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Chemistry LectureBasic Medical Course
Department of Medical ChemistryFaculty of Medicine
University of Debrecenwww.medchem.unideb.hu
email: [email protected]
Molecular ShapesValence Bond Theory of Covalent Bonding
László Virág
I. Molecular Shapes
II. The valence bond theory of covalent bonds
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Chemical BondsThree general types
MetallicElectrons free to move among several
neighboring atoms
IonicElectrostatic attraction between ions
CovalentSharing of electrons between 2 atoms
Core electrons
not involved in
bonding
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VALENCE ELECTRONS Electrons in the outermostshell of the atom
BONDING ELECTRONS Number of valenceelectrons shared with other atoms
NON-BONDING valence electrons, not shared with other atomsELECTRONS
Lewis Symbols
Lewis symbols show the valence electrons as dots arranged around the atomic symbol.
hydrogen
sodium
chlorine
Na
H
Cl
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Lewis Structures For Molecules
HF
H2O
NH3
CH4
H F
or H F
H O H
or H O H
H N HH
or H N H
H
H C HH
H
or H C H
H
H
Double and Triple Bonds• Atoms can share four electrons to form a
double bond or six electrons to form a triple bond.
• The number of electron pairs is thebond order.
O2
N2
=O O
N N
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Bond energy
Covalent bond
Molecular Geometry7
Why molecular geometry (shape) is important?
Especially for biologically importantmolecules,shape determines chemistry and therefore function
Enzyme-substratecomplementarity
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Molecular Shapes• Lewis electron-dot structures give us the
connectivity between atoms.• The bond angles in a molecule determine the
shape of the molecule.
C
F
F
F
F
Lewis Structures and Molecular Geometry: VSEPR Theory
• VSEPR theory
• This is used to predict the shape of the molecules
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The VSEPR ModelValenceShellElectronPairRepulsion Theory.
• Assumption: electron pairs repel, so the bonding pairs and lone pairs attached to a central atom are located as far apart from each other as possible.
• Remember to think in three-dimensions!
VALENCE ELECTRONSElectrons in the outermost shell of the atom
BONDING ELECTRONSNumber of valenceelectrons shared with otheratoms
NON-BONDING ELECTRONSValence electrons, not shared with other atoms
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VALENCE SHELL ELECTRON PAIR REPULSION: VSEPR
Electrons in bonds and in lone pairs can be thoughtas „charge clouds”.
Charge clouds repel each other and stay as far apartas possible
Shape of molecule is governed by pairs of valence electrons being as far apart as possible.
Order or repulsion magnitudesnon-bonding - non-bonding > bonding - non-bonding > bonding - bonding pairs
Non bonding (lone) pairs of electrons repel each other and bonding electron pairs more than bonding electrons do.
Lone pair electrons spread out more because they are not confined to the space between atoms.
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The VSEPR Model
• Electron domain geometry: arrangement of electron pairs (charge clouds) around the central atom.
• Molecular geometry: arrangement of atomsaround the central atom.
The VSEPR Model
• To apply VSEPR theory:
1. Draw the Lewis structure.2. Determine the electron domain geometry.3. Determine the molecular geometry.
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1. Write the Lewis structure.
2. Count the number of charge clouds (bonding and and nonbonding electron pairs) around the central atom.
• 2 legs (2 charge clouds) - linear
• 3 legs (3 charge clouds) - trigonal planar
• 4 legs (4 charge clouds) – tetrahedral
• 5 legs (5 charge clouds) – trigonal bipyramidal
• 6 legs (6 charge clouds) - octahedral
3. Look at the atoms and name the shape.
The basic procedure to follow to determine the shape
The VSEPR ModelPairs ofElectrons
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3
Electron-domain geometry
Linear
Trigonalplanar
Bond angles
180o
120o
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Pairs ofElectrons
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5
Electron-domain geometry
Tetrahedral
Trigonalbipyramidal
Bond angles
109.5o
120o and 90o
Pairs ofElectrons
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Electron-domain geometry
Octahedral
Bond angles
90o and 180o
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Determine the molecular geometry and bond angles in the ammonium ion, NH4
+.
Valence Electrons:N = 54H = 4 1 = 4
Charge = 1
Total = 8
The VSEPR Model
N
H
HH
H
N
H
HH
H
N
H
H
H
H
N
H
H
H
H
Four pairs of electrons around N:Electron domain geometry is tetrahedral.
