Lecture 18: Polyatomic Molecules and Hybridization

  • Polyatomic Molecules - Molecular dipole moment is the sum of the bond dipole vector
  • Molecular dipole and Molecular shape
  • BClBCl \rightarrow Is non-polar because it is trigonal planner
  • Molecular dipole and boiling point
  • If the dipole moment increases, the intermolecular interaction increases and boiling point increases
  • Explanation of why NF3NF_3 is less than NH3NH_3 (dipole moment)
  • Electronegative \rightarrow M=2.2,N=3.0,F=4.0M= 2.2, N=3.0, F=4.0
  • Even though FF is more electronegative, NH3NH_3 has a higher dipole moment
  • When there is a lone pair the vector of the dipole moment is away from the atom
  • In the case of NH3NH_3, HH is less electronegative than NN so all of the force goes up
  • But for NF3NF_3, FF is more electronegative than NN  so the direction is down
  • Therefore, there is a natural, neutral cancelation for NF2NF_2 so NH3NH_3 has a higher dipole moment
  • We consider
  1. Electronegativity
  2. Shape
  3. Lone Pair (Yes/No)
  • To determine dipole moment and polarity
  • Angle of the molecules
  • NF3NF_3  has a lower bond angle
  • This is because of the electro density distribution
  • It is easier to compress the angle because there is more electron repulsion, more space to expand
  • Hybridization - Mixing of 2 or more non-equivalent atomic orbitals
  • Hybrid orbitals have different shapes and orientations than their parent atomic orbitals
  • Number of hybrid orbitals = Number of atomic orbitals
  • Hybrid Atomic Orbitals
  • Why does hybridization happen?
  • This is a more stable state where the angles are larger and less compressed
  • SP3SP^3 Hybridization
  • If there is no hybridization we get 3 longs bonds at 90 C from pp orbitals and one small bond from ss orbital
  • One ss and 3 pp  orbitals will hybridize to form 4 equivalent orbitals
  • Each orbital will have 25% ss characteristics and 75% pp  character
  • These hybridized orbitals will overlap with 1ss orbital of HH atom to form 4 CMC-M bonds
  • From 90C \rightarrow 109.5C
  • Bending in Ethane (C2H6C_2H_6)
  • 2Sp3Sp_3 hybridized carbons
  • 3 hybridized orbitals on each cc atom from bonds with HH atoms
  • Fourth one on each CC now overlaps each other to form a carbon-carbon bond
  • The CCC-C bond is formed by the end to end overlap of the hybridized orbitals
  • This bond is called a sigma bond
  • This model is staggered and eclipsed (switching)
  • In this type of bond the electron density is centred in between the 2 atoms
  • Or we say that there is a cylindrical symmetry for the O bond

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