chemical bonding molecular geometries vsepr...
TRANSCRIPT
Ionic Bonding
Resonance Structures
VSEPR
Basic Shapes
3-D Notation
Hybridization (Lab)
Molecular Geometries
Octet Rule Polar Molecules
Lewis Structures Covalent Bonding
Types of Bonds
Chemical Bonding
• Chemical bond: attractive force holding two or more
atoms together.
• Covalent bond results from sharing electrons between
the atoms. Usually found between nonmetals.
• Ionic bond results from the transfer of electrons from a
metal to a nonmetal. Ionic compounds that form
electrolytes conduct electricity.
• Metallic bond: attractive force holding pure metals
together. Sea of electrons.
Chemical Bonds, Lewis Symbols, and the Octet Rule
Figure 8.3: Ionic Bonding
Figure 8.5: Covalent Bonding
Chemical Bonds
Bond Type Single Double Triple
# of e’s 2 4 6
Notation — =
Bond order 1 2 3
Bond
strengthIncreases from Single to Triple
Bond length Decreases from Single to Triple
Strengths of Covalent Bonds
Lewis Symbols
Chemical Bonds, Lewis Symbols, and the Octet Rule
The Octet Rule
• All noble gases except He has an s2p6 configuration.
• Octet rule: atoms tend to gain, lose, or share electrons
until they are surrounded by 8 valence electrons (4
electron pairs).
• Caution: there are many exceptions to the octet rule.
Chemical Bonds, Lewis Symbols, and the Octet Rule
Bond Polarity and Electronegativity
Electronegativity
• Electronegativity: The ability of one atoms in a
molecule to attract electrons to itself.
• Pauling set electronegativities on a scale from 0.7 (Cs) to
4.0 (F).
• Electronegativity increases
• across a period and
• down a group.
Figure 8.6: Electronegativities of Elements
Electronegativity
Bond Polarity and Electronegativity
Figure 8.7: Electronegativity and Bond Polarity
• There is no sharp distinction between bonding types.
• The positive end (or pole) in a polar bond is represented
+ and the negative pole -.
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Drawing Lewis Structures
Follow Step by Step Method (See Ng Web-site)
1. Total all valence electrons. [Consider Charge]
2. Write symbols for the atoms and guess skeleton structure
[ define a central atom ].
3. Place a pair of electrons in each bond.
4. Complete octets of surrounding atoms. [ H = 2 only ]
5. Place leftover electrons in pairs on the central atom.
6. If there are not enough electrons to give the central atom
an octet, look for multiple bonds by transferring
electrons until each atom has eight electrons around it.
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Lewis Structures – Examples - I
Lewis Structures – Examples - II
Exceptions to the Octet Rule
Central Atoms Having Less than an Octet
• Relatively rare.
• Molecules with less than an octet are typical for
compounds of Groups 1A, 2A, and 3A.
• Most typical example is BF3.
• Formal charges indicate that the Lewis structure with an
incomplete octet is more important than the ones with
double bonds.
Exceptions to the Octet Rule
Central Atoms Having More than an Octet
• This is the largest class of exceptions.
• Atoms from the 3rd period onwards can accommodate
more than an octet.
• Beyond the third period, the d-orbitals are low enough in
energy to participate in bonding and accept the extra
electron density.
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• There are five fundamental geometries for molecular
shape:
Molecular Shapes: VSEPR
Molecular Shapes – 3D Notations
VSEPR (Ballons)-Movie Clip
Figure 9.3
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Summary of VSEPR Molecular Shapes
e-pairs Notation Name of VSEPR shape Examples
2 AX2 Linear HgCl2 , ZnI2 , CS2 , CO2
3 AX3 Trigonal planar BF3 , GaI3
AX2E Non-linear (Bent) SO2 , SnCl2
4 AX4 Tetrahedral CCl4 , CH4 , BF4-
AX3E (Trigonal) Pyramidal NH3 , OH3-
AX2E2 Non-Linear (Bent) H2O , SeCl2
5 AX5 Trigonal bipyramidal PCl5 , PF5
AX4E Distorted tetrahedral
(see-sawed)
TeCl4 , SF4
AX3E2 T-Shaped ClF3 , BrF3
AX2E3 Linear I3- , ICl2
-
6 AX6 Octahedral SF6 , PF6-
AX5E Square Pyramidal IF5 , BrF5
AX4E2 Square Planar ICl4- , BrF4
-
See Ng Web-siteHyperChem CyberChm Gems
Examples: VSEPR Molecular Shapes - I
# electron
pairs on
Central
Atom A
Notation Example Lewis VSEPR & Name of Shape
2AX2
2 bp on A
3
AX3
3 bp on A
AX2E
2 bp and
1 lp on A
Examples: VSEPR Molecular Shapes – I – F08
Examples: VSEPR Molecular Shapes - II
# electron
pairs on
Central
Atom A
Notation Example Lewis VSEPR & Name of Shape
4
AX4
4 bp on A
AX3E
3 bp and
1 lp on A
AX2E2
2 bp and
2 lp on A
Examples: VSEPR Molecular Shapes – II – F08
Examples: VSEPR Molecular Shapes - III
# electron
pairs on
Central
Atom A
Notation Example LewisVSEPR & Name of
Shape
5
AX5
5 bp on A
AX4E
4 bp and 1
lp on A
AX3E2
3 bp and 2
lp on A
AX2E3
2 bp and 3
lp on A
Examples: VSEPR Molecular Shapes – III – F08
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Examples: VSEPR Molecular Shapes - IV
# electron
pairs on
Central
Atom A
Notation Example Lewis VSEPR & Name of Shape
6
AX6
6 bp on A
AX5E
5 bp and
1 lp on A
AX4E2
4 bp and
2 lp on A
The Effect of Nonbonding Electrons
• By experiment, the H-X-H bond angle decreases on
moving from C to N to O:
• Since electrons in a bond are attracted by two nuclei, they do
not repel as much as lone pairs.
