9.3
Electronegativity
Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.
Electronegativity values of t…
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.
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Q1: What is electronegativity and how does it differ from electron affinity?
Electronegativity is a dimensionless measure of an atom's tendency to attract electrons toward itself in a bond. Unlike electron affinity, which is a measurable physical quantity expressed in kJ/mol representing energy released when an isolated gas-phase atom gains an electron, electronegativity is calculated rather than measured. Linus Pauling derived the first electronegativity scale using thermochemical data from bond energies.
Q2: How does electronegativity determine bond polarity?
Electronegativity differences between bonded atoms determine how shared electrons distribute. In polar covalent bonds, electrons shift toward the more electronegative atom, creating partial charges. The greater the electronegativity difference, the more polarized the electron distribution and the larger the partial charges. This relationship between bond polarity dipole moment and percent ionic character helps predict bond characteristics.
Q3: Why is HCl considered a polar covalent bond rather than an ionic bond?
In HCl, electrons are unequally shared between hydrogen and chlorine rather than transferred completely. Chlorine's higher electronegativity attracts the shared electrons more strongly, creating higher electron density on chlorine and a partial negative charge, while hydrogen carries a partial positive charge. This unequal sharing defines a polar covalent bond, distinct from ionic bonding where electrons transfer entirely from metals to nonmetals.
Q4: What are the periodic trends in electronegativity values?
Electronegativity increases from left to right across a period and decreases down a group in the periodic table. Nonmetals in the upper right have the highest electronegativities, with fluorine being the most electronegative element at 3.98. Metals are less electronegative, and group 1 metals have the lowest values. Electronegativity decreases with increasing atomic size because larger atoms attract electrons less effectively.
Q5: How did Linus Pauling develop the electronegativity scale?
Pauling studied bond energies in heteronuclear molecules like HF and compared them to predicted values based on homonuclear bond energies. He discovered that HF's experimental bond energy (565 kJ/mol) exceeded predictions, suggesting ionic character in the bond. To explain this discrepancy, Pauling developed an electronegativity scale ranging from 0 to 4, based on thermochemical data from bond breaking energies.
Q6: Why are noble gases excluded from electronegativity values?
Noble gases are excluded because they typically do not share electrons with other atoms due to having a full valence shell. Since electronegativity measures an atom's tendency to attract electrons in bonds, and noble gases rarely form bonds under normal conditions, assigning them electronegativity values is not practical. Although rare noble gas compounds exist under extreme conditions, they do not fit the general electronegativity model.
Q7: What does the electric field orientation of HCl reveal about its molecular structure?
When gaseous HCl is placed in an electric field, hydrogen orients toward the cathode (negative pole) and chlorine toward the anode (positive pole), indicating partial charges on each atom. This behavior demonstrates that HCl is a polar molecule with unequal electron distribution. In contrast, nonpolar molecules like N2 orient equally between poles, showing no preferential charge distribution.