How do elemental ionization energies change across the Periodic Table? Are there any trends observed?
There could be...but in fact.......
In fact the ionization energy of an element derives from experiment; i.e. it is the energy required to form one mole of gaseous cations, and one mole of electrons from one mole of gaseous ATOMS, i.e. it is the energy associated with the following oxidation reaction.....
It is a well known Periodic property that ionization energies INCREASE across a Period from left to right as we face the Periodic Table, but DECREASE down a Group.