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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: RH111 on April 16, 2021, 05:07:36 PM

Title: Relative Ionic Radii (K+ vs Sr2+)
Post by: RH111 on April 16, 2021, 05:07:36 PM

Hi all,

I was wondering why the potassium ion (K+) is larger than the strontium ion (Sr2+). From my understanding, the addition of the n=4 electron shell should result in the strontium ion being larger. However, that is not the case. I think the difference has something to do with the additional two protons in strontium 2+ relative to it's nearest noble gas Kr (K+ only has one additional proton commpared to Ar).

However, without searching up the atomic radii (or anything outside of what's in the periodic table), how can I compare the radii of two ions with different charges like K+ and Sr2+?

Thanks!

Title: Re: Relative Ionic Radii (K+ vs Sr2+)
Post by: Orcio_87 on April 16, 2021, 05:42:22 PM

https://en.wikipedia.org/wiki/Atomic_radius

Quote

factor - nuclear charge - attractive force acting on electrons by protons in nucleus - increase along each period (left to right) - decreases the atomic radius

Positive charge will even increase this factor in favour of Sr²⁺.

Title: Re: Relative Ionic Radii (K+ vs Sr2+)
Post by: RH111 on April 16, 2021, 05:57:14 PM

Thanks!

A quick follow-up:
Why doesn't the same hold true for Ca2+ vs Na+ (Na+ is smaller) or Ba2+ vs Rb+ (Rb+ is smaller)?

Title: Re: Relative Ionic Radii (K+ vs Sr2+)
Post by: Orcio_87 on April 16, 2021, 06:07:17 PM

https://en.wikipedia.org/wiki/Ionic_radius

Here they wrote tha Ca²⁺ is smaller than Na⁺ and Ba²⁺ is smaller than Rb⁺.

Title: Re: Relative Ionic Radii (K+ vs Sr2+)
Post by: RH111 on April 16, 2021, 06:13:02 PM

Oh, my apologies, I am mistaken. Thank you.