Periodicity in Electronegativity

Electronegativity is defined as a measure of the ability of an atom to attract the electron pair in a covalent bond to itself. The value of electronegativity is assigned arbitrarily to one atom such as hydrogen. Then the value of electronegativity is assigned to all other atoms with respect to hydrogen.

In a homonuclear diatomic molecule such as hydrogen (H₂) or fluorine (F₂), the electron pair of the covalent bond in each molecule experiences equal attraction by each atom. Thus, none of the two atoms is able to shift the bond pair of electrons to itself.

In a heteronuclear diatomic molecule, the bond pair electrons get shifted towards the atom which is more electronegative than the other. For example, in HF or HCl the bond pair of electrons are not shared equally but the more electronegative atom F or Cl is able to shift the bond pair towards itself, resulting in the polarization of the molecule.

A large difference between electronegativity of the two atoms indicates highly ionic character of the bond between them. On the other hand, zero difference in the electronegativity between the two atoms indicates that the percentage ionic character is zero.

Trends

The most electronegative elements have been placed on the farthest right hand upper corner (noble gases are not included). The value of electronegativity decreases as you go down in any group and increases from left to right in the period. Thus, fluorine is the most electronegative and caesium is the least electronegative element.