Why does the ionization energy increase from group 1 to group 8A of the periodic table?

1 Answer
Feb 20, 2016

Ionization energy increases ACROSS a Period (left to right), but decreases DOWN a Group. Why?


It is easy to see why the ionization energy should decrease down a Group; the electron is farther removed from the nuclear core, and hence there is LESS electrostatic attraction to overcome.

It is less intuitive to account for the increase ionization energy going ACROSS the Period from LEFT to RIGHT.

Two factors are at work here: (i) increased nuclear charge, #Z#; and (ii) shielding by other electrons. When electrons add to the same shell, they shield each other very imperfectly from the increased nuclear charge. As #Z# ascends sequentially across the Period, the valence electrons experience increased effective nuclear charge.

The result? Across the Period, atomic radii decrease sequentially and markedly. Helium has an ATOMIC radius much lower than the HYDROGEN atom. The lithium atom is much larger than the neon atom. As a physical scientist you should look up the atomic radii to see that this decrease does indeed take place.

Given this property, we would expect that it would be harder to remove a valence electron, with the result that ionization energies should be larger for right hand elements on the same Period.