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Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: Goblik on October 16, 2019, 11:21:55 AM

Title: Atom and Ionic Radius
Post by: Goblik on October 16, 2019, 11:21:55 AM
Arrange by its radius:

8O, 8O2-, 9F, 9F-, 10Ne, 11Na, 11Na+, 12Mg, 12Mg2+

I understand the concept:
1. More electron shells, bigger the atom/ion
2. If atom/ion has same number of electron shells, then atom/ion with more proton has smaller radius
3. Anion radius > atom radius and Cation radius < atom radius

But I am not sure about the PRIORITY. And if I arrange with the concept, the result is wrong because according to radius data in wiki we know that the order of radius is:

Na > Mg > O2- > F- > Na+ > Mg2+ > O > F > Ne

What priority if we want arrange atom and ion by its radius?

In my case, how can we say Mg2+ radius is bigger than O radius? Because according to the concept this is a contradiction: Mg2+ has more protons so Mg2+ should have smaller radius than O?
Title: Re: Atom and Ionic Radius
Post by: oque on October 21, 2019, 12:09:25 AM
The concepts are right, but you are ignoring something.
You compare Mg2+ and O radius, but:
You can only use those rules if the two atoms/ions are isoelectric, from the same period and from the same group. Also if they are the same atom.

O and  Mg2+ doesn't fall in any category, so what do we do?
We can supose. If we had Mg and  O,  since the atomic radius increases from right to left, we can safely assume that Mg is bigger than O because they are far apart.  So we can conclude Mg2+ > O