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Question

(a) Explain the Pauling scale for the determination of electronegativity. Give the disadvantages of Pauling scale.
(b) How does Fluorine differ from other halogens?

Solution
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(a) Pauling's scale:
(i) Pauling scale is based on an emperical relation between the energy of a bond and the electronegativities of bonded atoms.
(ii) Consider a bond AB between two dissimilar atoms A and B of a molecule AB. Let the bond energies of AA,BB and AB bonds be represented as EAB, EBB and EAB respectively. It may be seen that the bond dissociation energy of AB is almost higher than the geometric mean of the bond dissociation energies of AA and BB bonds i.e.,
EAB>EAA×EBB
Their difference (Δ) is related to the difference in the electronegativities of A and B according to the following equation:
Δ=EABEAA×EBB=(XAXB)2
or 0.208 Δ=XAXB
Here XA and XB are the electronegativities of A and B respectively.
The factor 0.208 arises from the conversion of Kcals to electron volt.
(iii) Considering arbitarily the electronegativity of hydrogen to be 2.1, Pauling calculated electronegativities of other elements with the help of this equation.
Disadvantage of Pauling scale: The disadvantage of Pauling's scale is that bond energies are not known with any degree of accuracy for many solid elements.
(b) (i) Fluorine is the most reactive elements among halogens. This is due to the minimum value of FF bond dissociation energy.
(ii) Fluorine decomposes cold dilute alkali liberating F2O and with conc., alkali O2 is liberated. Under similar conditions, other halogens will give rise to hypohalites and halates respectively.
(iii) Fluorine has the greatest affinity for hydrogen, forming HF which is associated due to the hydrogen bonding. HF is a weak acid where as other hydrohalic acids are strong acids.
....HF.....HF......HF
(iv) It differs markedly from other halogens in that it can form two types of salts with metals NaF and NaHF2.
(v) The salts of HF are different from the salts of other hydracids. AgF is soluble in water but other silver halides are not soluble in water.
(vi) Fluorine can have only negative oxidation state due to strong electronegativity character while other halogens can have negative as well as positibve oxidation states.
(vii) HF attacks glass while others do not
(viii) Fluorine does not form polyhalide because of absence of d orbitals. WHere as the other halogens form poly halides such as I3,Br3,Cl3 ions.

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