Question

Which is the most stable halide of alkali metal?
A. Sodium fluoride
B. Sodium bromide
C. Sodium chloride
D. Sodium iodide

Answer 1

Hint: Alkali halide which is most stable among them is the compound which is colorless and appears as a white solid. It is used in toothpaste and prevents dental cavities. It is also used in a very small amount for the fluoridation of water. it is readily soluble in water.

Complete answer:
Alkali metal halides are the inorganic compound. They are composed of an alkali metal and a halogen. The general formula for alkali metal halide is given as $MX$. Where, \[M\] is the metal and $X$ represents a halogen.
The stability of alkali metal halide depends on the standard enthalpy of formation of that compound. Standard enthalpy of formation is the standard heat of formation defined as the change in enthalpy during the formation of one mole of the substance from its constituent elements.
The standard enthalpy of formation of compounds is directly proportional to the stability of compounds. In other words, higher the standard enthalpy of formation higher will be its stability.
Standard enthalpy of formation of alkali halides in $KJ/mol$ are:

Element	$MF$	$MCl$	$MBr$	$MI$
$Li$	$ - 612$	$ - 398$	$ - 350$	$ - 271$
$Na$	$ - 569$	$ - 400$	$ - 360$	$ - 288$
$K$	$ - 563$	$ - 428$	$ - 392$	$ - 328$
$Rb$	$ - 549$	$ - 423$	$ - 389$	$ - 329$
$Cs$	$ - 531$	$ - 424$	$ - 395$	$ - 337$

The order of sodium halides are : $F > Cl > Br > I$
It says, sodium fluoride is most stable among all the sodium halides.

Hence, option (A) is correct.
Note:
The alkali metal halides are colourless crystalline solids. They appear as finely ground powders which is white. They melt at high temperatures, generally several hundred degrees, to colorless liquids. Due to the high melting point they have high lattice energies. At still higher temperatures, these liquids evaporate to give gases composed of diatomic molecules. Alkali halides dissolve in polar solvents to give ionic solutions and they contain highly solvated anions and cations. Alkali halides dissolve large amounts of the corresponding alkali metal: caesium is completely miscible at all temperatures above the melting point.