7.0 Electronegativity

  Periodic Table

• The measure of the capacity or tendency of an atom to attract the shared pair of electrons of the covalent bond toward itself is called electronegativity of that atom.
• Electronegativity is a relative value that indicates the tendency of an atom to attract shared electrons more than the other atom bonded to it. Therefore it does not have any unit. Pauling was the first scientist to put forward the concept of electronegativity.
• On Pauling’s scale, the electronegativity is expressed in Paulings.
• The numerical value of electronegativity of an atom depends on its ionization potential and electron affinity values.

• Factors Affecting Electronegativity
  
• Atomic size: Electronegativity of a bonded atom decreases with increase in size. Therefore, with increase in size the force of attraction on valence shell electrons decreases and hence the electronegativity decreases.
• Effective nuclear charge: When effective nuclear charge is increases, the nucleus will attract the shared pair of electrons with greater strength and hence the electronegativity increases.
• Hybridization state of atom: Electronegativity increases with increase in the s character of the hybrid orbital. This is because the s orbital is nearer to the nucleus and thus suffers greater attraction resulting in increase of electronegativity.
• The number of covalent bonds present between two bonded atoms is known as bond order. With increasing bond order, the bond distance decreases, effective nuclear charge increases and thus electronegativity increases. Increasing order of electronegativity is as follows: $C-C<C=C<C?C$
• Oxidation number: The electronegativity value increases with increase in oxidation number because radius decreases with increase in oxidation number. The increasing order of electronegativity is as follows: $Fe < F{e^{ + 2}} < F{e^{ + 3}}$
• Electronegativity does not depend on stability of fully filled or half-filled orbitals because it is simply the capacity of nucleus to attract bonded pair of electrons.

• Trends in Electronegativity
  
• In a period: Atomic size decreases on going from left to right in a period and thus electronegativity increases.
• In a group: Atomic size increases on going from top to bottom in a group and thus electronegativity decreases.
• Inert gas: The electronegativity value of inert gas is zero, because they do not form covalent bonds.
• F has maximum electronegativity value in the periodic table, while Cs ha minimum electronegativity.
• According to Pauling scale, the electronegativity value of $F$ is 4.0, $O$ is 3.5 and $Cl$ is 3.1.
• Exceptions

a. The elements of group $II\:B$, i.e. $Zn, Cd \:and\: Hg$ show increase in electronegativity value on going from top to bottom in a group.
b. The elements of group $III A$, i.e. $Al$ to $Ga$ show increase in electronegativity value on going from top to bottom in a group.
c. The elements of group $IV A$, show no change in electronegativity value on going from top to bottom in a group from $Si$ onwards.