Lecture Notes: Chemical Bonding - Lewis Dot Representation
Introduction
- Lecture Focus: Introduction to Lewis Dot Representation and Formal Charge
- Previous Topics: Periodic classification, periodic properties, and initial concepts of chemical bonding (electron sharing and transfer)
Today's Topics
- Lewis Dot Representation and Lewis Theory
- Calculation of Formal Charge
- NCERT Highlights and Key Concepts
Lewis Theory
- Purpose: Used for covalent compounds to represent structures
- Lewis Dot Structure: Shows valence electrons using dots around element symbols
- Example: Hydrogen (H) is represented by 'H.'
Valence Electrons
- Definition: Electrons in the outermost shell
- Importance: Participate in bond formation
- Examples:
- Sodium (Na): 1 valence electron
- Carbon (C): 4 valence electrons
- Oxygen (O): 6 valence electrons
Calculating Valence Electrons in Molecules
- H₂: 2 valence electrons
- CH₄: 8 valence electrons
- NO₃⁻: 24 valence electrons
- NH₄⁺: 8 valence electrons
Lewis Octet Rule
- Octet: 8 electrons in the outer shell for stability
- Duplet: 2 electrons (common for Hydrogen)
- Tendency: Atoms aim to achieve noble gas configuration (Group 18)
Rules for Lewis Dot Structures
- Calculate total valence electrons for bonding in a molecule
- Least electronegative atom is central (unless another rule applies)
- Represent shared and unshared pairs
- Adjust structure to satisfy the octet rule
Examples of Lewis Dot Representation
- H₂: Simple single bond
- NF₃: Nitrogen as central atom, three fluorine atoms attached, lone pairs adjusted
- NH₄⁺: Central nitrogen with four hydrogens, positive charge at central atom
- O₂: Double bond representation
- NO₃⁻: Nitrogen central, resonance structures considered
Formal Charge
-
Definition: Charge on an atom in a molecule
-
Formula:
[
\text{Formal Charge} = \text{Valence Electrons} - \text{Lone Pair Electrons} - \frac{\text{Bonding Electrons}}{2}
]
-
Application: Helps in determining stability and correctness of Lewis structures
Homework Problems
- Calculate formal charge in H₃PO₄ (on phosphorus)
- Calculate formal charge in PH₃ (on all atoms)
NCERT Highlights
- Octet Rule: Stability through noble gas-like electronic configuration
- Lewis Symbols: Represent valence electrons; inert electrons not shown
- Atoms in a molecule: Only outer shell electrons participate in bonding
- Lewis Postulate: Stable configuration when atoms are linked
Conclusion
- Next Steps: Continue practicing Lewis structures and understand formal charges
- Encouragement: Keep smiling, keep learning!
These notes should serve as a comprehensive guide to today's lecture on chemical bonding with a focus on Lewis structures and formal charges.