 said though I'll emphasise 'in gaseous form'

Answer 3

I'd agree with what both @Photon336 and @mww say, but just to mention also that the electron being removed will be the most loosely bound electron in the outermost shell of the gaseous atom.

Other electrons may be present in the outermost shell, but it is the one which requires the least energy to remove (i.e. the one most screened from the attractive positive charge of the nucleus) which will be ejected.

Answer 4

@Ciarán95 maybe you can tell me how they come with such a measure? I assume the theoertical 1st IE recorded is an average value of IEs?

Answer 5

@mww Ionization energies (IEs) are very easy to calculate theoretically within the frozen molecular orbital (MO) approximation (we assume the orbitals of the ion are identical to those of the neutral molecule).

We define the IE as the minimum energy required to eject an electron out of a neutral atom or molecule in its ground state. You can then show for a N-electron system and the corresponding system with 1 electron removed from an orbital within the frozen MO approximation the IE is equal to the orbital energy, which in turn is equal to the difference in energy between the two states. This is simply Koopmans' theorem.