Transcript CHEMICAL BONDING - Welcome to Westford Academy

Chemical Bonding
Set 5
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Credits
• Thank you to Mr. Neil
Rapp who provided the
bulk of this powerpoint
on his website
www.chemistrygeek.co
m
• Other information
comes from Zumdahl, Steven,
and Susan Zumdahl. Chemistry.
Boston: Houghton Mifflin, 2003.
MOLECULAR
GEOMETRY
MOLECULAR GEOMETRY
VSEPR
• Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion between
electron pairs.
Molecule adopts the shape
that minimizes the electron
pair repulsions.
Some Common Geometries
Linear
Trigonal Planar
Tetrahedral
VSEPR charts
• Use the Lewis structure to determine the geometry
of the molecule
• Electron arrangement establishes the bond angles
• Molecule takes the shape of that portion of the
electron arrangement
• Charts look at the CENTRAL atom for all data!
• Think REGIONS OF ELECTRON DENSITY rather than
bonds (for instance, a double bond would only be 1
region)
Other VSEPR charts
Structure Determination by VSEPR
Water, H2O
2 bond pairs
2 lone pairs
The molecular geometry is
BENT.
The electron pair geometry is
TETRAHEDRAL
Structure Determination by
VSEPR
Ammonia, NH3
The electron pair geometry is
tetrahedral.
lone pair of electrons
in tetrahedral position
N
H
H
H
MOLECULAR GEOMETRY — the positions of
the atoms — is TRIGONAL PYRAMIDAL.
The
You will need to memorize:
• Shapes:
– Linear
– Bent
– Trigonal planar
– Tetrahedral
– Trigonal pyramidal
– Trigonal bipyramidal
– Octahedral
Angles:
180
104.5
120
109.5
107.3
2 @ 180, 3 @ 120
90
Hybridization
• A hybrid occurs when two things are
combined and the result has
characteristics of both
– EX: hybrid car (uses gas and electricity)
• During chemical bonding, different
atomic orbitals undergo
hybridization.
Carbon’s Hybridization
• Consider methane, CH4
• The carbon atom has four valence electrons
with the electron configuration of [He]2s22p2.
– You may expect the two unpaired p electrons to
bond with other atoms and the two paired s
electrons to remain as a lone pair
– However, carbon undergoes hybridization, a
process in which atomic orbitals mix and form
new, identical, hybrid orbitals.
The hybrid orbitals in carbon
Note that each hybrid orbital contains one electron that it can share with
another atom, giving carbon 4 bonding sites. The hybrid orbital is called
an sp3 orbital because the four orbitals form from one s and three p
orbitals.
Other Hybridizations
Anything with a tetrahedral
geometry is a result of sp3
hybridization. Carbon is the
most common example.
Anything with a
trigonal planar
shape is a result of
sp2 hybridization.
AlCl3 and nitrate ion
are examples. Can
result from a double
bond.
Anything with a
linear shape is a
result of sp
hybridization. CO
and CO2 are
common examples.
These often result
from double and
triple bonds.
Lone Pairs & Hybridization
• Lone pairs also occupy hybrid orbitals.
• Compare the hybrid orbitals of BeCl2 (linear)
and H2O (bent).
• Why does the water molecule have sp3
orbitals and the BeCl2 has sp?
– The two O-H bonds occupy two of the sp3 orbitals
in water, and the two lone pairs occupy the other
two.
– Beryllium doesn’t have any lone pairs, so it’s
geometry remains liner with sp hybridization.
Homework Questions
1) Although the VSEPR model is correct in
predicting that CH4 is tetrahedral, NH3 is
trigonal pyramidal, and H2O is bent, the model
in its simplest form does not account for the
fact that these molecules do not have exactly
the same bond angles (<HCH is 109.5 degrees,
as expected for a tetrahedron, but <HNH is
107.3 degrees, and <HOH is 104.5 degrees).
Explain these deviations from the tetrahedral
angle. Use outside resources if necessary.
More Homework
2) Predict the molecular structure and bond angles for
the following:
– HCN, PH3, CHCl3
3) Compare the molecular shapes and hybrid orbitals of
PF3 and PF5 molecules. Explain why their shapes
differ. HINT: PF5 has sp3d hybridization (why would
that be?)
4) List, in a table, the Lewis structure, molecular shape,
bond angle, and hybrid orbitals for molecules of CS2,
CH2O, H2Se, CCl2F2, and NCl3.