CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.3.13 Polarity & Dipole Moments

Polarity & Dipole Moments

  • 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

Chemical Bonding Nonpolar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Polar Diatomic Molecule, downloadable AS & A Level Chemistry revision notes

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:

 

Chemical Bonding Dipole, downloadable AS & A Level Chemistry revision notes

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 way that the individual dipole moments cancel each other out

Chemical Bonding Polar Molecule, downloadable AS & A Level Chemistry revision notes

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

Chemical Bonding Nonpolar Molecule, downloadable AS & A Level Chemistry revision notes

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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