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VSEPR Theory
Electron pairs surrounding an atom repel each other. This is referred to as Valence Shell Electron Pair Repulsion (VSEPR) theory.
NOT based on how many electron pair there are, but where they are.
Where they are: electron groups (charge clouds)
Each group of valence electrons around a central atom is located as far as possible from the others, to minimize repulsions.
C
H
H
H
HO C O
C
O
H H
N
HH H
OH H
C NH
What counts as an electron group?
1) A connection between the atom of interest and any other atom
a) single, double, and triple bonds each count one electron group
2) A lone pair on the atom of interest
SO O
S
O
O
O
O HH
VSEPR Theory
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VSEPR Theory
The electron pair geometry gives the arrangement of atoms AND the lone pair electrons around the central atom.
There are five basic arrangements of electron groups around a central atom.
VSEPR Theory
The electron pair geometry gives the arrangement of atoms AND the lone pair electrons around the central atom.
There are five basic arrangements of electron groups around a central atom.
The molecular shape is the three–dimensional arrangement of nuclei joined by the bonding groups. This is defined only by the relative positions of the nuclei.
Classified by AXE
AXmEn
A =
X =
E =
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For CO2, the central C atom has two atoms attached (the two oxygen atoms) and has no lone pair.
2 electron groups –
Hybridization –
The electron pair geometry is –
The molecular shape is –
Bond angle is –
Linear Molecules
O C O
Trigonal Planar For H2CO, the central C atom has three atoms attached and has no lone pair.
3 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angle is
C
O
H H
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Trigonal Planar In ozone, O3, the central O atom has two atoms attached (the other 2 oxygens)
and has one lone pair.
3 electron groups
Hybridization –
The electron pair geometry is
The molecular shape is
Bond angle is
OO O
S
O O
Tetrahedral In SiCl4, the central Si atom has four atoms attached (four chlorine atoms) and
has no lone pair.
4 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angle is
Cl
Si
Cl
Cl
Cl
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Tetrahedral In NH3, the central N atom has three atoms attached (three hydrogen atoms) and
has one lone pair.
4 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angle is
N
HH H
Tetrahedral In SCl2, the central S atom has two atoms attached (two chlorine atoms) and has
two lone pair.
4 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angle is
Cl
S
Cl
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Trigonal Bipyramidal In PCl5, the central P atom has five atoms attached (five chlorine atoms) and has
no lone pair.
5 electron groups
Hybridization
The electron pair geometry is t
The molecular shape is
Bond angles
Cl
P
Cl
Cl
Cl
Cl
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Trigonal Bipyramidal In SF4, the central S atom has four atoms attached (four fluorine atoms) and has
one lone pair.
5 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angles
F
S
F
F
F
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Trigonal Bipyramidal In BrF3, the central Br atom has three atoms attached (three fluorine atoms) and
has two lone pair.
5 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angles
Br
F
F
F
Trigonal Bipyramidal In XeF2, the central Xe atom has two atoms attached (two fluorine atoms) and has
three lone pair.
5 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angle is
Xe
F
F
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Octahedral In SF6, the central S atom has six atoms attached (six fluorine atoms) and has
no lone pair.
6 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angles
S
F
F F
FF
F
Octahedral In BrF5, the central Br atom has five atoms attached (five fluorine atoms) and has
one lone pair.
6 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angles
Br
F
F F
FF
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Octahedral In XeF4, the central Xe atom has four atoms attached (four fluorine atoms) and
has two lone pair.
6 electron groups
Hybridization
The electron pair geometry is
The molecular shape is
Bond angles
XeF F
FF
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Electronegativity and Polarity
A covalent bond in which the shared electron pair is not shared equally, but remains closer to one atom than the other, is a polar covalent bond.
If “X” and “Y” share bonding e- equally:
If “X” and “Y” do NOT share bonding e- equally:
Unequal sharing of bonding e- leads to polar covalent bonds
Polar Covalent Bonds
Polar: having poles
N S
One end opposite from the other
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Electronegativity and Polarity
A covalent bond in which the shared electron pair is not shared equally, but remains closer to one atom than the other, is a polar covalent bond.
Unequal sharing of electrons causes the more electronegative atom of the bond to be partially negative and the less electronegative atom to be partiallypositive.
The ability of an atom in a covalent bond to attract the BONDING electrons towards itself is called its electronegativity.
If electronegativity (EN) difference is:Electronegativity difference is < 0.5 then the bond is considered to be pure covalent
C C C S I HBr Br
2.0 > electronegativity difference ≥ 0.5 then the bond is considered to be polar covalent
C O Si OF H N C
Electronegativity difference is ≥ 2.0 then the bond is considered to be ionic
Al :F Ca :O Na :Cl Rb :N
Polarity Revisited
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The lowercase Greek letter delta, , is used to indicate a polar bond.
The MORE EN element has extra e–, so it is negative and is indicated by the symbol –.
The LESS EN element is short of e–, so it is positive and is indicated by the symbol +.
H–Cl
Electronegativity and Polarity
Give delta notation and polarity arrows for the following:
C O Si OF H N C
Electronegativity and Polarity
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Molecular Polarity
A molecule is polar if
- it contains one or more polar bonds and
- the individual bond dipoles do not cancel (shape).
Overall molecular polarity depends on both shape and bond polarity.
The polarity of a molecule is measured by its dipole moment (μ), which is given in the unit debye (D).
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