Notes on Lewis structures, octet rule, and resonance

From the transcript: once you have a Lewis structure, the goal is that the octet rule is satisfied and all electrons are accounted for. There are cases where you could find an alternative Lewis structure.
This implies two core ideas:
- Lewis structures are a bookkeeping tool for valence electrons, bond formation, and lone pairs.
- The octet rule acts as a guideline for completing the valence shell of most atoms in organic and inorganic compounds.

A Lewis structure shows valence electrons as dots and bonds.
- Bonding pairs are shown as lines (each line represents two electrons).
- Lone pairs are shown as pairs of dots around an atom.
Purpose: visualize how electrons are distributed to satisfy bonding requirements and the octet rule.
In most stable, typical molecules, we aim to place electrons to give each atom (except H) a valence shell of eight electrons.
Hydrogen is an exception, needing only 2 electrons to satisfy its valence shell.

The octet rule: in many compounds, atoms tend to have eight electrons around them in the valence shell.
- This is achieved by forming bonds and placing lone pairs as needed.
Practical implications:
- Shared electrons in bonds count toward the octet for each bonding atom.
- Placement of electrons must balance bonding and lone pairs to reach 8 around each atom where applicable.
Note on exceptions and variations (contextual understanding):
- Some atoms can have expanded octets or incomplete octets depending on the element and molecule; these cases are often discussed in more advanced sections.

There are cases where more than one valid Lewis structure can be drawn for the same arrangement of atoms.
These different diagrams are called resonance forms or alternative Lewis structures.
Key features of resonance:
- The atoms (skeleton) remain the same; only the placement of electrons changes between forms.
- The real electronic structure is a resonance hybrid, an average of the contributing forms, with delocalized electrons.
Why resonance matters:
- It explains partial multiple-bond character (bond orders between 1 and 2, or higher in other cases).
- It helps rationalize stability and reactivity better than any single Lewis form.
Common examples discussed in introductory contexts (illustrative, not exhaustive):
- Nitrate anion: NO
- Carbonate anion: CO$_3^{2-}$
- Benzene skeleton: C$6$H$6$ (delocalized π electrons)

Same connectivity: atoms in the same positions; only electrons move.
Each form must be a valid Lewis structure: obeys octet rule where possible, with feasible lone pairs and bonds.
Total charge remains the same across all resonance forms; sum of formal charges equals the molecule’s formal charge.
Not every molecule has resonance forms; resonance arises when electron delocalization can occur across multiple valid structures.

Total valence electrons available for the molecule:
- $V = ext{sum of valence electrons of all atoms}$
Formal charge for an atom:
- $FC = V - \left(L + \frac{B}{2}\right)$
- where:
- $V$ = number of valence electrons on the atom in the free state
- $L$ = number of electrons assigned to the atom in lone pairs
- $B$ = number of electrons shared in bonds around the atom (each bond counted as two electrons)
When constructing Lewis structures with potential resonance:
- Ensure that the total valence electrons used equals $V$.
- Adjust lone pairs and bonds to satisfy as many octets as possible and place formal charges logically.
- Identify if moving electrons (e.g., lone pair into a bond or π electrons) creates an alternative valid form.

Conceptual link: electron counting, bond formation, and formal charges underpin many branches of chemistry, including organic, inorganic, and physical chemistry.
Real-world relevance:
- Resonance explains bond length variations and partial bond orders observed experimentally.
- Delocalization contributes to molecular stability and reactivity patterns.
Ethical/philosophical note (conceptual): models like Lewis structures and resonance are idealizations that guide predictions; actual electron distribution is a quantum mechanical probability distribution, with resonance forms serving as useful approximations.

Draw a skeleton of the molecule with correct connectivity.
Count total valence electrons: $V = \sum v_i$ .
Distribute electrons to satisfy octets where possible, using bonds and lone pairs.
Check if more than one valid electron distribution exists; identify potential resonance forms.
Compute formal charges for each form:
- $FC = V - \left(L + \frac{B}{2}\right)$
Determine if a resonance hybrid best explains observed properties (bond lengths, stabilities).