Which one of these has the largest radius?

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When assessing ionic radii, several trends help us compare these ions. 
Option 1: Cl- (chloride) has gained one electron relative to neutral chlorine, increasing electron-electron repulsion and slightly enlarging the radius compared to Cl, but it is still a relatively small anion within the same period. 
Option 2: Ca2+ (calcium ion) is a cation with a +2 charge; strong effective nuclear charge pulling electrons closer reduces its radius compared with many neutral atoms and with singly charged anions in the same period, making Ca2+ typically smaller than many anions and other cations with lower charges. 
Option 3: S2- (sulfide) is a negatively charged ion with two extra electrons; in the same period, anions are larger than their neutral counterparts due to increased electron-electron repulsion, and among the period-3 and period-4 species, the doubly charged sulfide usually has a noticeably larger radius than chloride because of both the additional electron and the general trend that anions are bigger than their cationic neighbors. 
Option 4: K+ (potassium ion) is a +1 charged cation; while it is relatively large for a cation, its radius is still smaller than the radii of most anions in this set due to the loss of electrons and the strong pull of the nucleus, yielding a smaller size than S2- and often smaller than Cl-. 
Putting these observations together, the comparison shows that S2- is the largest among the listed species, since it is a doubly charged anion with expanded electron cloud, whereas the cations (Ca2+ and K+) are contracted by higher effective nuclear charges, and Cl- sits between them but is still smaller than S2- due to losing the extra electron repulsion present in S2-.

CHEM 110 Section 3 (PM) SP25 Pre-Lecture 10 Quiz C04-4-5

Which one of these has the largest radius?

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