Four atoms around N:Molecular geometry is tetrahedral.
No lone pairs:Bond angles are 109.5o.
N
H
HH
H109.5o
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The VSEPR Model• Lone pairs repel more than bonding pairs.• Therefore, the lone pairs cause the bond
angles to close up.• The following angles are computed using
HyperChem using molecular mechanics:
C
H
HHH
NHH
H
O
H
H107.643o
105.611o
109.471o
104.5º
Determine the molecular geometry in ammonia, NH3.
Valence Electrons:N = 53H = 3 1 = 3
Total = 8
The VSEPR Model
N
H
HH
N
H
HHN H
H
H
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N H
H
H
Four electron pairs around N:
Electron domain geometry is tetrahedral.
Three atoms around N:
Molecular geometry is trigonal pyramidal.
Determine the molecular geometry and bond angles in water.
Valence Electrons:O = 62H = 2 1 = 2
Total = 8
The VSEPR Model
O HH
O HH
O HH
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Four pairs of electrons around O:Electron domain geometry is tetrahedral.
Two atoms around O:Molecular geometry is bent.
Two lone pairs:Bond angle is less than 109.5o.
O HH
O
H
H
105.6o104.5º
Determine the molecular geometry and bond angles in CO2.
Valence Electrons:C = 42O = 2 6 = 12
Total = 16
The VSEPR Model
O C O
O C O
O C O
C OO
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In VSEPR, we count multiple bonds as a single electron domain (single charge cloud).Two pairs of electrons around C:
Electron domain geometry is linear.Two atoms around C:
Molecular geometry is linear.No lone pairs on C:
Bond angle is 180o.
C OO
CO O180o
Determine the molecular geometry and approximate bond angles in XeOF4.
Valence Electrons:Xe = 8O = 64F = 4 7 = 28
Total = 42
The VSEPR Model
Xe
O
F
FF
F
O
F
FF
FXeXe
OF
F F
F
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4 single bond+1double bond+1 lone pair:
Six charge clouds around Xe:Electron domain geometry is octahedral.
Five atoms around Xe:Molecular geometry is square pyramidal.
One lone pair on Xe:O-Xe-F bond angle is less than 90o.
Covalent Bonding and Orbital Overlap
The VSEPR model is a simple method which allows us to predict molecular geometries, but it does not explain why bonds exist between atoms.
How can we explain molecular geometries and the basis of bonding at the same time?
There are 2 theories explaining the formation of covalent bonds:
valence bond theory (discussed today)molecular orbital theory (discussed next year)
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VALENCE BOND THEORY
VALENCE BOND THEORY
The electron-dot structures provide a simple way
to predict the distribution of valence electrons in a molecule.
The VSEPR model provides a simple way to predict molecular shape.
Valence bond theory provides an easily visualised orbital picture of how electron pairs are shared in a covalent bond.
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Valence bond theory
• Valence bond theory is the simplest approach to an orbital picture of covalent bonds
• Combines Lewis' idea of electron pair bonds with electron orbitals (quantum mechanics)
• Each covalent bond is formed by an overlap of atomic orbitals from each atom
• The bond strength is proportional to the amount of orbital overlap
• Each of the bonded atoms maintains its ownatomic orbitals, but the electron pair in theoverlapping orbitals is shared by both atoms.
1s orbital 1s orbital
+
+
2p orbital 2p orbital
1s orbital
+
2p orbital
+
2p orbital 2p orbital
bonds
bond
A covalent bond results when two atoms approach each other closely enough so that a single occupied valence orbital on one atom overlaps a single occupied valence orbital on the other atom.The now- paired electrons in the overlapping orbitals are attracted to the nuclei of both atoms
and thus bond the two atoms together
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Chemical bonds formed due to overlap of atomic orbitals
s-s s-p s-d p-p p-d d-d
H-HLi-H
H-CH-NH-F
H-Pd inPalladiumhydride
C-CP-PS-S
F-Sin SF6
Fe-Fe
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Overlap of two 1s orbitals in H2
• Overlap of two 2p orbitals directed along the bond axis (sigma bond)
• Overlap of p and s orbitals
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Problems with tetrahedral bonds
• In CH4 the bonds are all equivalent and at angles of 109.5°
• The 2p orbitals in C are at 90° - far from optimum for overlap
• The ground state configuration is 2s22p2
• Reconcile these facts with the known structure
Hybridization
• The wave mechanics permits mixing of the atomic orbital set to produce “hybrid” orbitals
• We can say that an imaginary mixing process converts a set of atomic orbitals to a new set of hybrid atomic orbitals or hybrid orbitals.