• Therefore, the bond angle decreases as the number of lone pairs
increases
VSEPR Model
104.5O107O
NHH
H
C
H
HHH109.5O
OHH
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Figure 9.10: Shapes of Larger Molecules
• In acetic acid, CH3COOH, there are three central atoms.
VSEPR ModelHyperChem
Shapes of Larger Molecules
• In glycine (simplest amino acid), NH2CH2CO2H, there are four possible central atoms.
• Draw the Lewis Structure and the 3D VSEPR Molecular Geometry for glycine. Indicate the name of
the shape for all possible central atoms, including estimation of bond angles.
• Hint 1: Designate the 2nd carbon in the formula as the central atom in skeleton structure.
• Hint 2: The acid portion of glycine is the same as that of acetic acid.
Lewis-VSEPR HW assigned 10/29/10 . Due 11/1/10.
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Solution Key
Figure 8.10: Drawing Lewis Structures
Resonance Structures
Figure 9.12
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Figure 9.11: Molecular Shape and Molecular Polarity
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Figure 9.13: Molecular Shape and Molecular Polarity
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• Lewis structures and VSEPR do not explain why a bond
forms.
• How do we account for shape in terms of quantum
mechanics?
• What are the orbitals that are involved in bonding?
• We use Valence Bond Theory:
• Bonds form when orbitals on atoms overlap.
• There are two electrons of opposite spin in the orbital overlap.
Covalent Bonding and Orbital Overlap
Gems - Movie Clip
Figure 9.14: Covalent Bonding and Orbital Overlap
• To determine the electron pair geometry:
• draw the Lewis structure,
• count the total number of electron pairs around the central atom,
• arrange the electron pairs in one of the above geometries to
minimize e--e- repulsion, and count multiple bonds as one
bonding pair.
VSEPR Model (Figure 9.6)
VSEPR
Model
Drawing Lewis Structures
Formal Charge
• Consider:
• For C:
• There are 4 valence electrons (from periodic table).
• In the Lewis structure there are 2 nonbonding electrons and 3
from the triple bond. There are 5 electrons from the Lewis
structure.
• Formal charge: 4 - 5 = -1.
C N
Drawing Lewis Structures
Formal Charge
• Consider:
• For N:• There are 5 valence electrons.
• In the Lewis structure there are 2 nonbonding electrons and 3
from the triple bond. There are 5 electrons from the Lewis
structure.
• Formal charge = 5 - 5 = 0.
• We write:
C N
C NCyberChm Gems
Easy ways to remember shapes• Steric number- the number of atoms
bonded to the central atom of a molecule
plus the number of lone pairs on the central
atom. It is often used in VSEPR theory
(valence shell electron-pair repulsion
theory) in order to determine the particular
shape, or molecular geometry, that will be
formed.
Molecular Geometry Steps1. Memorize the structures in groups
according to steric number/ Bonding
number.
A. Steric number 2 = 1 structure
B. Steric number 3 = 2 structures
C. Steric number 4 = 3 structures
D. Steric number 5 = 4 structures
E. Steric number 6 = 3 structures.
Molecular Geometry Steps2. Now you have to memorize the structures in each
steric number group.
A. Steric number 2 = Linear (180 degrees)
B. Steric number 3 = Bent and Trigonal Planar (Both 120
degrees)
C. Steric number 4 = Tetrahedral, Trigonal Bipyramidal
and Bent again. (109.5, 107.3 and 104.5 degrees)
D. Steric number 5 = Trigonal Bipyramidal, See-Saw, T-
Shaped and Linear (Mixture of 90, 120 and 180
degrees)
E. Steric number 6 = Square Planar, Square Pyramid and
Octahedral (All 90 degrees) (Last one kind of breaks
the rule but I still find this helps. )
Molecular Geometry Steps3. Try to find a relationship between all the groups.
A. For groups 2 and 3, shapes pretty straightforward.
B. Group 4 are all tetrahedral shaped molecules (Think
group "4" / "Tetrahedral") with slight variations on the
bond angles based on the amount of lone pairs.
• Tetrahedral is the most uniform shape and it has a bond
angle closest to 120, with a slight repulsion from the
uppermost atom making a bond angle of 109.5.
• Trigonal Bipyramidal has a lone pair of electrons which
pushes the "side atoms" a little further away from 120
forming an angle of 107.3.
• Lastly, this variation of bent has two lone pairs of
electrons pushing the "side atoms" even further away
making an angle of 104.5.
Molecular Geometry Steps cont.C. Group 5, are the "weird shapes". This includes the
trigonal bipyramidal, see-saw, T-shape and linear. T
shape is 90 degrees because in the letter T, only 90
degree angles are present. Linear is 180 degrees. See-saw
has 3 angles, 1 angle along the top of the seat (180), 1
between the seats and the legs (90), and one between the
two legs (120). and trigonal Bipyramidal has 2 angles, 90
and 120 degrees.
D. Group 6 is the "square group" including: Square planar,
square pyramid and octahedral. (I know the last one isn’t
a square but, close enough). All squares consist of only
90 degree angles so these all only have 90 degree angles.
Using All of this info you can intuitively put together the
notation for each structure.
Ionic Bonding
Resonance Structures
VSEPR
Basic Shapes
3-D Notation
Hybridization (Lab)
Molecular Geometries
Octet Rule Polar Molecules
Lewis Structures Covalent Bonding
Types of Bonds
Chemical Bonding
Lewis
AXE notation
VSEPR shapes
Polarity