Here's the definition of ionization energy that I am learning at this level:
energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of uni positively charged gaseous ions.
I would like to know, why did they specify that the removal of electrons have to be at gaseous state and the ions that form are gaseous as well? Does it have to do with the amount of energy supplied that causes the elements to vapourize?
In fact, it is not restricted at all. You may compare the energies needed to remove one electron out of a solid; these are also meaningful, albeit in a different way, and known for a wide range of substances. But when you are talking about atoms, you want to measure atoms, and the only way to have an undisturbed lone atom is to put it in a gaseous state. And of course, the resulting ion should be in that state, too; after all, you can't make a solid out of positive ions alone, and if you want to add negative ions to counterbalance them, an ambiguity arises as to which ions should you use.
The reason why ionisation energy is measured as the energy required to remove 1 mole of electrons from 1 mole of substance is that it allows the ionisation energy of different substances to be compared. This is because the ionisation energy of a substance is going to be different in different states.Therefore you can't compare the ionisation energy of a substance in a solid state to the ionisation energy of another substance in the liquid state. For this reason, the ionisation energy of all substances are measured in the gaseous state. It is like the standard solubility of a substance is taken at 25 degrees. It doesn't make sense to compare of the solubility of one substance at 25 degrees and the solubility of another substance at 100 degrees.
Now you might ask why did they choose to measure the ionisation energy of substances in the gaseous phase, why not the solid or liquid phase? The reason why ionisation energy is measure in the gaseous phase is because in the gaseous phase, there is very little attraction between other particles which effects the ionisation energy. Therefore, if the ionisation energy was measured for a substance in the solid, since the particles are close together, the intermolecular forces will distort the value of the ionisation energy.
