Why is Sr2+ smaller than S2-??

[virtualmaster999]

Hey everyone!

I thought this would be the other way around. Sr2+ would be placed at Kr, and S2- would be Ar. So Sr2+ is further down, so larger atomic radius; but why isnt it bigger??

Thanks in advance!

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[BYU4you]

To quote Dr. Romano

From the chemical literature values Sr++ has an ionic radius of 132pm, S-- has a value of 170pm, as a general rule, an anion is larger than it's neutral counterpart, while the cation is smaller. Go to a general chemistry text book such as Chang, Zumdahl or Masterson. These books have very nice tables to show you what happens when a neutral atom becomes an ion. Hopefully you will see the more positive an atom becomes the smaller it's radius, and the more negative an atom becomes the larger the radius. If you don't have a GenChem book handy you can find some nice tables on the internet.

Hope this helps

Dr. Romano

Essentially - means extra electrons so they stuck out farther and + less electrons so they are smaller in Laymans terms

 

[cacajuate]

I still think that's a bulls hit question. They aren't in the same area so you can't make a good prediction about it. And looking up the radius in a book to show the answer is correct is not helpful.

Chad specifically says that they have to be in the same series to make a good prediction, this is clearly not the case. I understand anions are generally larger but how would we know that the radius of Sr isn't that much larger to start?