ELECTRONEGATIVITY - SUMMARY
Electronegativity Difference (∆x) &
• Covalent versus Ionic Bonds:
xA & xB = electronegativity of atoms A & B, respectively.
!x = | xA - xB | = magnitude of the electronegativity
difference between atoms A & B.
If
!x = 0 (Exactly)
AB bond is purely covalent (or non-polar).
If
0 < !x ≤ 1.7
AB bond is polar covalent, i.e., the electrons are
“polarized” toward with larger x.
If !x > 1.7
AB bond is ionic, i.e., the electrons in the bond are
“totally owned” by atom with larger x (anion) & “totally
lost” by atom with smaller x (cation).
WRITING LEWIS STRUCTURES Begin page 4 of Lewis /VSEPR
1. ORDER OF CONNECTION of the atoms.
• More than one… ISOMERS.
2. COUNT TOTAL # of VALENCE ELECTRONS.
• ANION: PO4-3 = 5 + (4•6) + 3 = 32
• CATION: NH4+ = 5 + (4•1) - 1 = 8
EVALUATE BONDING REGIONS:
4. IONIC: [1A (not H), 2A (not Be)] w/ [6A, 7A].
Cations (1A, 2A) & Anions (6A, 7A).
5. COVALENT (non-polar or polar):
• TRY SINGLE BONDS FIRST
• ADD LONE PAIRS (:) as needed (octet/duet).
• DO BOOKKEEPING - MUST MATCH w/ TOTAL.
IF NO MATCH…
1
2
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
WRITING LEWIS STRUCTURES - (continued)
ELECTRON COUNT DOESN’T MATCH …
• USUALLY “OVERDRAWN”…used too many.
• XS electrons =
(e- total from single bonded structure) - (tot. val. e-)
• XS e-’s ÷ 2 = # of ADDITIONAL BONDS (sharing) needed.
• Put in MULTIPLE BONDS in all possible ways …
RESONANCE structures.
• MORE THAN ONE ARRANGEMENT of ELECTRONS
among a FIXED ARRANGEMENT of ATOMS.
H2O (water) - O is the central atom.
• 2(1) + 6 = 8 total valence electrons.
• Each O-H bond polar covalent.
• Single bonded structure works:
• Octet for O & Duet for each H.
H
••
O
••
H
SOME EXAMPLES …
3
4
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
O2 :
+
H
H
N
• (2)(6) = 12 total valence electrons.
• Single bonded structure does NOT work!!
TOTAL e-’s USED
TOTAL VALENCE eXS e-’s
XS e-’s ÷ 2
H
H
••
••
••
••
O
O
••
is the central atom.
5 + (4)(1) - 1 = 8 total valence electrons.
Each N-H bond polar covalent.
Single bonded structure works:
• Octet for N & Duet for each H.
••
NH4+ N
•
•
•
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
= 14
= 12
= 2
= 1 ---> CREATE 1 MORE BOND
MORE …
5
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
O2 : FINAL STRUCTURE:
••
O
••
6
CN- : • 4 + 5 + 1 = 10 total valence electrons.
• Single bonded structure does NOT work!!
••
O
••
•
•C
••
••
A DOUBLE BOND … O-O BOND ORDER = 2.0 .
TOTAL e-’s USED
TOTAL VALENCE eXS e-’s
XS e-’s ÷ 2
MORE ….
••
N
••
!
•
•
= 14
= 10
= 4
= 2 ---> CREATE 2 MORE BONDS
MORE …
7
8
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
LEWIS STRUCTURES - EXAMPLES (pp. 7 - 9) :
CN- : FINAL STRUCTURE:
Li2O : •
•
•
•
•
!
N
••
••
C
••
Li +1
MORE ….
-2
••
Li +1
O
••
A TRIPLE BOND … C-N BOND ORDER = 3.0 .
O is the CENTRAL ATOM.
(2)(1) + 6 = 8 total valence electrons.
Li is in 1A & O is in 6A.
