Question

Arrange the following compounds in the increasing orders of their melting point.
$\text{I}.AgI$ $\text{II}.NaI$ $\text{III}.CuI$ $\text{IV}.NaCl$

• A. $IV>II>III>I$
• B. $I<III<II<IV$
• C. $III>IV>II>I$
• D. $I<IV<III<II$

Answer 1

The correct option is B $I<III<II<IV$

$\text{Melting point}\propto \text{ionic nature of halide}$

We know that,
Greater the electronegativity difference between the cation and the anion in a compound, higher will be the ionic character.

The electronegativities of $Na$, $Cu$, $Ag$, $Cl$ and $I$ are $0.93,1.90,1.93,3.16$ and $2.66$ respectively.

The electronegativity difference in $NaCl$ = $3.16-0.93$ = $2.23$

The electronegativity difference in $NaI$ = $2.66-0.93$ = $1.73$

The electronegativity difference in $CuI$ = $2.66-1.90$ = $0.76$

The electronegativity difference in $AgI$ = $2.66-1.93$ = $0.73$

From the above calculations we can see that the ionic character decreases in the following order: $NaCl>NaI>CuI>AgI$

$\therefore$ The increasing order of melting point is $\underset{{558}^{0}C}{AgI}<\underset{{606}^{0}C}{CuI}<\underset{{661}^{0}C}{NaI}<\underset{{801}^{0}C}{NaCl}$