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Exceptions to the Periodic Trends

Posted: Wed Oct 12, 2016 5:02 pm
by Matt_Coopersmith_4H
Hello All. I was reviewing notes from today's lecture and started noticing various exceptions to periodic trends. This made me question: What factors leads to irregularities in periodic trends?

When looking at the trends of atomic radius, the radius should decrease across a period. However, there are a few exceptions, such as with Thallium and Lead, where the atomic radius increases. Why doesn't the periodic trend hold up in this situation?

When looking at ionization energy, group 3A has many exceptions to the trend that ionization energy increases across a period. Does this have to deal with the electrons filling the "p" orbital? If not, what explains this irregularity?

All in all, do these exceptions have fundamental reasons in terms of electron/molecular structure in which the trends do not pertain? If so, what are they? Are these exceptions random?

Thank you!

Re: Exceptions to the Periodic Trends

Posted: Wed Oct 12, 2016 7:46 pm
by Maggie Bui 1H
Hi Matt,

I don't have an answer for all the specific examples you suggested, but in general, I believe irregularities in atomic radius are due to electron-electron repulsion and effective nuclear charge: electron-electron repulsion may cause electrons to spread out and effective nuclear charge affects how tightly electrons are held by the nucleus' positive charge (due to protons).

Re: Exceptions to the Periodic Trends

Posted: Thu Oct 13, 2016 2:23 pm
by Banik_Housepian_2K
Hello Matt,
I was wondering about that as well. For instance i know that as we move down the periodic table the radius increases due to the addition of electron shells, but the charge of the nucleus is also greater meaning it pulls the electrons in with a greater force. The trends are great for generalizing, but they cannot be totally correct for all atoms.

Re: Exceptions to the Periodic Trends

Posted: Fri Oct 14, 2016 2:17 pm
by Rory_Grant_3E
Hey Matt,

I'm not exactly sure why these exceptions occur, but like the person above I imagine it has to do with the interactions between electrons within an atom, and the interactions between the electrons and the nucleus. I don't think we have to know what these exceptions are, so we're chillin in that regard, but like you I'm definitely curious about the specific reasonings behind them. Hopefully someone with more expertise can comment on the matter below!