Bond Polarity (Oxford AQA International A Level Chemistry)

Electronegativity

• Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself

• The Pauling scale is used to assign a value of electronegativity for each atom

First three rows of the periodic table showing electronegativity values

• Fluorine is the most electronegative atom on the Periodic Table, with a value of 4.0 on the Pauling Scale

Table showing the factors that effect electronegativity

Trends in Electronegativity

• Electronegativity varies down the groups and across periods of the Periodic Table

Down a group

• There is a decrease in electronegativity going down the group

• The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii

• Overall, there is decrease in attraction between the nucleus and outer bonding electrons

Trend in electronegativity down a group

Across a period

• Electronegativity increases across a period

• The nuclear charge increases with the addition of protons to the nucleus

• The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the period of the periodic table

Trend in electronegativity across a period

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 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 bigger the difference in electronegativity, the higher the polarity of the covalent bond

The electron distribution in an HCl molecule

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

  • For example; 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 

The electron distribution in a chlorine molecule

• 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 within the molecule

  • How the bonds are arranged in the molecule (i.e the geometry of 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

A polar molecule 

A nonpolar molecule