Which ion has the larger radius, Cl- or K+? Why?

Answer 1

I started answering this question confident of my answer, but as I thought about it I realized there are really several factors to consider.

First, realize that Cl- and K+ have the same number of electrons: 18. So the only difference is the number of protons. Well, it's clear that K+ has two more protons than Cl-, according to the periodic table.... but what does that mean?

Does having more protons mean the ion itself will be bigger? Well, not necessarily, because in fact the protons will PULL more on the electrons, making the ion SMALLER. THAT would mean that Cl+ is the bigger ion.

HOWEVER... we have another complication. Potassium has one more energy level than chlorine... does that contribute to its size? Do the electrons have more space to hang out? I don't think this factor matters because once you take an electron away you're really taking the orbital away, but... it's another thing to consider.

My best guess would be that Cl- is bigger because there are fewer protons and the nucleus thereby exhibits less force on the valence electrons, which means those electrons are freer to move around and the ion's larger. There's always the possibility that more protons just mean a bigger ion, but I don't think that's probable.

Answer 2

K+ and Cl- are said to be isoelectronic, that is, the same number of electrons and exactly the same electron arrangement: 1s2 2s2 2p6 3s2 3p6.. Now K+ has two more protons than Cl-, and this makes a dramatic difference to the radius. Two more protons atrracting the same number of electrons makes K+ very much smaller than Cl-.

Answer 3

Cl- because the extra electron puts it in a higher orbital and the K+ has one less electron making the final electron difference: 1
take Cl:0
and K:1
so
K+:0
and Cl-:1