Each Li-O BOND is IONIC:
Li “GIVES” & O “TAKES” - FORM IONS.
••
• NO BOND ORDERS … IONIC BOND.
9
LEWIS STRUCTURES - EXAMPLES (page 9) :
O3 :
LEWIS STRUCTURES - EXAMPLES (page 9) :
• (3)(6) = 18 total valence electrons.
• Single bonded structure does NOT work!!
O
••
••
= 20
= 18
= 2
= 1 ---> CREATE 1 MORE BOND
••
••
O
O
O
••
••
••
O
O O
••
••
I
II
••
TOTAL e-’s USED
TOTAL VALENCE eXS e-’s
XS e-’s ÷ 2
O
••
••
••
••
••
O
••
••
MORE THAN ONE WAY TO DO THIS…RESONANCE…
••
••
10
HOW DO WE CHOOSE?…FORMAL CHARGE…
MORE THAN ONE WAY TO DO THIS…RESONANCE…
11
12
PAGE 6 - LEWIS/VSEPR :
PAGES 7-9 - LEWIS/VSEPR :
RETURN TO PREVIOUS Examples - Check FC assignment.
FORMAL CHARGE on an ATOM in LEWIS STRUCTURE
measures STRAIN on the VALENCE of and ATOM due to
OCTET.
• H2O - WITH FORMAL CHARGES SHOWN:
H
••
O
••
H
• FC on O = 6 - [4 + (1/2)•(4)] = 0 (ZERO)
• FC on EACH H = 1 - [0 + (1/2)•(2)] = 0 (ZERO)
FCX = (valence of X) - [ (# bonds to X) + (# non-bonded e-‘s around X) ]
• NH4+ - WITH FORMAL CHARGES SHOWN:
H
• SUM OF FC’s = NET CHARGE on species:
• NEUTRAL molecule - SUM = ZERO (0).
• ION - SUM = NET CHARGE on ION.
• NH4+ - SUM = +1 &
• PO4-3 - SUM = -3
H
N
+
H
H
• FC on N = 5 - [0 + (1/2)•(8)] = + 1
• FC on EACH H = 1 - [0 + (1/2)•(2)] = 0 (ZERO)
13
PAGES 7-9 - LEWIS/VSEPR :
RETURN TO PREVIOUS Examples - Check FC assignment.
PAGES 7-9 - LEWIS/VSEPR :
RETURN TO PREVIOUS Examples - Check FC assignment.
• O2 - WITH FORMAL CHARGES SHOWN:
••
14
Li2O -WITH FORMAL CHARGES SHOWN:
••
O•• O
••
• CN- - WITH FORMAL CHARGES SHOWN:
!
••
••
Li+1
• FC on EACH O = 6 - [4 + (1/2)•(4)] = 0 (ZERO)
••
O
••
-2
Li+1
••
••
• FC on O = 6 - [8 + 0] = -2
• FC on EACH Li = 1 - [0 + 0] = +1
C N
• FC on N = 5 - [2 + (1/2)•(6)] = 0 (ZERO)
• FC on C = 4 - [2 + (1/2)•(6)] = - 1
BACK TO O3 ….
15
16
PAGE 9 - LEWIS/VSEPR : BACK to OZONE … O3
••
••
PAGE 11 - LEWIS/VSEPR : ISOCYANATE ANION… OCN• 16 total valence electrons - All bonds covalent.
••
••
••
I
• EQUAL “STRAINS” in BOTH I & II.
• PAIR OF ELECTRONS in DOUBLE BOND - DELOCALIZE
Spread out ELECTRON DENSITY over - 3 OR MORE ATOMS.
• NEED QUANTUM MECHANICS to understand…
• EACH O-O bond “SHARES” the 2nd bond so: Each O-O
bond has BO = 1 (of its own) + 1/2 (of 2nd bond) = 1 1/2.
