Bond Polarity (AQA A-Level Chemistry): Revision Notes

1.5.2 Bond Polarity

Electronegativity

Trends in Electronegativity

• Across a Period: Electronegativity increases from left to right. As you move across a period, the atomic radius decreases, and nuclear charge increases, leading to a stronger pull on the bonding electrons.
• Down a Group: Electronegativity decreases as atomic radius increases and additional electron shells provide shielding, reducing the effective nuclear attraction.

Most and Least Electronegative Elements

• Fluorine (F) is the most electronegative element (value of 4.0 on the Pauling scale).
• Caesium (Cs) is one of the least electronegative elements.

Non-polar and Polar Covalent Bonds

The polarity of a bond depends on the difference in electronegativity between the two atoms involved:

Non-polar Covalent Bond

• If both atoms have equal or very similar electronegativity, the electrons are shared equally.

Polar Covalent Bond:

• If there is a significant difference in electronegativity, the electron pair is unequally shared, leading to partial charges.

Partial Charges ($δ⁺$and $δ^-$)

Polar bonds result in partial charges:

• The more electronegative atom attracts the electron density, becoming slightly negative ($δ⁻$)
• The less electronegative atom becomes slightly positive ($δ⁺$).
• These charges are represented by $δ⁺$(partial positive) and $δ⁻$(partial negative) to show the dipole in the molecule.

Dipoles and Permanent Dipoles

A dipole is a separation of charge within a bond, due to differences in electronegativity:

Bond Dipole: Occurs within a polar bond, where one end is δ⁺ and the other is δ⁻.

Molecular Dipole (Permanent Dipole)

• If the dipoles in a molecule do not cancel out due to the shape of the molecule, the entire molecule has a permanent dipole.

Why Some Molecules with Polar Bonds Are Non-polar

• Some molecules contain polar bonds but are non-polar overall if the shape of the molecule causes the dipoles to cancel out.
• Symmetrical Shapes: Linear, trigonal planar, or tetrahedral molecules with symmetrical polar bonds (e.g. $CO₂, BF₃$) are non-polar because the individual bond dipoles cancel out, leaving no overall dipole.

Summary Table of Bond Types

Type of Bond	Electronegativity Difference	Example	Polarity
Non-polar Covalent	0 – 0.4	$H₂, CH₄$	Non-polar
Polar Covalent	0.5 – 1.7	$H₂O, HCl$	Polar
Ionic	> 1.7	$NaCl$	Ionic

This understanding of bond polarity is essential for predicting molecular behaviour, intermolecular forces, and solubility in different substances.