Drawing Chemical Structures: Rules, Exceptions, and Formal Charges

Method for Drawing Chemical Structures

The instructor outlines a systematic approach for drawing chemical structures, emphasizing rules, exceptions, cleanup procedures, and the use of formal charges to resolve ambiguities.

Core Steps for Structure Assignment

1. Identify Substances: Begin with the given chemical formula of the substance, such as $CH<em>2O</em>2^{2-}$, $NO^+$, or $OF_2$.

2. Electron Assignment - Initial Structure: Initially assign electrons based on a preliminary structure.

   • Count Used Electrons: Determine the number of electrons already used in the bonds and lone pairs in the initial structure (e.g., in an example, $8$ electrons were implicitly used: $2$ for bonds, $6$ for lone pairs around an atom, totaling $8$). The goal is to compare this to the total available valence electrons (not explicitly stated how to calculate total valence electrons, but implied as a prerequisite).

3. Assign Remaining Electrons to Terminal Atoms: Any electrons not used in the initial structure are then assigned to the terminal atoms.

   • Rule: Assign in a manner that does not exceed the octet rule (or duo rule for hydrogen).

   • Example Implication: If $12$ electrons remain (from an unstated total), these would be distributed to terminal atoms, perhaps $6$ on one and $6$ on another, provided it doesn't violate the octet rule.

   • Visual Check: The structure on paper should visually represent the electron distribution accurately.

Cleanup and Formatting

• The primary intention after initial assignment is to "clean up" the structure, which includes ensuring all atoms satisfy bonding rules and electron counts.

• A "filter" method is mentioned, though not detailed, as part of the cleanup process.

Special Considerations for Central Atoms (Third Period and Beyond)

For molecules where the central atom is from the third period or below, additional orbital considerations come into play, allowing for expanded octets.

• Periodic Table Reference: Attention is drawn to the periodic table, specifically the electron configuration.

• Orbital Setup: For third-period elements, the relevant orbitals are:

  • $3s^2$

  • $3p^6$

  • $3d$

• Significance of $3d$ Orbital: The presence of the empty $3d$ orbital is crucial, as it allows third-period elements (and beyond) to accommodate more than eight valence electrons, thus permitting expanded octets.

• Octet Fulfillment: Elements most assured of fulfilling the octet rule are generally noble gases; however, many other elements also strive for this (e.g., arrangements like three lone pairs and a single bond, characteristic of halogens in certain compounds, achieve octet).

• Problem of Multiple Structures: When following the initial drawing method, it's possible to arrive at multiple plausible structures (e.g., "three structures" were generated in an example, likely indicating resonance structures or isomers).

Resolving Multiple Structures: Formal Charges

To determine the most plausible or stable structure among multiple possibilities, formal charges are used.

• Method: Calculate the formal charges for each atom in each possible structure.

• Rules for Formal Charges: While specific rules were not detailed in the transcript, their existence is implied as a system for evaluating structures.

Examples and Concepts Discussed

• Carbon: As a central atom, carbon typically forms four bonds because it has $4$ valence electrons.

• Nitrogen: Has $5$ valence electrons.

• Sulfur: Has $6$ valence electrons.

• Dynamic Nature of Electrons (Resonance): The concept that electrons are "running around all the time" and that structures can exist in a state of "musical chairs" implies resonance. This means that multiple valid Lewis structures (resonance forms) contribute to the overall description of the molecule, and the actual molecule is a hybrid of these forms.

• The discussion around specific examples like a carbon-centered molecule, nitrogen, and sulfur reinforces the application of these rules to common elements in chemical structures.