-
••
O C N
••
II
-
-2
••
O C N
••
••
O
C N
••
••
-
••
II
••
••
I
••
••
O
O O
••
••
••
O
O
O
••
••
••
III
SEPARATING GOOD Formal Charge from BAD:
• “MINIMIZE”....BUILD-UP SHOULD BE AS LOW AS
POSSIBLE...“SPREAD OUT THE BURDEN” .
• IF THERE MUST BE “BURDEN”
• NEGATIVE FC on MOST ELECTRONEGATIVE ATOM.
• POSITIVE FC on LEAST ELECTRONEGATIVE ATOM.
17
18
PAGE 12 - LEWIS/VSEPR : OCN- - the RESULTS:
••
-
••
••
II
O C N
••
NOTE - Do NOT CONFUSE:
• RESONANCE STRUCTURES - MOVING ELECTRONS... with
• ISOMERS - MOVING ATOMS.
-
III
BEST
MAJOR
WORST
MINOR
Resonance structures of OCN- isomer:
-2
••
••
C
N
••
C
N
O
II
C
••
N
••
-
III
ISOMERS: OCN- versus NOC- versus ONC-
••
••
O
-
••
O
••
••
I
••
N
••
••
“THE” structure of OCN- is (approximated) to be:
C
••
O
••
••
"In Between"
MINOR
-2
••
••
••
••
O C N
I
••
O
C N
••
••
••
PAGE 12 - LEWIS/VSEPR : OCN- - the RESULTS:
19
20
PAGE 13 - LEWIS/VSEPR : IMPORTANT & USEFUL:
BONDING PATTERNS that lead to ZERO (0) Formal Charge
PAGE 13 - LEWIS/VSEPR : IMPORTANT & USEFUL:
BONDING PATTERNS that lead to ZERO (0) Formal Charge
• Column 4A [X could be C, Si, Ge, ...] FCX = 4 - (4 + 0) = 0
(4 bonds & 0 lone pairs)
• Column 5A [X could be N, P, As, ...] FCX = 5 - (3 + 2) = 0
(3 bonds & 1 lone pair)
••
or
X
X
••
or
X
or
X
••
X
or
X
21
PAGE 13 - LEWIS/VSEPR : IMPORTANT & USEFUL:
BONDING PATTERNS that lead to ZERO (0) Formal Charge
PAGE 13 - LEWIS/VSEPR : IMPORTANT & USEFUL:
BONDING PATTERNS that lead to ZERO (0) Formal Charge
• Column 6A [X could be O, S, Se, ...] FCX = 6 - (2 + 2•2) = 0
(2 bonds & 2 lone pairs)
••
••
or
• Column 7A [X could be F, Cl, Br, ...] FCX = 7 - (1 + 3•2) = 0
(1 bond & 3 lone pairs)
••
••
X
••
X
22
X
••
••
23
24
EXCEPTIONS TO OCTET - EXPANDED OCTET (pp. 14 - 17) :
EXPANDED OCTET (pp. 14 - 17) :
PO4-3 - Phosphate anion
• 5 + 4•6 + 3 = 32 total valence electrons.
• All bonds (polar) covalent - FC’s shown.
••
•
•O
••
!1
P
+1
•
•O •
•
••
••
•O
•
••
! 3
!1
••
•
•O •
•
!1
P
+1
•
•O •
•
••
••
••
O ••
!1
!1
••
••
O ••
!1
What about FC build-up….NOT OPTIMUM for a -3 anion…
• -3 should be “spread out” as much as possible (3 • -1).
• On most electronegative atom (Oxygen).
• Zero (0) FC everywhere else.
CAN WE DO BETTER.?
!1
TOTAL e-’s USED
TOTAL VALENCE eXS e-’s
! 3
!1
••
•
•O •
•
= 32
= 32
= None - a valid Lewis structure
25
26
EXPANDED OCTET - PO4-3 (continued):
Page 14 - Expanded Octet :
CAN THERE BE MORE THAN 8?
-3
• P atom is in the third row of the periodic table.
