# Lecture 18: Polyatomic Molecules and Hybridization

• Polyatomic Molecules - Molecular dipole moment is the sum of the bond dipole vector
• Molecular dipole and Molecular shape
• ﻿$BCl$﻿ ﻿$\rightarrow$﻿ 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 ﻿$NF_3$﻿ is less than ﻿$NH_3$﻿ (dipole moment)
• Electronegative ﻿$\rightarrow$﻿ ﻿$M= 2.2, N=3.0, F=4.0$﻿
• Even though ﻿$F$﻿ is more electronegative, ﻿$NH_3$﻿ 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 ﻿$NH_3$﻿, ﻿$H$﻿ is less electronegative than ﻿$N$﻿ so all of the force goes up
• But for ﻿$NF_3$﻿, ﻿$F$﻿ is more electronegative than ﻿$N$﻿ so the direction is down
• Therefore, there is a natural, neutral cancelation for ﻿$NF_2$﻿ so ﻿$NH_3$﻿ 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
• ﻿$NF_3$﻿ 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
• ﻿$SP^3$﻿ Hybridization
• If there is no hybridization we get 3 longs bonds at 90 C from ﻿$p$﻿ orbitals and one small bond from ﻿$s$﻿ orbital
• One ﻿$s$﻿ and 3 ﻿$p$﻿ orbitals will hybridize to form 4 equivalent orbitals
• Each orbital will have 25% ﻿$s$﻿ characteristics and 75% ﻿$p$﻿ character
• These hybridized orbitals will overlap with 1﻿$s$﻿ orbital of ﻿$H$﻿ atom to form 4 ﻿$C-M$﻿ bonds
• From 90C ﻿$\rightarrow$﻿ 109.5C
• Bending in Ethane (﻿$C_2H_6$﻿)
• 2﻿$Sp_3$﻿ hybridized carbons
• 3 hybridized orbitals on each ﻿$c$﻿ atom from bonds with ﻿$H$﻿ atoms
• Fourth one on each ﻿$C$﻿ now overlaps each other to form a carbon-carbon bond
• The ﻿$C-C$﻿ 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