Answer : The correct formula unit of an ionic compound is Ca(OH)2
Explanation :
An ionic compound is formed when a metal atom donates one or more electrons to a non metal atom.
During this process, metal gains a positive charge and a nonmetal gets a negative charge.
In an ionic compound, these charges on the ions are balanced so that the overall compound is neutral.
Let us check which of the given compounds have a neutral charge.
1) Al2F3 : Charge on Al is +3 and that on F is -1.
The total charge on 2 Al is 2x3 = +6.
The total charge on 3 F atoms is 3x(-1) = -3
The overall charge on the compound is +6-3 = +3.
That means the given formula unit is not correct.
2) Ca2O : Charge on Ca is +2 and that on O is -2
Total charge on Ca is 2x2 = +4
The overall charge is +4-2 = +2
Therefore the given formula unit is not correct.
3) Ca(OH)2 : The charge on Ca ion is +2 and that on OH is -1.
The total charge on 2 OH ions is 2x(-1) = -2
The overall charge is +2-2 = 0
This gives a neutral compound.
Therefore the given formula unit is correct.
