Electronegativity

Nonmetals form covalent bonds by sharing electrons. But are these electrons shared equally among both atoms, or does one atom attract the electrons more than the other?

The Lewis model depicts all covalent bonds as equally shared electrons; however, this is not always the case. For instance, if gaseous nitrogen is placed in an electric field, it will orient equally between the poles. 

But when gaseous hydrogen chloride, a neutral molecule, is placed in an electric field, the hydrogen orients towards the cathode and chlorine towards the anode, indicating that hydrogen has a partial positive charge and chlorine has a partial negative charge. 

The ability of an atom to attract electrons towards itself is called electronegativity. Chlorine is thus said to be more electronegative than hydrogen, attracting the shared electrons towards itself, while resisting the removal of its own electrons.

This, however, does not make the bond ionic. In an ionic bond, electrons are transferred from metals to nonmetals, while in HCl, the electrons are unequally shared. The electron density is higher on the chlorine than on the hydrogen atom forming a polar covalent bond. 

The greater the difference in electronegativity between two atoms, the more polar the bond will be. Thus, in addition to nonpolar covalent or ionic bonds, polar covalent bonds are found across a large variety of compounds. 

The American Chemist Linus Pauling studied energies required to break bonds in molecules such as diatomic chlorine or hydrogen. He established an electronegativity scale based on thermochemical data, which helps predict bond types. 

Electronegativity is associated with the ionization energy and electron affinity of the atoms. In the periodic table, electronegativity values increase from left to right — metals are less electronegative compared to nonmetals, with exception to transition metals. 

Additionally, the electronegativity values decrease down the column and with increasing atomic size, because atoms are less able to attract electrons to themselves.

Fluorine, the most electronegative element, has the arbitrarily assigned electronegativity value of 3.98. Francium, on the other hand, is the least electronegative element with the electronegativity value of 0.7. 

Electronegativity has no unit; it cannot be determined experimentally.