Following the footsteps of Gilbert N. Lewis in understanding the nature of the covalent bond, Linus N. Pauling stumbled upon an interesting observation.
Pauling noticed that the nature of the chemical bond could be explained using a scale or continuum.
The atoms on the left side of the scale had more tendency to lose an electron and form an ionic bond. On the right end of the scale were atoms that preferred electron sharing to form the covalent bond.
The preference to lose or share their electrons depended on the atoms' electronegativity.
Pauling, therefore, devised an electronegativity scale ranging from 0 to 4. Lower values are for the least electronegative atom (e.g., metals), and higher values are for the most electronegative atom (e.g., halogens). Amongst the metals, Caesium (Cs) has the lowest electronegativity, and Fluorine (F) has the highest of all the halogens.
The atoms with higher electronegativity values and a higher number were nonmetals and formed covalent bonds. He found that they attracted most of the electrons of the covalent bond.
By calculating the electronegativity difference between the two atoms in a bond, Pauling also found different bonding interactions that led to various polar and nonpolar covalently bonded compounds.
If the atoms forming a covalent bond (bond between heteroatoms) have electronegativity differences in the range of 0.5-1.7, then such a bond is called a polar covalent bond. A few examples are- H2O, NH3, and HCl. Higher electronegativity differences led to more bond polarity.
If there is no electronegativity difference between two bonded atoms (E.g., Cl2, H2) or the electronegativity difference is less than 0.5 (e.g., CH4), then such a bond is a nonpolar covalent bond.
In polar covalent compounds, there is only partial separation of charges; however, when the electronegativity difference is above 1.7, such a bond is no longer covalent. The bond is ionic in such compounds – like in NaCl and NaF.
So, calculating the electronegativity difference between two atoms made it possible to predict their bonding type.
In addition to predicting the nature of the bond, electronegativity is used in predicting the direction of the electron shift in a covalent bond, and subsequent reactivity; it can predict relative acidity and metallic and non-metallic character.
Related Reading- Electronegativity