## periodic trend exceptions [ENDORSED]

Joshua Hughes 1L
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### periodic trend exceptions

How do we know if something is an exception to a period trend? Like can someone explain why Be and O would be an exception to the ionization trend ( I may have messed that up, I think it was O and Be)? I think it was briefly explained in class but I didn't understand it. I think for the most part I understand why the noble gasses can be an exception to trends.

Scott Chin_1E
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Joined: Sat Jul 22, 2017 3:00 am

### Re: periodic trend exceptions

I think the exceptions have to do with Oxygen's electron configuration. Given its configuration, $1s^{2}2s^{2}2p^{4}$
and Nitrogen's $1s^{2}2s^{2}2p^{3}$, Nitrogen's completed half shell of the p subshell makes it slightly more stable than oxygen's (which is has one more electron). Nitrogen's relative stability with a completed half shell makes it harder to remove an electron from it's structure thus increasing its ionization energy. Since Oxygen is more unstable with its addition electron, it will more readily give away it's electron to become more stable, thus its lower (but not by much) ionization energy.

I would think that this concept could be applied to Beryllium (given its configuration $1s^{2}2s^{2}$). This is because Beryllium has a full s-subshell so it would be much more stable thus requiring more energy to remove its electron. On the other hand, both Lithium and Boron would be much more unstable and more likely to lose one electron to either have a full 2s shell (for Boron) or a full 1s shell (for Lithium).

I hope this makes some sense. Let me know if you need more clarification.

Chem_Mod
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### Re: periodic trend exceptions  [ENDORSED]

We can experimentally determine certain conditions, which is how we determine exceptions to the rules. You are only responsible for the exceptions mentioned in class, though, so if it was not mentioned in class, you will not be expected to simply know the exceptions. For more information on some of the specific exceptions, including the one you mentioned, please reference this previous post: viewtopic.php?f=25&t=22704&p=66536&hilit=oxygen+ionization+energy&sid=fd03a0bce96fa7934fb15f32ae4ec228#p66536