The exception of ionization energies

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Kristy 1F
Posts: 31
Joined: Fri Sep 28, 2018 12:19 am

The exception of ionization energies

Postby Kristy 1F » Fri Nov 02, 2018 2:05 am

The first ionization energies generally increase across the period, however, the first ionization energy of oxygen is lower than before, so as the S and Se element. Is there any explanation for this decrease? Should we remember that?

David S
Posts: 54
Joined: Fri Sep 28, 2018 12:15 am

Re: The exception of ionization energies

Postby David S » Fri Nov 02, 2018 8:03 pm

In this case, the key here is that Oxygen and the other elements in it's group have just one p orbital with a paired electron. Since paired electrons experience more electron-electron repulsion, they are less tightly held by the atom and thus the ionization energy is lower (it is easier to remove that one paired electron). However, the effect of this is small enough that when we move on to the halides, the increased positivity of the nucleus leads to more tightly held valence electrons, and the ionization energy rises once again, overriding the little bump caused by the electron repulsion within electron pairs.

Also note that half filled and fully filled subshells are notably stable for symmetry reasons. Since N has a half filled p subshell which is pretty stable, we will need more energy to break the symmetric arrangement via removing of e-.

I think it's unlikely for a question on a test to ask us to explain such exceptions, but doesn't hurt to know.

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