What influences elemental ionization energies?

Consider a graph of the ionization enthalpies (in #"eV"#) of the elements versus atomic number, #Z#.

The graph clearly shows an elemental periodicity that reflects the electronic structure. Ionization energies INCREASE across the Period from left to right as we face the Table, and DECREASE down a Group, a column of the Periodic Table.

Ionization energies, the energies associated with the following reaction......

#M(g) + Delta rarr M^(+) + e^(-)#

.......are a result of (i) nuclear charge, which means as #Z# increases, the ionization energy should increase, and this is certainly true from the graph (compare the ionization energies of helium compared to hydrogen), and (ii) shielding by other electrons, which would tend to detract from the ionization energy.

Incomplete electronic shells tend to SHIELD the nuclear charge very ineffectively, which means that ionization energy demonstrably increases across the Period, from left to right. When we start a new Period, electronic shielding of the nuclear core becomes more effective with the result that the given graph reflects electronic and Periodic structures......