Question

# A.Explain the Pauling scale for the determination of electronegativity. Give the disadvantages of the Pauling scale.B.How does Fluorine differ from other halogens?

Hint: Try to find the expression for calculation of electronegativity which uses bond energies. Based on the equation we can find the quantities which cannot be calculated exactly. Try to find exceptions in different types of compounds of halides.

A.Pauling scale of electronegativity estimates the difference between electronegativities of 2 elements. We require bond dissociation energy of compounds of both elements and bond dissociation energies of molecules of both elements.
For this calculation, we require the following condition to be true.
${ E }_{ A-B }>\sqrt { { E }_{ A-A }\times { E }_{ B-B } }$
${ E }_{ A-A }$ stands for A-A bond dissociation energy.
$\left( { { X }_{ A }-{ X }_{ B } } \right) ^{ 2 }\quad =\quad { E }_{ A-B }-\sqrt { { E }_{ A-A }\times { E }_{ B-B } } \quad =\quad \triangle \quad$
${ X }_{ A }$ stands for electronegativity of element A.
${ X }_{ A }-{ X }_{ B }\quad =\quad 0.208\sqrt { \triangle }$
0.208 is a conversion factor.
-Pauling scale is based on bond dissociation energies so they will not be accurate. The electronegativity of an element may differ when we use different elements for calculation. This is the disadvantage of the Pauling scale.
B.Fluorine differs from other halogens in some physical and chemical properties. The bond energy of fluorine is less compared to other elements in the group deviating the original trend. Fluorine molecules have a short bond and 3 lone pairs on each fluorine atom resulting in an unstable F-F bond.
Hydrogen halides of halogens are highly acidic whereas HF is weakly acidic due to the high bond dissociation energy of HF. Fluorine forms fluorides of oxygen whereas other halogens form oxides. Silver Fluoride is soluble in water, unlike other silver halides which are insoluble in water. Other halogens exhibit more than one oxidation state whereas Fluorine exhibits only 1 oxidation state.

Note:
The first elements of s and p block elements deviate from remaining elements because of their very small size and high charge density. That’s why some fluorine properties deviate from rest and we need to be careful with them.