Lecture Notes: Lewis Structures and Valence Electrons

Introduction

• Focus on Lewis structures (Lewis formulas) in this section.
• Review of valence electrons: outer shell electrons, important in chemical bonding.

Valence Electrons and Periodic Trends

• Valence Electrons: Found in outer S and P orbitals.
• Periodic trend: Group number (ones place) indicates the number of valence electrons.
  • Example: Group 1 elements have 1 valence electron (e.g., 1s1 for hydrogen).

Drawing Lewis Dots

• Represents valence electrons using dots.
• One dot per valence electron, placed around the element symbol.
• Follow Hund's Rule: One dot per side before doubling up.
  • Examples:
    • Boron (Group 13): 3 valence electrons, 3 dots (one on each side).
    • Oxygen (Group 16): 6 valence electrons, 6 dots (two pairs, two singles).
    • Silicon (Group 14): 4 valence electrons, 4 dots (one on each side).

Total Valence Electrons in Molecules

• Calculate total valence electrons to verify structural formulas.
  • Example: Carbon dioxide (CO2) has 16 valence electrons.
  • Example: Phosphorus pentoxide (P2O5) has 40 valence electrons.

Writing Lewis Structures

• Steps:
  1. Draw skeletal structure with the least electronegative element at the center.
  2. Count total valence electrons.
  3. For ions, adjust for charge (+/- electrons).
  4. Ensure all elements have an octet (except hydrogen and helium).
  5. Form double/triple bonds if there are too many electrons.

Example: Nitrogen Trifluoride (NF3)

• Nitrogen is the central atom.
• Valence electrons: Nitrogen (5) + Fluorine (3x7) = 26.
• Draw single bonds and complete octets.
• Check electrons match calculated valence electrons.

Simplifying Lewis Structures

• Charge considerations from periodic table charges.
• Nitrogen typically forms three bonds, one unshared pair.
• For ions, adjust electron count with central atom.

Example: Carbon disulfide (CS2)

• Carbon requires four bonds, sulfur two.
• Use process of elimination to find correct structure.

Advanced Considerations

• Polar bonds and molecules: Polarity can arise within molecules.
• Molecules may have double/triple bonds or not follow the octet rule.
  • Examples:
    • Phosphorus pentachloride (PCl5) has an expanded octet.
    • Boron trifluoride (BF3) does not have an unshared pair of electrons.

Influence on Molecular Shape and Polarity

• Shape and polarity are interconnected.
• Understanding Lewis structures aids in predicting molecular geometry and behavior.