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Expanded valence shells (extended octets)
more than
8e
-^
around a central atom
- Extended octets are formed only by atoms with
vacant
d
-orbitals in the valence shell (
p
-elements
from the
third or later periods
- Extended octets form when:
- There are too many electrons (
n
need
< n
rem
) or
more than 4 atoms are bonded to the central atom–
electron-rich structures
→
place the extra
electrons at the central atom
lower formal charges
can be
achieved by forming an extended octet
Example:
Write the Lewis structure of
XeF
4
n
tot
= 8(Xe) + 4
×
7(F) = 36
n
rem
n
need
= 0(Xe) + 4
×
6(F) = 24
n
need
< n
rem
extra
e
-^
add
lone pairs at
Xe
Example:
Write the Lewis structure of
I
3
-.
n
tot
×
7(I) + 1(charge) = 22
n
rem
n
need
= 4(I) + 2
×
6(I) = 16
n
need
< n
rem
extra
e
add
extra lone pair at the central
I
atom after
completing the octets for all atoms
Example:
Select the favored resonance
structure of the
PO
4
3-
anion.
Formal charges:(a) O
P
(b) O–
P
O=
b
) has an extended octet (
e
the
P
atom
b
) is more favored (contributes more
to the resonance hybrid) due to the lowerformal charges
9.4 Using Lewis Structures and BondEnergies to Calculate
H
of Reaction
- Any reaction can be represented as a two step
process in which:– All
reactant bonds break
to give individual atoms
product bonds form
from the individual atoms
Average bond enthalpies can be used to estimatethe enthalpy changes of reactions in the gas phase(only approximate values)
H
o r
H
B
(broken)
H
B
(formed)
- Energy is absorbed (+) to break the bonds of the
reactants and emitted (-) during forming the bonds of theproducts
Example:
Estimate the standard enthalpy of the
reaction
CH
(g) 4
+ 2F
2
(g)
CH
F 2
2
(g)
+ 2HF
(g)
Lewis structures are needed to get the bond order
Bonds broken (reactants):
4 C–H
(412 kJ/mol)
, 2 F–F
(158 kJ/mol)
Bonds formed (products): 2 C–H
(412 kJ/mol)
, 2 C–F
(484 kJ/mol)
, 2 H–F
( kJ/mol)
H
o^
H
B
(broken)
H
B
(formed)
= [
×
×
158]
- [
×
×
×
565] = -958 kJ
(this value is only an estimate, the exact value canbe calculated using
H
o f data)
AX
2
,^
AX
3
,^
AX
4
,^
AX
5
AX
6
- All electron-groups surrounding the central atom
are
bonding groups
- If all surrounding atoms (
X
) are the same, the
bond angles
are equal to the characteristic angles
of the arrangement
molecular shape
and electron-group
arrangement have the same name:
Examples:
Linear shape, bondangle of 180
Trigonal planar shape,bond angles of 120
Examples:
Tetrahedralshape, bondangles of109.
Trigonalbipyramidalshape, bondangles of120 and 90
- The VSEPR model treats multiple bonds in
the same way as single bonds (a singlebonding electron-group)
Example: CO
3
2-
Three
atoms attached to a central atom (
AX
Trigonal planar shape,
bond angles of 120
Any one of the resonance structures can be used to predict the molecular shape
double bonds
on
bond angles
Example: Cl
2
CO
Three
atoms attached to the
central atom, no lone pairs (
AX
Trigonal-planar shape →
The
bond angles deviate
from
the ideal values →
The double bond has greater
e
density and repels the singlebonds stronger ⇒∠
Cl-C-Cl < 120
Cl-C-O > 120
