Electronegativity and Bond Polarity
Quick Notes
- Electronegativity is an atom’s ability to attract the bonding electrons in a covalent bond.
- Trends in electronegativity:
- Increases across a period (more protons, same shielding)
- Decreases down a group (more shielding and distance)
- A polar bond forms when two atoms in a bond have different electronegativities.
- More electronegative atom gains δ⁻ (partial negative charge)
- Less electronegative atom gains δ⁺ (partial positive charge)
- A molecule may contain polar bonds but still be non-polar overall if the dipoles cancel due to symmetry.
- CO2 has polar bonds but is non-polar overall (linear, dipoles cancel)
- H2O has polar bonds and is polar overall (bent, dipoles don’t cancel)
Full Notes
Electronegativity
Electronegativity is defined as the ability of an atom to attract the bonding electrons in a covalent bond.
It is measured on a relative scale called the Pauling scale, where:
- Fluorine (F) is the most electronegative element (value = 4.0)
- Group 1 metals are the least electronegative (values ~0.7–1.0)
Trends in Electronegativity have been covered in more detail here.
Electronegativity increases across a period and decreases down a group.
Across a period: More protons in the nucleus and similar shielding means greater attraction to bonding electrons.
Down a group: Atomic radius increases and shielding increases means weaker attraction to bonding electrons.
This trend explains why atoms like oxygen and nitrogen tend to form polar bonds when joined to hydrogen or carbon.
Polar Covalent Bonds
A polar covalent bond occurs when there is a difference in electronegativity between the two bonded atoms.
The bonding electrons are pulled closer to the more electronegative atom, making the electrons unevenly shared. This creates partial charges (δ⁺ and δ⁻) at either end of the bond:
- The more electronegative atom becomes δ⁻
- The less electronegative atom becomes δ⁺
Example Hydrogen Chloride (HCl)
Chlorine (Cl) is more electronegative than Hydrogen (H). The bonding electrons are pulled closer to Cl, giving it a partial negative charge (δ⁻). H loses electron density, giving it a partial positive charge (δ⁺).
If both atoms are the same (e.g. H2, O2), the bond is non-polar because electrons are shared equally.
Polar and Non-Polar Molecules
A molecule is polar or non-polar depending on whether it contains polar bonds and its symmetry.
Non-Polar Molecules (No Permanent Dipole)
If polar bonds are arranged symmetrically then dipoles cancel out and the molecule is non-polar.
Example CO₂
Each C=O bond is polar, but the molecule is linear, so dipoles cancel. No overall dipole = non-polar molecule.
Example CCl₄ (Tetrachloromethane)
Each C-Cl bond is polar, but tetrahedral shape means dipoles cancel. CCl₄ is non-polar despite having polar bonds.
Polar Molecules (Have a Permanent Dipole)
If dipoles do not cancel due out, the molecule is polar.
Example H₂O
O-H bonds are polar and form a bent shape (104.5°). Dipoles do not cancel meaning water is polar.
Example CHCl₃ (Chloroform)
The C-H and C-Cl bonds have different polarities. Dipoles do not cancel meaning CHCl₃ is polar.
Summary
- Electronegativity is the ability of an atom to attract bonding electrons.
- Electronegativity increases across a period and decreases down a group.
- Polar covalent bonds form when atoms have different electronegativities, creating δ⁺ and δ⁻ charges.
- Molecular polarity depends on both bond polarity and symmetry of the molecule.