Bond Polarity
Quick Notes
- Electronegativity = ability of an atom to attract the bonding electrons in a covalent bond.
- If bonded atoms have different electronegativities, the electrons are unevenly distributed, creating a polar bond.
- Polar bonds have partial charges (δ⁺ and δ⁻).
- The more electronegative atom gains a partial negative charge (δ⁻)
- The less electronegative atom gains a partial positive charge (δ⁺)
- Molecules are polar if there is an overall dipole moment (one end δ⁺, the other δ⁻).
Full Notes
Electronegativity has been covered in more detail
here.
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Electronegativity and Bond Polarity
Electronegativity is the ability of an atom to attract bonding electrons in a covalent bond.
The Pauling Scale is often used to compare the electronegativities of different elements:
- Most electronegative: Fluorine (F) = 4.0
- Least electronegative: Group 1 metals (~0.7–1.0)
Electronegativity Trends in the Periodic Table
- Increases across a period (greater nuclear charge, similar shielding).
- Decreases down a group (greater shielding).
Non-Polar and Polar Covalent Bonds
In a non-polar covalent bond (e.g., Cl2, O2), electrons are shared equally between atoms of the same electronegativity.
In a polar covalent bond (e.g., HCl, H2O), electrons are shared unequally, leading to partial charges (shown as δ⁺ and δ⁻).
The atom with the higher electronegativity ends up with a partial negative charge (δ-) and the other a partial positive charge (δ+).
Example: Hydrogen Chloride (HCl)
- Cl is more electronegative than H.
- Bonding electrons pulled closer to Cl → Cl δ⁻, H δ⁺.
Polar and Non-Polar Molecules
A molecule is polar or non-polar depending on its symmetry.
Non-Polar Molecules
If polar bonds are arranged symmetrically, dipoles cancel → molecule is non-polar.
Examples: CO2 and CCl4 — polar bonds present, but symmetry cancels dipoles.
Polar Molecules
If dipoles do not cancel due to asymmetry, molecule is polar.
Examples: H2O (bent) and CHCl3.
- H2O has a bent shape (104.5°) and O-H bonds are polar, meaning dipoles do not cancel → Water is polar
- CH3Cl has a tetrahedral shape (109.5°) however the C-H and C-Cl bonds have different polarities, meaning dipoles do not cancel → CHCl₃ is polar.
Summary
- Electronegativity determines how electrons are shared in a bond.
- Polar bonds form when atoms have different electronegativities.
- Polar molecules exist if bond dipoles do not cancel.
- Non-polar molecules exist if bond dipoles cancel due to symmetry.
- δ⁺ and δ⁻ notation plus arrows indicate bond polarity.