3rd row - valence shell is n = 3: 3s, 3p, & 3d subshells.
• More than 8 electrons can be accommodated.
••
•• O
••
P
••
••
O ••
O
••
••
•• O ••
••
• Possibility may become reality if:
• The atom is a central or internal atom.
• Better formal charges result. Let’s check this out.
P
•• O ••
P “fits” the role … [Example: p. 15-17, “Lewis-VSEPR”]
P
••
O ••
••
•• O ••
••
-3
••
•• O ••
••
•• O
••
-3
•• •
•• O
•
•• O ••
••
O ••
••
-3
•• •
•• O
•
••
•• O
••
P
••
••
O
•• O ••
••
BEST FORMAL CHARGES …ALL “EQUAL” …
27
28
PAGE 17 - EXPANDED OCTET:
Pages 18 - 20 Exceptions to Octet : Electron Deficient
QUANTUM MECHANICS RETURNS - DELOCALIZATION
1 bond delocalized over
• P-O Bond Order = 1 + 4 bonding regions or sites
1
= 1+4
• Beryllium (Be) - QUARTET (4) …NOT OCTET. FC ratrionalizes
BeCl2 - 16 total valence e-’s ; covalent bonds.
1
=14
+1
••
We might depict the structure in the non-Lewis form:
O
P
+1
••
Cl•• Be Cl••
-3
O
-2
O
-2
••
••
••
+2
Cl Be Cl
••
••
+2
-2
••
Cl Be Cl•• ••
• “BAD” Formal Charge… QUARTET of e-’s for Be:
O
••
Four equivalent P-O bonds
each with Bond Order = 1 1/4
••
••
••
••
Cl Be Cl
•
•
29
Pages 18 - 20 Exceptions to Octet : Electron Deficient
BE CAREFUL…GOOD FC DOES NOT WARRANT
“LESS THAN 8” :
• Boron (B) - SEXTET (6)…NOT OCTET. FC rationalizes
BCl3 - 24 total valence e-’s ; covalent bonds.
-1
••
••
••
+1
••
B ••Cl••
Cl
••
-1
•• ••
••
••
Cl
••
••
•
B Cl
•• •
Cl
••
Cl ••
+1
••
••
Cl
••
-1
• Octets more important.
••
• Must be experimental evidence for “less than 8” species.
(Be & B).
B Cl
••
+1
•• Cl ••
••
• “BAD” Formal Charge… SEXTET of e-’s for B:
•• ••
Cl
••
B
••
30
NEXT ….GEOMETRY…VSEPR…
•• •
•
Cl
••
Cl ••
••
31
32
Pages 21-23 LEWIS/VSEPR - VSEPR Model:
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
• Electrons in bonds - Bonding Pairs (BP) &
• Non-bonded electrons - Lone Pairs (LP)
• LP’s need “more room”… push BP’s “closer together”…
“jockey for position” to achieve a stable GEOMETRY:
SN = 2
IDEAL = LINEAR 180º
X A X
Valence Shell Electron Pair Repulsion model:
• PARAMETER to assign “Ideal” Geometry -
LP = 0
• Steric Number (SN) - For atom “X” in Lewis structure:
O C O
H Be H
= (# of atoms bonded around X) + (# of LP’s around X)
33
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
SN = 4
SN = 3 Trigonal Planar (120º)
3 Trigonal sp2 (Later) 0 Trigonal planar
planar
X
(120o angles)
X
sp2 (Later)
1
A
Bent (< 120o)
O
IDEAL = TETRAHEDRAL
X
SO2 (
S
B
A
X
H
H
LP = 0
F
C
H
109º28'
X
X
BF3
F
X
34
109º28'
H
tetrahedral
F
LP = 1
119.5o)
N
H
