Bond Polarity (SL IB Chemistry)

What is electronegativity?

• Electronegativity refers to the ability of an atom to draw an electron pair towards itself in a covalent bond
• Different atoms have different electronegativities, shown by the Pauling scale below 
• The higher the value, the more electronegative the element is 

The Pauling Scale 

First three rows of the periodic table showing electronegativity values

• In diatomic molecules the electron density is shared equally between the two atoms
  • Eg. H₂, O₂ and Cl₂

• Both atoms have the electronegativity value and have an equal attraction for the bonding pair of electrons leading to formation of a covalent bond
• The covalent bond is nonpolar 

Diagram to show the electron distribution in a chlorine molecule

The two chlorine atoms have identical electronegativities so the bonding electrons are shared equally between the two atoms and the bond is nonpolar

What is meant by a polar bond?

• 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 extend of polarity in a covalent bond varies, depending on how big a difference exists in the electronegativity values of the two bonded atoms 
  • The bigger the difference in electronegativity, the higher the polarity of the covalent bond

Diagram to show the electron distribution in an HCl 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

What is a dipole?

• 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:

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