What NEET Asks
- Direct questions on drawing Lewis structures for simple molecules/ions.
- Identification of molecules/ions that obey or violate the octet rule.
- Questions involving formal charge calculation for specific atoms within a structure.
- Conceptual understanding of why atoms bond to achieve stability.
Key Points
- Lewis Dot Structure: Represents valence electrons as dots around element symbols, showing bonding and lone pairs.
- Valence Electrons: Outermost shell electrons participating in bonding. Group number often equals valence electrons (for main group elements).
- Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight electrons in their outermost shell for stability (like noble gases).
- Duplet Rule: For Hydrogen (H) and Helium (He), stability is achieved with two electrons.
- Steps to Draw: Count total valence electrons; identify central atom; draw single bonds; distribute remaining electrons as lone pairs; form multiple bonds if central atom lacks octet.
- Octet Rule Exceptions:
- Incomplete Octet: Be, B, Al (e.g., BeCl₂, BF₃)
- Expanded Octet: Elements from 3rd period onwards (P, S, Xe, Cl) can accommodate >8 electrons (e.g., PCl₅, SF₆).
- Odd-Electron Molecules: Molecules with an odd number of valence electrons (e.g., NO, NO₂).
Must-Know Formula / Reaction
Formal Charge (FC) = (Total number of valence electrons in free atom) - (Total number of non-bonding electrons) - (1/2 * Total number of bonding electrons)
- Explanation: Helps in choosing the most plausible Lewis structure by minimizing formal charges.
Common Mistakes
- Students often miscount total valence electrons, especially for ions (add electrons for negative charge, subtract for positive).
- Don't confuse lone pair electrons with lone pairs (e.g., 2 lone pairs = 4 lone pair electrons).
- Forgetting to check for octet rule exceptions, particularly expanded octets for 3rd-period elements.
Rapid Revision
Lewis structures depict valence e- arrangement for bonding. Octet rule (8 e-, duplet for H) drives stability. Follow steps: count valence e-, place central atom, form bonds, add lone pairs, adjust for octets. Remember exceptions for Be, B, Al (incomplete) and 3rd period+ (expanded).