Which IE/EN is Larger?

Question

With K+, Cl-, S2-

K+ has the smallest radius size and S2- has the largest?

K+ < Cl-, S2+

So for IE/EN, will the ranking smallest to biggest just the opposite of the above?

S2+ < Cl- < K+ ( smallest to largest EN/IE value)

Answer 1

Your radius ranking is correct, and your reasoning about ionization energy is correct. But electronegativity does not belong in this comparison at all — and that is the part worth sorting out.

First, why these three behave as a set

K⁺, Cl⁻, and S²⁻ are isoelectronic — every one has 18 electrons in the argon configuration. Only the nuclear charge differs:

S²⁻: 16 protons  |  Cl⁻: 17 protons  |  K⁺: 19 protons

Same electron cloud, different pull on it. Every trend here follows from that single fact.

Radius — you have it right

More protons pull the same 18 electrons in tighter:

K⁺ < Cl⁻ < S²⁻  (smallest to largest)

Ionization energy — yes, it is the reverse

IE is the energy to pull one more electron off. The nucleus holding those electrons most tightly makes removal hardest:

S²⁻ < Cl⁻ < K⁺  (smallest to largest IE)

Your instinct is right, and it is worth knowing why rather than just as an inversion rule: both trends come from the same nuclear-charge argument, so they run opposite by construction. Note also that removing an electron from an anion like S²⁻ is especially easy — it is already electron-rich and the extra electrons repel one another.

(The general "radius up, IE down" pattern is reliable, but do not treat it as universal — it can break across a period at half-filled and filled subshells, as with N and O.)

Electronegativity — the correction

Electronegativity is defined as the ability of an atom in a chemical bond to attract shared electrons. It is a property of atoms within molecules, not of free ions.

So "the electronegativity of K⁺" is not a defined quantity. There are no tabulated EN values for ions, and asking which of K⁺, Cl⁻, S²⁻ has the highest EN is a category error rather than a hard question.

If your problem intends the parent atoms, then use Pauling values:

K = 0.82  |  S = 2.58  |  Cl = 3.16

K < S < Cl  (smallest to largest EN)

Notice this is not simply the reverse of the ionic radius order — chlorine outranks sulfur. Neutral-atom electronegativity follows periodic position (up and to the right), not the isoelectronic proton-count logic that governs the ions.

One small thing: you wrote S²⁺ a couple of times where you meant S²⁻. Sulfur gains two electrons to reach the argon configuration. Worth being careful with, since a sign flip changes the species entirely.