< 109º28'
H
H
trigonal pyramidal
LP = 2
O
H
O
H
< 109º28'
bent
35
36
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
SN = 5 IDEAL = TRIGONAL BIPYRAMIDAL
SN = 5 IDEAL = TRIGONAL BIPYRAMIDAL
Axial (A)
X
Equatorial (E)
Equatorial (E) X
A X
X Equatorial (E)
X
Axial (A)
Axial (A)
X
X Equatorial (E)
Equatorial (E) X
A
X Equatorial (E)
X
Axial (A)
Cl
LP = 0
Cl
Cl
Cl
P
Cl
trigonal bipyramidal
LP = 1
F
S
F
LP = 2
120º E
90º A
F
Cl
F
F
T-shaped
F
LP = 3
F
F
Xe
F
Linear
See-Saw
37
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
38
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
SN = 6 IDEAL = OCTAHEDRAL
X
X
LP = 0
F
F
X
A
X
F
S
F
SN = 6 IDEAL = OCTAHEDRAL
X
X
X
X
F
LP = 2
F
F
F
I
A
X
X
X
F
Octahedral
LP = 1
X
F
F
Xe
F
F
Square Planar
F
F
Square Pyramidal
39
40
Pages 21-23 - LEWIS/VSEPR - VSEPR Model:
POLAR BOND IF
Page 25 : Polar versus Non-Polar Molecules :
∆XAB = |XA - XB| ≠ 0
!
B
!
A
µNET in Debyes (D)
0
1.94
1.46
0
1.86
1.60
1.03
Molecule
H2
H2 O
NH3
CH4 or CCl4
CH3Cl
CH2Cl2
CHCl3
Bond Dipole Moment in direction shown
IF XB > XA .
µ = dipole moment = ! • r
1 Debye (D) = 3.34 x 10-30 C•m
µNET = vector sum of µbond 's = 0
NON-POLAR
µNET = vector sum of µbond 's ≠ 0
POLAR
41
Pages 24 - 26: Polar versus Non-Polar Molecules :
SN = 2
IDEAL = LINEAR
42
Pages 24 - 26: Polar versus Non-Polar Molecules :
SN = 3
180º
IDEAL = TRIGONAL PLANAR 120º
X
X
A
X
NON-POLAR
X
2 Identical Bonds to 2 Identical Atoms
A
X
NON-POLAR
3 Identical Bonds to 3 Identical Atoms
OTHERWISE … POLAR
OTHERWISE … POLAR
43
44
Pages 24 - 26: Polar versus Non-Polar Molecules :
Pages 24 - 26: Polar versus Non-Polar Molecules :
SN = 5 IDEAL = TRIGONAL BIPYRAMIDAL
SN = 4
Axial (A)
IDEAL = TETRAHEDRAL 109º28'
X
Equatorial (E)
X
X
A
X
A
Equatorial (E)
Equatorial (E)
X
X
Axial (A)
NON-POLAR
X
X
X
NON-POLAR
5 Identical Bonds to 5 Identical Atoms
OR
4 Identical Bonds to 4 Identical Atoms
Axial (A)
NON-POLAR
Y
Equatorial (E)
X
A
Y
OTHERWISE … POLAR RECALL: CH2Cl2 (1.60 D)
X
X
Equatorial (E)
Equatorial (E)
Axial (A)
3 Identical A-X Bonds Equatorial +
2 Identical A-Y Bonds Axial
45
46
Pages 24 - 26: Polar versus Non-Polar Molecules :
Pages 24 - 26: Polar versus Non-Polar Molecules :
SN = 6 IDEAL = OCTAHEDRAL
SN = 6 IDEAL = OCTAHEDRAL
X
X
X
A
X
Y
X
OR
X
6 Identical Bonds to 6 Identical Atoms
Y
OR
X
X
A
Y
X
X
Z
X
A
Y
X
Z
NON-POLAR
2 Identical A-X Diangonal + 2 Identical A-Z Diagonal
+ 2 Identical A-Y Axial
NON-POLAR
4 Identical A-X Square Planar + 2 Identical A-Y Axial
OTHERWISE … POLAR
MORE ….
47
48