• Hybridization alters the shape and energy of the original
• In the case of C, the differences between the 2s and 2p are smoothed out and a homogeneous collection of four sp3 hybrid orbitals is produced
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C
METHANE CH4
hybridization
1s
2s
2p
Ener
gy
ground state excited state sp3-hybrid state
2sp3
Hybridization of CarbonHow can carbon form four bonds if two of its valence electrons are already paired ?
sp3 Hybridization
2s2 2p2
sp3 hybrid orbitals
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Tetrehedral Shape of Methane
Tetrehedron is a geometrical solid whose 6 faces are equilateral triangles.
sp3 hybridization
• Formally, one of the 2s electrons is promoted to the empty 2p orbital (an energy cost, which is repaid on bond formation)
• The four basis orbitals are then “hybridized” to yield the set of four sp3
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Tetrahedral directions and sp3 hybrids
Valence bond picture of CH4
• Each C sp3 hybrid contains one electron
• Each H 1s contains one electron
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Lone pairs occupy sp3 hybrid orbitals
• Valence bond picture of the tetrahedral electronic geometry provides same results for the molecules with lone pairs
Molecular geometry: trigonal pyramidal
Ammonia
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sp Hybridization
sp hybridization of carbon in ethyne
2s2 2p2 sp hybrid orbitals 2 p orbitals
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sp hybridization for linear geometry
• One s and one p orbital
sp2-hyridization of the carbon atom
hybridization
2py orbital2px orbital2pz orbital
2s orbital
x
yz
90o
sp2
sp2
sp2
p
side view
p
sp2
sp2sp2
90o
top view
The spatial arrangements of the orbitals in sp2-hybridized carbon
2s2 2p2
sp2 hybrid orbitals1 p orbital
sp2 Hybridization
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sp2 Hybridization
Geometry: trigonal planar
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Sigma and pi bonding
• The hybridized orbitals describe the electronic geometry: bonds along the internuclear axes (sigma bonds)
• The “unused” p orbitals overlap in a parallel arrangement above and below the internuclear axis (pi bonds)
Comparison of pi and sigma bonding
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Pi bonding accounts for bond multiplicity
• Two unused p orbitals in sp hybrid (linear geometry)– Two pi bonds
– N≡N triple bond (one sigma, two pi)
• One unused p orbital in sp2 hybrid (trigonal planar geometry– One pi bond
– C=C double bond (one sigma, one pi)
Valence bond picture of ethylene H2C=CH2
• Sigma bonds between C and H (blue/red) and C and C (blue)– Six electrons around C
• Pi bond between C and C (green)– Two electrons around C
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Valence bond picture of acetylene HC≡CH
• Sigma bonds between C and H (red and blue) and C and C (blue)– 4 electrons around C
• Two pi bonds between C and C (green)– 4 electrons around C
Beyond coordination number 4
• Invoke empty d orbitals (impossible for second row elements)– One d orbital for trigonal
bipyramidal
– Two d orbitals for octahedral
• Number of orbitals in hybrid always equals number of charge clouds
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Trigonal bipyramid – sp3d
Octahedral –sp3d2
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Shortcomings of valence bond
• The orbitals still maintain atomic identity
• Bonds are limited to two atoms
• Cannot accommodate the concept of delocalized electrons – bonds covering more than two atoms
• Problems with magnetic and spectroscopic properties
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„AXE” Method
• The "AXE method" of electron counting is commonly used when applying the VSEPR theory.
• The A represents the central atom and always has an implied subscript one.
• The X represents how many sigma bonds are formed between the central atoms and outside atoms. Multiple covalent bonds (double, triple, etc) count as one X.
• The E represents the number of lone electron pairspresent outside of the central atom.
• The sum of X and E, sometimes known as the steric number, is also associated with the total number of hybridized orbitals used by valence bond theory.
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