Resonance occurs when a single Lewis structure cannot accurately represent a molecule’s true bonding. In many molecules and ions, such as the carbonate or nitrite ion, electrons are spread out, or delocalized, across multiple atoms. To show these different possible electron arrangements, chemists draw multiple Lewis structures, known as resonance structures.
These individual structures are hypothetical blueprints, not real representations of the molecule. The true structure is the resonance hybrid, which is a single, weighted average of all valid resonance structures.
Fundamental Rules for Electron Movement
Determining valid resonance structures requires understanding the rules governing electron movement. The primary constraint is that only electrons are permitted to move; the position and connectivity of the atoms must remain fixed. If atoms were to move, the resulting structure would represent a different molecule entirely.
This means that strong single bonds (sigma bonds) must remain untouched when drawing resonance forms. Only two types of electrons possess the mobility needed for delocalization: lone pairs and pi electrons, which form the second or third bond in a multiple bond.
All valid resonance structures must contain the exact same total number of valence electrons as the original structure. Conserving the total electron count is essential, meaning the overall net charge of the molecule or ion cannot change. For example, if the starting species is a negative ion, every subsequent resonance form must also carry that same negative charge.
The connectivity of the atoms must be preserved, meaning every atom must remain bonded to the same neighboring atoms in all structures. The only change between structures is the location of the pi electrons and lone pairs.
The Step-by-Step Drawing Process
The practical method for generating new resonance structures involves systematically moving specific electrons using a tool called the curved arrow formalism. A curved arrow always indicates the movement of a pair of electrons, with the tail starting at the electron source (a lone pair or pi bond) and the head pointing toward the destination (an adjacent atom or bond that can accept the electrons).
The process begins by identifying an electron-rich site, such as a lone pair or a pi bond, adjacent to an electron-deficient site, like a positive charge or another pi bond. This adjacency allows for delocalization. Common patterns include a lone pair next to a double bond, or a positive charge next to a double bond.
When moving electrons, a single curved arrow can show a lone pair becoming a new pi bond between two atoms. Alternatively, an arrow can show a pi bond shifting to an adjacent location, or a pi bond breaking to form a lone pair on an adjacent atom. Electrons move toward regions of lower electron density, often toward a positive charge or a more electronegative atom.
A strict rule for second-row atoms (carbon, nitrogen, oxygen) is that they can never be surrounded by more than eight valence electrons (the octet rule). If an arrow forms a new bond on an atom that already has four bonds, a second arrow must simultaneously break an existing pi bond. This prevents the atom from exceeding its octet.
After drawing the arrows, the next step is to calculate and assign the formal charge to every atom in the resulting structure. Formal charge is the hypothetical charge an atom would have if all bonds were perfectly shared. These charges help verify that the net charge of the new structure matches the original.
Evaluating Which Structure Contributes Most
Once a set of valid resonance structures has been drawn, the next step is to determine which one most closely resembles the true resonance hybrid. Not all structures contribute equally to the hybrid; some are minor contributors, while others are major, and the most stable structures are the ones that contribute the most.
Octet Fulfillment
The primary rule for stability is maximizing the number of atoms that satisfy the octet rule. Structures where every atom, especially carbon, nitrogen, and oxygen, has a complete valence shell of eight electrons are more stable than those with incomplete octets. A structure with an incomplete octet, such as a carbon atom with only six electrons, will be a minor contributor.
Charge Minimization
The second factor is the separation of formal charges. Structures with the fewest formal charges are generally more stable. If formal charges are unavoidable, structures where the charges are closer together are preferred over those where the charges are widely separated.
Charge Placement
If two structures have the same number of complete octets and formal charges, the third rule considers the electronegativity of the charged atoms. The more stable structure places any negative formal charge on the most electronegative atom, such as oxygen or nitrogen. Conversely, any positive formal charge is more stable when it resides on the least electronegative atom, typically carbon.
For example, a structure with a negative charge on an oxygen atom will be a more significant contributor than an otherwise identical structure with the negative charge on a carbon atom. By applying these three rules in order—octet fulfillment, charge minimization, and charge placement—one can evaluate the relative importance of each resonance structure to determine the characteristics of the overall resonance hybrid.