AS.030.103 Study Guide - Midterm Guide: Valence Bond Theory, Pauli Exclusion Principle, Trigonal Planar Molecular Geometry
Lecture 25:
Quantum Mechanics and Molecular Structure (slide 31 start):
~conceptual framework/language of describing chemical bonds
~explains bonding patterns of molecules (size, shape, energy, spectrum)
Advanced Theories of Chemical Bonding:
Atomic orbitals -> molecules
Molecular Orbital Theory (Robert Mullikan, 1896-1986): valence e- are
delocalized, valence e- in orbitals spread over entire molecule
Valence Bond Theory (Linus Pauling): valence e- are localized between atoms (or
are lone pairs), half-filled atomic orbitals overlap to form bonds
*******Sigma Bond:
Central Themes of Valence Bond Theory (Localized Electron Model):
Basic Principle of Valence Bond Theory:
Covalent bond forms when the orbitals from two atoms overlap and a
pair of e- occupies region betw two nuclei
1. Maximum overlap: bond strength depends on coulombic attraction betw the
shared e- and two nuclei; the greater the orbital overlap, the stronger the
bond
2. Spins pair: two e- in the overlap region occupy the same space and therefore
have opposite spins (Pauli exclusion principle)
3. Hybridization: valence atomic orbitals in a molecule are different from those in
the isolated atoms
Lewis Diagrams:
1. Determine total # of valence e-
2. Use pairs of e- to connect other atoms to central atom, and then
distribute the rest as lone pairs
Paramagnetic: magnetic state of an atom with one or more unpaired e- (ex: O2,
clings to magnet)
Diamagnetic: characterized by paired e-, no unpaired e- (ex: N2, not attracted to
magnet)
Lecture 26:
*Shapes of molecules are not predicted using Lewis diagrams!
Formal charge = group number - # of e- in lone pairs – ½ # of e- in bonding pairs
Resonance forms:
Lewis diagrams suggests that O3 has a single and double bond, but the
bonds are the same length in reality (lewis fails)
Actual bonding is resonance hybrid!
Breakdown of Octet Rule:
1. Odd – e- molecules
2. Octet-deficient molecules
3. Valence shell expansion
*******VSEPR (Valence Shell Electron Pair Repulsion):
Structure determined by minimizing repulsion of e- pairs
*******Predictions:
CO2=linear
BF3=trigonal planar
CH4=tetrahedral
PF5=trigonal bipyramidal
SF6=octahedral
*some modification of angles occurs with lone pairs are present
3 types of repulsion (when lone pairs present):
1. Bonding pair against bonding pair
2. Bonding pairs against lone pair
3. Lone pair against lone pair
Lone pairs occupy more space (closer to nucleus)
Sigma Bond Formation:
sp Hybridization:
Pauling-modify valence body (VB) approach with orbital hybridization
Document Summary
Quantum mechanics and molecular structure (slide 31 start): ~explains bonding patterns of molecules (size, shape, energy, spectrum) Molecular orbital theory (robert mullikan, 1896-1986): valence e- are delocalized, valence e- in orbitals spread over entire molecule. Valence bond theory (linus pauling): valence e- are localized between atoms (or are lone pairs), half-filled atomic orbitals overlap to form bonds. Central themes of valence bond theory (localized electron model): Lewis diagrams: determine total # of valence e, use pairs of e- to connect other atoms to central atom, and then distribute the rest as lone pairs. Paramagnetic: magnetic state of an atom with one or more unpaired e- (ex: o2, clings to magnet) Diamagnetic: characterized by paired e-, no unpaired e- (ex: n2, not attracted to. *shapes of molecules are not predicted using lewis diagrams! Formal charge = group number - # of e- in lone pairs # of e- in bonding pairs.
