Question

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 bee included.

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

Note: $1pm={10}^{-10}cm$
Which of the four compounds listed would have the least exothermic lattice energy?

• A. $KCl$
• B. $K_{2}S$
• C. $MgC{l}_2$
• D. $MgS$

Answer 1

The correct option is A $KCl$

A compound has least exothermic lattice energy if it has a cation of lower ionisation energy and anion of of higher negative electron affinity. Because, on adding these energies, the total will become less value. So cation of lower ioisation energy =K.

Anion of higher negative electron affinity = Cl.

Compound=KCl.