Illustrated Glossary of Organic Chemistry

Electron density: An atom or group with higher electron density means some aspect of molecular structure (such as resonance or inductive effects) is shifting negative charge towards this spot in the molecule. An atom or group with lower electron density means that some aspect of molecular structure is shifting negative charge away.

Ethoxide ion:

Acetate ion:

Trifluoroacetate ion:

The oxygen atom of ethoxide ion has higher electron density than the oxygen atoms of acetate ion, due to electron delocalization by resonance. The electron-withdrawing inductive effect of trifluoroacetate ion's trifluoromethyl group amplifies this effect. These effects can be seen by comparing the color around each oxygen atom in the electrostatic potential maps. Red indicates higher electron density than orange, which in turn, is higher electron density than yellow.