This produces a polar covalent bond, and may cause a molecule to have a…

The idea

A polar bond has a dipole: a separation of charge with a direction. Whether the whole molecule has a permanent dipole depends on how those bond dipoles are arranged in space, because they add as vectors. If the polar bonds point symmetrically outwards so their dipoles cancel, the molecule is non-polar overall despite its polar bonds: carbon dioxide is linear, tetrachloromethane is tetrahedral and boron trifluoride is trigonal planar, and all three cancel. If the arrangement is unsymmetrical, usually because of a lone pair on the central atom or different atoms attached, the dipoles do not cancel and the molecule has a permanent dipole. Water and ammonia are polar for exactly this reason.

What earns the marks

• Identifying the individual polar bonds and their direction.
• Considering the shape or symmetry of the whole molecule.
• Stating whether the bond dipoles cancel or reinforce.
• The conclusion about whether the molecule has a permanent dipole.

Common mistakes and how to put them right

• Deciding molecular polarity from the bonds alone without considering the shape.

• Saying carbon dioxide is polar because each carbon to oxygen bond is polar.

• Treating a molecule with lone pairs on the central atom as symmetrical.

• Confusing a permanent dipole with the temporary induced dipole of a London force.

Exam tips

• Draw the shape before deciding polarity; symmetry cannot be judged from a formula.
• Say explicitly that the dipoles cancel or do not cancel, then state the conclusion.
• Watch for molecules where symmetry is broken by one different atom, such as a chlorinated methane.