The electron distribution in a covalent bond between elements with…

The idea

When two atoms of different electronegativity share a pair of electrons, the pair is not held midway. The more electronegative atom pulls it closer, so the electron density around that atom increases and around the other decreases. The result is a partial negative charge, written δ⁻, on the more electronegative atom, and a partial positive charge, δ⁺, on the other. These are fractions of a charge, not full ionic charges; no electron has been transferred. The larger the electronegativity difference the more unsymmetrical the distribution, and at a large enough difference the bond is better described as ionic. When the two atoms are identical, as in chlorine or oxygen, the distribution is symmetrical and the bond is non-polar.

What earns the marks

• The bonding pair being drawn closer to the more electronegative atom.
• δ⁻ placed on that atom and δ⁺ on the other.
• Identifying which atom is more electronegative, using data where supplied.
• Stating that a bond between two identical atoms is non-polar because the distribution is symmetrical.

Common mistakes and how to put them right

• Drawing full positive and negative charges instead of partial charges.

• Putting δ⁻ on the less electronegative atom.

• Describing the process as electron transfer, which would produce ions.

• Assuming any bond between two different elements must be fully ionic.

Exam tips

• Always write δ⁺ and δ⁻ rather than plain plus and minus signs on a polar bond.
• Justify the labels by naming the more electronegative element, not just by asserting the polarity.
• Check diatomic molecules first: identical atoms always give a non-polar bond.