Electronegativity & Bonding (Cambridge International A-Level Chemistry)

Using Electronegativity to Predict Bond Formation

• The differences in Pauling electronegativity values can be used to predict whether a bond is covalent or ionic in character

Electronegativity & covalent bonds

• Single covalent bonds are formed by sharing a pair of electrons between two atoms

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

  • Eg. H₂, O₂ and Cl₂

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

• The equal distribution leads to a non-polar molecule

Electronegativity and non-polar molecules

The electronegativity values are equal resulting in the formation of a nonpolar covalent bond

Electronegativity & polar covalent bonds

• When atoms of different electronegativities form a molecule, the shared electrons are not equally distributed in the bond
• The more electronegative atom (the atom with the higher value on the Pauling scale) will draw the bonding pair of electrons towards itself
• A molecule with partial charges forms as a result
• The more electronegative atom will have a partial negative charge (delta negative, δ-)
• The less electronegative atom will have a partial positive charge (delta positive, δ+)
• This leads to a polar covalent molecule

Electronegativity and polar covalent molecules

The electronegativity values are not equal so a polar bond forms

• If there is a large difference in electronegativity of the two atoms in a molecule, the least electronegative atom’s electron will transfer to the other atom

• This in turn leads to an ionic bond – one atom transfers its electron and the other gains that electron

  • The cation is a positively charged species which has lost(an) electron(s)
  • The anion is a negatively charged species which has gained(an) electron(s)

Electronegativity and ionic bonding

Large differences in electronegativity values lead to the formation of ionic bonds

Exam Hint Electronegativity difference (ΔEN) gives a guide to how polar a bond is (it’s a continuum, not a strict rule):

• ΔEN ≈ 0.0–0.4 means non-polar covalent
• ΔEN ≈ 0.4–1.7 means polar covalent
• ΔEN ≳ 1.7 means mostly ionic

Remember: a polar* bond *doesn’t always mean a polar molecule, the shape of the molecule determines whether bond dipoles cancel out.