Bond Polarity & Dipole Moments (CIE AS Chemistry)

• Electronegativity is the ability of an atom to draw a pair of electrons towards itself in a covalent bond
• Electronegativity increases across a Period and decreases going down a Group

Polarity

• When two atoms in a covalent bond have the same electronegativity the covalent bond is nonpolar

Bonding electrons in a chlorine molecule

The two chlorine atoms have similar electronegativities so the bonding electrons are shared equally between the two atoms

• When two atoms in a covalent bond have different electronegativities the covalent bond is polar and the electrons will be drawn towards the more electronegative atom
• As a result of this:
  • The negative charge centre and positive charge centre do not coincide with each other
  • This means that the electron distribution is asymmetric
  • The less electronegative atom gets a partial charge of δ+ (delta positive)
  • The more electronegative atom gets a partial charge of δ- (delta negative)

• The greater the difference in electronegativity the more polar the bond becomes

Bonding electrons in a hydrogen chloride molecule

Cl has a greater electronegativity than H causing the electrons to be more attracted towards the Cl atom which becomes delta negative and the H delta positive

Dipole moment

• The dipole moment is a measure of how polar a bond is
• The direction of the dipole moment is shown by the following sign in which the arrow points to the partially negatively charged end of the dipole:

Representing dipoles

The sign shows the direction of the dipole moment and the arrow points to the delta negative end of the dipole

Assigning polarity to molecules

• To determine whether a molecule with more than two atoms is polar, the following things have to be taken into consideration:
  • The polarity of each bond
  • How the bonds are arranged in the molecule

• Some molecules have polar bonds but are overall not polar because the polar bonds in the molecule are arranged in such a way that the individual dipole moments cancel each other out

Polarity in chloromethane

There are four polar covalent bonds in CH₃Cl which do not cancel each other out causing CH₃Cl to be a polar molecule; the overall dipole is towards the electronegative chlorine atom

Polarity in tetrachloromethane

Though CCl₄ has four polar covalent bonds, the individual dipole moments cancel each other out causing CCl₄ to be a nonpolar molecule