Kössel-Lewis Approach to Chemical Bonding

🔑 Key Idea

Atoms bond to achieve stable electron arrangements like noble gases (8 electrons in outer shell, called an octet). Helium is stable with 2 electrons (duplet).

⚡️ How Bonding Works

  • Ionic Bond: Atoms transfer electrons to form positive and negative ions (e.g., Na⁺ and Cl⁻ form NaCl). The bond is due to electrostatic attraction between ions.
  • Covalent Bond: Atoms share electron pairs to complete their octet (e.g., two Cl atoms share electrons to form Cl₂).

✍️ Lewis Symbols & Structures

Lewis symbols use dots to show valence electrons:

  • Li· (1 dot), ·Be· (2 dots), ·B· (3 dots), ·C· (4 dots), ·N· (5 dots), ·O·· (6 dots), ··F·· (7 dots), ··Ne·· (8 dots)
  • Number of dots = valence electrons!

Drawing Lewis structures:

  1. Count total valence electrons (add extra electrons for negative ions; subtract for positive ions).
  2. Place the least electronegative atom in the center (e.g., C in CH₄, N in NF₃).
  3. Connect atoms with single bonds.
  4. Add lone pairs or double/triple bonds to satisfy the octet rule (H needs duplet).

Example: CO molecule
Total valence electrons = 4 (C) + 6 (O) = 10. Structure: :C≡O: (triple bond).

⚠️ Octet Rule Exceptions

  • Incomplete octet: Some atoms have <8 electrons (e.g., Be in BeH₂, B in BCl₃).
  • Odd-electron molecules: Molecules with unpaired electrons (e.g., NO, NO₂).
  • Expanded octet: Central atoms with >8 electrons (e.g., P in PF₅, S in SF₆).

🧮 Formal Charge

Formula:
\[ \text{Formal charge} = \left( \begin{array}{c} \text{Valence} \\ \text{electrons} \\ \text{in free atom} \end{array} \right) – \left( \begin{array}{c} \text{Non-bonding} \\ \text{electrons} \end{array} \right) – \frac{1}{2} \left( \begin{array}{c} \text{Bonding} \\ \text{electrons} \end{array} \right) \]

Example (O₃): Central O has formal charge = +1, one terminal O = 0, other = -1.

⚡️ Ionic Bond Formation Factors

  • Low ionization energy (easy to form cations).
  • High negative electron gain enthalpy (easy to form anions).
  • Strong electrostatic attraction in crystal lattice (e.g., NaCl has rock salt structure).

🎯 NEET Must-Knows

  1. Octet rule exceptions (incomplete octet, expanded octet, odd-electron molecules).
  2. Lewis structures for molecules like CO, NO₂⁻, and O₃.
  3. Formal charge calculation and its significance.
  4. Ionic bond formation using ionization energy & electron gain enthalpy.
  5. Covalent vs. ionic bonds (electron transfer vs. sharing).

Keep practicing Lewis structures—you’ll ace it! 💪✨