Bond Polarity (DP IB Chemistry): Revision Note

Bond polarity

What is electronegativity?

• Electronegativity is the ability of an atom to attract the shared pair of electrons in a covalent bond

• Different atoms have different electronegativities, shown on the Pauling scale

  • Electronegativity values are listed in the IB Chemistry Data Booklet (Section 9)

• The higher the value, the more electronegative the element

The Pauling Scale

• In diatomic molecules, the electron density is shared equally between the two atoms

  • E.g. H₂, O₂ and Cl₂

• These atoms have the same electronegativity, so the bonding electrons are shared equally

• This results in a nonpolar covalent bond

Diagram to show the electron distribution in a chlorine molecule

What is meant by a polar bond?

• When two atoms in a covalent bond have different electronegativities, the bonding electrons are drawn closer to the more electronegative atom

  • This creates a polar covalent bond with an uneven distribution of electron density

• As a result: 

  • The centres of positive and negative charge no longer coincide

    • The electron distribution becomes asymmetric

  • The less electronegative atom gains a partial positive charge (δ⁺)

  • The more electronegative atom gains a partial negative charge (δ^-)

• The degree of polarity depends on the difference in electronegativity between the bonded atoms

  • A larger difference produces a more polar bond

Diagram to show the electron distribution in an HCl molecule

What is a dipole?

• A dipole forms when there is a separation of charge across a covalent bond

• The dipole moment is a measure of how polar a bond is

  • The greater the difference in electronegativity, the greater the dipole moment

• The direction of the dipole is represented using a special arrow symbol:

  • The crossed tail is placed near the partially positive atom (δ⁺)

  • The arrowhead points toward the partially negative atom (δ⁻)