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Consider the table below showing several properties of four representative elements, potassium, calcium, sulfur and chlorine, all from period 4 of the periodic table. Notice that the ionization energies of the nonmetals and the electron affinities of the metals have not been included.

Atom Ionization Energy (kJ/mol) Electron Affinity (kJ/mol) Atomic Radius (pm) Most Stable Ion Ionic Radius (pm)
$K$ 419 N/A 227 $K^+$ 133
$Ca$ (1) 589
(2) 1145
N/A 197 $Ca^{2+}$ 99
$S$ N/A (1) $-$201
(2) + 532
104 $S^{2-}$ 184
$Cl$ N/A $-$ 348 99 $Cl^-$ 181

$\$
Note:

• 1 pm = 10$^{-10}$ cm

Which of the following would have the smallest exothermic lattice energy?

A

$KCl$

B

${ K }_{ 2 }S$

C

${ CaCl }_{ 2 }$

D

$CaS$

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