Rules of Resonance in Chemistry

The rules of resonance in chemistry govern how resonance structures are generated and which structures are considered valid representations of a molecule or ion. Here are the key rules of resonance:

 

Movement of electrons: 

In resonance, only electrons are allowed to move, not the nuclei (atoms). The position of the atomic nuclei remains the same in all resonance structures.

 

Same positions of atoms:

Resonance structures must have the same positions for all the atoms in the molecule or ion. Only the distribution of electrons changes.

 

Valence electrons participate:

Only valence electrons (outer shell electrons) of atoms are involved in resonance. Core electrons are not considered to move between resonance structures.

 

Maintain charge:

Resonance structures must retain the same overall charge as the real molecule or ion. The distribution of charges may vary, but the net charge remains the same.

 

An equal number of electrons:

The total number of electrons in all resonance structures must be the same as the number of valence electrons in the real molecule or ion.

 

Equivalent resonance structures:

Resonance structures are considered equivalent when they have the same arrangement of atoms and similarly distribute electrons. Equivalent resonance structures supply equally to the resonance hybrid.

 

Maximizing octets:

Resonance structures should try to satisfy the octet rule for all atoms when possible. However, certain molecules like hypervalent species (e.g., sulfur hexafluoride, SF6) may have resonance structures that exceed the octet rule for some atoms.

 

Preferred structures:

In some cases, certain resonance structures contribute more to the overall resonance hybrid than others. The molecule,s electronic structure depends upon the stability of the resonance structure.

 

Delocalization of electrons:

Resonance structures should demonstrate the delocalization of electrons through π (pi) bonds or lone pairs, resulting in the spreading of electron density over multiple atoms.

 

Non-crossing arrows:

When drawing resonance structures, avoid crossing arrows. Arrows indicate the movement of electrons, and they should not intersect to prevent confusion.

 

By following these rules, chemists can generate valid resonance structures for molecules and ions, leading to a better understanding of their electronic structure, stability, and reactivity. Resonance is an important concept in organic chemistry and is used to explain various chemical phenomena and behaviors of molecules.