Answer to Question #160884 in General Chemistry for John

Question

Plot a graph of first ionisation energy against atomic number for the elements Li → Ar. Explain the trends observed in the above graph for the period 3 elements Na →Ar.

Answer 1

(a) Graph

So, for Period 2, the Group1 Alkali Metal (lithium, lowest Z) has the lowest 1st ionisation energy and the Group 0/18 Noble Gas (neon, highest Z) has the highest 1st ionisation energy value and most values follow the general trend of increasing from left to right across period 2

So, for Period 3, the Group1 Alkali Metal (sodium, lowest Z) has the lowest 1st ionisation energy and the Group 0/18 Noble Gas (argon, highest Z) has the highest 1st ionisation energy value and most values follow the general trend of increasing from left to right across period 3.

(b) The first ionization energy varies in a predictable way across the periodic table. It increases from left to right across a period.

From left to right across a period, more protons are being added to the nucleus, but the number of electrons in the inner, lower-energy shells remains the same. The valence electrons feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons. The valence electrons are therefore held more tightly, the atom decreases in size (atomic radius), and it becomes increasingly difficult to remove them, corresponding to a higher value for the first ionization energy.