9.7: Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.

Electronegativity values of the elements were proposed by one of the most famous chemists of the twentieth century: Linus Pauling. Pauling investigated the energies required to break bonds in heteronuclear molecules such as hydrogen and fluoride. Based on the values, he proposed that the energy required to break a bond will be the average of bond energies of H₂ (436 kJ/mol) and F₂ (155 kJ/mol), i.e., 296 kJ/mol. However, the experimentally obtained bond energy of HF is 565 kJ/mol, which is much higher than the predicted value. To account for this difference, Pauling suggested that the bond must have an ionic character, which is determined by the concept of electronegativity.

Electronegativity is a measure of the tendency of an atom to attract electrons (or electron density) towards itself.

Electronegativity determines how the shared electrons are distributed between the two atoms in a bond. The more strongly an atom attracts the electrons in its bonds, the larger its electronegativity. Electrons in a polar covalent bond are shifted toward the more electronegative atom; thus, the more electronegative atom is the one with the partial negative charge. The greater the difference in electronegativity, the more polarized the electron distribution and the larger the partial charges of the atoms.

Electronegativity and the Periodic Table

• Electronegativity increases from left to right across a period in the periodic table and decreases down a group.
• The electronegativity values derived by Pauling follow predictable periodic trends, with the higher electronegativities toward the upper right of the periodic table.
• Thus, the nonmetals, which lie in the upper right, tend to have the highest electronegativities, with fluorine the most electronegative element of all (EN = 4.0).
• Metals tend to be less electronegative elements, and the group 1 metals have the lowest electronegativities.
• Noble gases are excluded from the electronegativity list because these atoms usually do not share electrons with other atoms since they have a full valence shell. (While noble gas compounds such as XeO₂ do exist, they can only be formed under extreme conditions, and thus they do not fit neatly into the general model of electronegativity.)

Electronegativity versus Electron Affinity

Be careful not to confuse electronegativity and electron affinity. The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol. Electronegativity, on the other hand, describes how tightly an atom attracts electrons in a bond. It is a dimensionless quantity that is calculated, not measured. Pauling derived the first electronegativity values by comparing the amounts of energy required to break different types of bonds. He chose an arbitrary relative scale ranging from 0 to 4.

This text is adapted from Openstax, Chemistry 2e, Section: 7.2 Covalent Bonding.

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.