[RH111]

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!

[Orcio_87]

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

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⁺.

[RH111]

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)?

[Orcio_87]

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

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

[RH111]

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