91Ó°ÊÓ

"The ability of an atom in a molecule to attract electrons to itself," was how Linus Pauling defined electronegativity. Basically, the electronegativity of an atom is a relative value of that atom’s ability to attract electron density toward itself when it bonds to another atom.

Electronegativity values of given atoms

B = 2.0

H = 2.1

P = 2.1

Se = 2.4

C = 2.5

S = 2.5

Br = 2.8

Cl = 3.0

excess bond energy is the difference between the actual bond energy X-Y and the

average bond energies of X-X and Y-Y. It can be formulated as:

excess bond energy =${\mathbf{BE}}_{X-Y}\mathbf{-}\frac{1}{2}\mathbf{(}\mathbf{B}{\mathbf{E}}_{X-X}\mathbf{+}{\mathbf{BE}}_{Y-Y}\mathbf{)}$

when X and Y atoms are similar or have the same electronegativity, then excess bond energy will be zero.

Electronegativity of P is 2.1 and electronegativity of H is 2.1. Since, both atoms have same electronegativities then the excess bond energy will be zero.

Electronegativity of C atom is 2.5 and electronegativity of S atom is 2.5. Since, both the atom has the same electronegativities then the excess bond energy will be zero.

Electronegativity of Bromine atom is 2.8 and electronegativity of Cl atom is 3.0. Since, both the atom has different electronegativities then the excess bond energy will not be zero.

Electronegativity of B atom is 2.0 and electronegativity of H atom is 2.1. Since there is a small difference between electronegativities of atoms, the excess bond energy will not be zero.

${\mathbf{Se}}_8$ contains 8 Se atoms. The electronegativity of all Se atoms is 2.4, hence the excess bond energy will be zero.