GC question

[sbdento]

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I'm having some trouble with a question from Destroyer. It is number 233 in the GC section and is a "which of the following is false?" question.

The answer is:

The strontium cation is larger than the sulfur anion since negative ions gain electrons.

I do understand that anions are larger because they gain electrons and cations are smaller because they lose them. But, when you look at these two ions the sulfur ion is equivalent to Ar and the Sr ion is equivalent to Kr. Kr has an extra orbital, so it should in my mind, (despite the cation and anion size rule) be a true statement.

Can somebody give me some incite?

 

[Kahr]

I had to re-watch Chad's video on this.

Honestly, it seems like the question should be comparing the calcium (Ca++) cation to the sulfur (S--) anion, not strontium. Why? Because they're in an isoelectronic series (same number of electrons), and then you would say sulfur is larger than calcium based on the effective nuclear charge (based on the number of protons pulling the electrons in).

So yeah, it kind of befuddles me too, since I've always seen questions about ionic radii being based on a series of cations only, anions only, isoelectronic series, or just the normal atomic radius to the ionic radius of the same element. (I'd be interested to know if anyone out there has seen the DAT actually ask a question comparing a cation and anion of different elements NOT in an isoelectronic series)

 

[amalgamgrillz]

I think that statement is false because of the reasoning. Yes, Sr cation is bigger than the S anion, but NOT simply because cations lose electrons and anions gain electrons. Sr is bigger because it has more electrons/shells/subshells