Identify Partial Charges Study guide
Partial charges (δ+ and δ−) mark where the resonance hybrid carries real charge character: an atom bearing a formal charge in a significant contributor keeps a fraction of that charge in the hybrid.
Tap every atom with the requested partial-charge character in each molecule below.
Common Questions
What is a partial charge?
A partial charge is the fraction of a full formal charge that an atom actually carries in the resonance hybrid. If an atom bears a negative formal charge in one significant contributor and no charge in another, the real molecule holds something in between, written δ−. Partial charges are not a separate bookkeeping system from formal charges: they are what formal charges average to once you stop pretending the molecule is any single drawing.
How do you find which atoms carry δ+ or δ−?
Draw the significant resonance contributors and read the formal charges off each one. Every atom that carries a charge in at least one significant contributor carries that character in the hybrid. Note that this includes the atom the charge is drawn on in the structure you started from: in acetate both oxygens carry δ−, not just the one drawn with the minus sign.
Why do equivalent atoms always share the charge?
Atoms that are equivalent by symmetry sit in identical chemical environments, so nothing distinguishes them in the real molecule. The two oxygens of a carboxylate are equivalent, therefore each carries half a negative charge, and experiment agrees: the two C–O bonds of acetate are the same length.
Are partial charges the same as electronegativity differences?
No, although both produce them. An inductive δ+ comes from a neighboring atom pulling electron density through a sigma bond, which falls off quickly with distance. A resonance δ+ comes from delocalization through a pi system and can appear several atoms away, which is why the ortho and para carbons of aniline carry δ− while the meta carbons do not.