Molecular Shapes, Polarity, and Hybridization - Prof. Julia Metzker, Exams of Chemistry

Download Molecular Shapes, Polarity, and Hybridization - Prof. Julia Metzker and more Exams Chemistry in PDF only on Docsity!

Molecular Shapes

Molecular Orbitals

CHEM 1212 Dr Metzker

Lecture Goals

o Review Molecular Geometry o V-SEP-R Theory o Polarity o Discuss Molecular Orbital Theory

Molecular Geometry

V-SEP-R

o Valence Shell Electron Repulsion Theory o All we need to know: o Electron pairs “need” to be as far away as possible from other electron pairs. o Bonding pairs require less space than non-bonding pairs <--

Domain Geometries

Molecular Polarity

o To determine molecular polarity we sum up the bond dipoles in 3D space. o Some molecules with polar bonds are non- polar o If all atoms exert equal “pull” on the electron density, the molecule will be non-polar o E.g., CCl 4 Cl C Cl Cl Cl

1. Carbon: 4 electron pairs
2. Domain Geometry=Tetrahedral
3. Carbon: no lone pairs
4. Molecular Geometry = Tetrahedral But, is the molecule polar?

o Cl is more electronegative than C o Cl “pulls” harder on electron density o Thus, C-Cl bond has dipole o 4 Cl arranged symmetrically in space each “pulling” with the same force o Thus, no net “pull” for the entire molecule o CCl 4 is nonpolar!! Polarity can be modeled by drawing arrows (+-->) o The nuclei are no longer “pulling” equally in all directions o CHCl 3 is polar! CHCl 3 Replace one chlorine with hydrogen Is the molecule still nonpolar?

Therefore …

o The electron occupies volume (shape) o a.k.a.: the electron orbital o Atomic orbital shapes and energies are determined by the electron quantum numbers ( n,l,ml)

Quantum Number Review

o n ---> shell 1, 2, 3, 4, ... o l ---> subshell 0, 1, 2, ... n - 1 o ml ---> orbital -l ... 0 ... +l o ms ---> electron spin +1/2 and -1/ http://hercules.gcsu.edu/~jmetzker/Reviews/QuantumAtom-EC/index.html

Atomic Orbital Energies

n=1 n=2 n=3 n=

Energy l= l= l= l= l= l= l= l= l= l= ml = 1 0 - ml = 2 1 0 -1 - ml = 3 2 1 0 -1 -2 3 ml = 0

Atomic Orbital Labels

n=1 n=2 n=3 n=

Energy l= l= l=1 l= l= l= l= l= l= l= 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f ml = 1 0 - ml = 2 1 0 -1 - ml = px pz py ml = dxy dxz dx2-y2 dz2 dyz

Atomic Orbital Shapes

Energy 3s 3p 3d

Test Yourself

o What are n and l for an electron in 3d? o (9)_________________ o How many electrons can occupy the 2p subshell? o (10)_________________ o Draw the three p orbitals o (11)_________________

s-Orbital Overlap

How bonds are formed! o“Shapes must agree!!” o e.g., H 2 - 2 Hydrogen atoms (1s^1 ) o σ (sigma) = head on overlap o Atomic orbitals add to form bonding molecular orbital o Atomic orbitals subtract to form anti-bonding orbital o Aufbau filling o lowest energy to highest o Bond Order = o 1/2(bonding - antibonding)

+ = o = 1/2(2 - 0) = 10

• =

p-Orbital Overlap

σ (sigma) = head-on overlap π (pi) = side-on overlap

o Draw the lewis structure for O 2 2+ o (13)_________________ o Use MO diagram to predict bond order o (14)_________________ 2s 2s 2p 2p o Note: The 1s orbitals have been omitted from diagram!

Hybridization

combines orbitals on ONE atom

o What if orbital shapes don’t match molecular shapes? o e.g. carbon: o Tetrahedral geometry 109.5° o px, py, pz are orthogonal (90°) o one s and three p orbitals => 4sp^3 orbitals (109.5°)

A look to organic chemistry: Hybridization

sp

sp

sp:

109.5° 120° 180°

Other hybridization schemes

sp

d:

sp

d

90°,120° 90°,90°