Question

Rank these elements according to first ionization energy (highest to lowest) : Ne, F, O, N, C, B, Be, Li. HINT: There is a general trend for ionization energy within a period of the periodic table. However, elements with a filled
`s` or half-filled
`p` subshell have a higher ionization energy than expected by the general trend.

Answer 1

Original question: “Which element has the highest 3rd ionization energy?”

This is an interesting question in terms of chemical concepts, but also seems like it might be someone’s homework, so I’m going to try to answer this abstractly but in enough detail to put the pieces together.

Ionization energy is the energy required to remove an electron from an atom in the gas phase, so the third ionization energy is the energy required to remove a 3rd electron from the atom:

X2++energy→X3++e−
X
2
+
+
e
n
e
r
g
y
→
X
3
+
+
e
−


This eliminates either H or He from consideration since they only have 1 and 2 electrons, respectively. There are 3 important factors then: (1) All IE’s increase within a period as the nuclear charge increases and decrease down a group as the radius increases; (2) The successive ionization energies always increase for a given element as more electrons are removed; and (3) The ionization energy to remove the first core electron is drastically larger than that to remove the final valence electron. Thus, an element whose 3rd removed electron is a valence electron will have a lower 3rd IE than an element with 2 valence electrons, whose 3rd removed electron is a core electron.

If the OP (or anyone else) would like to post their choice and reasons in the comments, I will be happy to acknowledge if you are correct or guide your thinking if not.