valence-shell electron-pair repulsion(vsepr) theory
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Theory which predicts the geometric arrangement
of atoms in a molecule. The valence electron pairs
occupy positions as far as possible from one
another. All electron pairs are considered.
In VSEPR theory double and triple bonds are
treated as single bonds.
Valence-Shell Electron-Pair
Repulsion(VSEPR) Theory:
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Number of
Electron Pairs
Number of
Lone Pairs
Geometry Example
4 0 Tetrahedral
4 1 Trigonal pyramidal
4 2 Bent
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Valence-Shell Electron-Pair
Repulsion(VSEPR) Theory:
In VSEPR Theory double and triple bonds are
treated the same as single bonds.
Lone-pair
Lone-pair
repulsion
Lone-pair
Bonding-pair
repulsion
Bonding-pair
Bonding-pair
repulsion
>>
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Valence Bond Theory:
A covalent bond results when a singly occupied
valence orbital of one atom overlaps a singly
occupied valence orbital of another atom.
Consider H2 molecule.
H: 1s1 1s1:H
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Hybridization:
Consider H2. According to valence bond theory
each atom contains a single unpaired electron.
Each atom can form a single bond.
Consider oxygen(O2). Each O atom should be able
to form two bonds.
O1:
1s 2s 2p
O2:
1s 2s 2p
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Consider CH4.
Hybridization:
4H:
1s
C:
1s 2s 2p
According to Valence Bond theory, C should
only form 2 bonds. We know it form 4 bonds.
Why??
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Hybridization:
Hybridization is the mixing of atomic orbitals.
sp3 ¼ s character
¾ p character
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Reference: http://davidjohnewart.com/Chemistry/chemtheft/12.html
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Reference: http://classnotes.org.in/class11/chemistry/chemical-bonding-molecular-structure/hybridisation/
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If two equal and opposite charges are separated by
a known distance, the dipole moment is defined as
follows.
Dipole Moment() = Q r
Q: charge r: distance between charges
Ex: + -
HF polar molecule
Expressed in Debye Units(D).
1 D = 3.3310-30 Cm
Dipole Moment:
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Factors Determining the Polarity of a Molecule:
1. Dipole moment.
2. Shape of the molecule.
Polar vs. Non-polar Molecules:
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MO Theory:
Consider oxygen(O2).
According to Lewis and Valence Bond
Theory, O2 has all paired electrons. Not so!!!
O1:
1s 2s 2p
O2:
1s 2s 2p
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MO Theory:
When atoms bond, atomic orbitals(AO’s) combine
to form molecular orbitals(MO’s).
# AO’s = # MO’s
Orbital interaction can be additive or subtractive.
Subtractive – Do not promote bonding. Higher
energy. Ex: σ*
Additive – Promote bonding. Lower energy. Ex: σ
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Consider MO diagram for H2.
Reference: https://www.pinterest.ca/pin/322851867015938399/
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=# 𝐛𝐨𝐧𝐝𝐢𝐧𝐠 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐧𝐬 − # 𝐚𝐧𝐭𝐢𝐛𝐨𝐧𝐝𝐢𝐧𝐠 𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐧𝐬
𝟐
Bond Order:
Bond Order is a measure of the number of electron
pairs shared between atoms.
Bond
Order
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Molecular Orbitals from p orbitals:
Reference: https://chem.libretexts.org/LibreTexts/Mount_Royal_University/Chem_1201/Unit_4%3A_Chemical_Bonding_II_-
_Advanced_Bonding_Theories/4.11%3A_Multiple_Bonds_in_MO_Theory
Same for
px and py.
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MO Diagram for Oxygen(O2) :
O O