Inductive Effect & Resonance

The inductive effect and resonance both relate to the distribution of electrons in a chemical bond, but are two different and distinct bonding processes.

The Inductive Effect

The inductive effect, sometimes written as "the -I Effect" in literature, is the distance-dependent phenomenon by which the charge of a chemical bond affects orientation on adjacent bonds in a molecule, producing a permanent state of polarization.

How It Works

The electron density of a σ bond is not uniform when atoms of two different elements participate in the bond. Electron clouds in a bond tend to orient themselves toward the more electronegative atom involved in the bond.

The inductive effect occurs in water molecules. The chemical bonds within a water molecule are more positively charged near the hydrogen atoms and more negatively charged near the oxygen atom. Thus, water molecules are polar. Note, however, the induced charge is weak and the inductive effect is only active across short distances, so other factors can quickly overcome it.

Inductive Effect and Acidity and Basicity

The inductive effect affects the stability as well as acidity or basicity of a chemical species. Electronegative atoms draw electrons toward themselves, which can stabilize a conjugate base. Groups that have -I effect on a molecule decrease its electron density, making the molecule electron deficient and more acidic.

Resonance

Resonance is the bonding of multiple Lewis structures within a molecule as a result of a double bond formed with equal probability between different atoms.

For example, ozone (O₃) has resonance forms. One might wonder whether the bond formed between one oxygen atom might be a different length from another since single bonds are usually weaker/longer than double bonds.

In reality, each bond is the same length and strength because resonance forms (drawn on paper) don't represent what's really going on within the molecule -- it doesn't have a double bond and a single bond. Rather, the electrons are distributed evenly across the atoms, forming intermediate bonds between single and double bonds.