drawing good lewis structures molecular shapechemistry.bd.psu.edu/jircitano/413ch5.pdf ·...

1

Always.

Drawing Good Lewis Structures

1. # valence e– in atoms (± charge) must = # e– in structure.

2. Determine connectivity: least EN usually central, avoid small

rings, H always terminal (1 e–).

6. Minimize formal charge (# and distribution of + and –).

4. Remove required e– in pairs from central atom.

5. Move e– pairs from outside atoms to bond with central atom

to complete octet again.

3. Complete octet for each atom (except H); check against #1.

Valence-Shell Electron-Pair Repulsion

VSEPR

regions of e– ρ around central atom repelled as far as possible

Molecular Shape

1957

Ronald J. Gillespie

English

Ronald S. Nyholm

Australian

regions of e ρ around central atom repelled as far as possible

•• = = =

linear trigonal planar tetrahedral trigonalbipyramidal

octahedral

x x x xx

5 6180o 90o 90o 90o 90o

2 3 4

linear

180o

bent120o

trigonal planar

120o

bent109.5o

trigonal pyramidal tetrahedral

109.5o

VSEPR and Deviations

linear T-Shaped see-saw trigonal bipyramidal

120o

square planar square pyamidal octahedral

90o

deviate from ideal when lone-pair involved

bonding-bonding < Lp-bonding < Lp-Lp

0

–2

H2 Molecule

ergy

(eV

)

i t

1

23

0.5

–4

r (Å)1.0

En experiment

Overlap Symmetry

crosssection

crosssection

s p

dSσ > Sπ > Sδ

Valence Bond Treatment: CH4

½(s + px + py + pz)

½(s + px – py – pz)

½(s – px + py – pz)

½(s – px – py + pz)H

4 sp3 orbitals

sp3

2s

2p

weighted average E

E

C

HH

H

2

sp2 Hybrid Orbitals

sp2

trigonal planar

√3

1s +

√3

√2px

√3

1s –

√6

1px +

√2

1py

√3

1s –

√6

1px –

√2

1py

sp2

2s

2pE

2p

sp Hybrid Orbitals

sp

linear

√2

1(s + pz)

√2

1(s – pz)

sp2s

2pE

2p

Other Hybrid Orbitals

s – px + py

s + px

s – px – py

pz + dz2

pz – dz2

s – px – dz2

s + px + dz2

s + pz – dz2 – dx

2– y

2

s + py – dz2 – dx

2– y

2

s – px – dz2 + dx

2– y

2

s – p – d 2 – d 2 2

d2sp3dsp3

axial

equatorial

s – py – dz2 – dx

2– y

2

trigonalbipyramidal

octahedral

Hybrid Orbitals and Bond Strength

bond strength S

sp > sp2 > sp3

SC-C

SC-H

3 2

0.4

0.5

0.6

0.7

0.8

0.9

S

s character S

25% s33% s50% s

sp3 sp2 sp

0.30 20 40 60 80 100

% s character

Multiple Bonds

Multiple bonds from π (and δ) overlap.

ClO3–

O Cl O

O

•••• ••

•• ••

•••• – Cl has low E d orbitals

ClO

O

Osp3

sp2

sp2

sp2

Non-VSEPR Molecule

N(SiH3)3

D3h not C3v

Si

SiSi

SiN

more bonds, lower E

sp2 N

low E d on SiSi Si

N

3

MO Treatment H2

E

gy

no e– density between nuclei – antibonding (u)

1s

E = E

1s 1s

E

En

erg

H2 lower energy

than 2 H by 2 x E.

e– density between nuclei – bonding (g)

1s

MO Treatment H2

1s

Bond Order = (# of bonding e– – # of antibonding e–)/2

gy

1s 1s

1s

En

erg

Bond Order = (2 – 0)/2 = 1

MO Treatment He2

1s

Bond Order = (# of bonding e– – # of antibonding e–)/2

gy

1s 1s

1s

En

erg

Bond Order = (2 – 2)/2 = 0

No energy advantage: He2 does not exist.

–

Molecular Orbitals

π*2p g–

S depends on E and symmetry: SAB > 0, bonding: E stabilizedSAB < 0, antibonding: E destabilizedSAB = 0, nonbonding: no stabilization

Sσ > Sπ > Sδ

+

–

+

+

–

π2p u

*2p u

2p g

*1s u

1s g

+

s-p Energy Separation in First Row Elements

B C N O F

En

ergy 2s

2s

2p2p

2p2p

2p

Complicated by s-p mixing

when s and p close in E.

Changes relative MO E’s.

B C N O FB C N O F

2s

2s

2s

E (eV) 5.7 8.8 12.4 16.5 21.6

En

ergy

*2p2p

*2s2s

Homonuclear Diatomic MO Diagram

2px 2py 2pz 2pz 2py 2px

*2p

*2p

2p

2s 2s

2p

2p

*2s

2s

Complicated by s-p mixing

when s and p are closer in

E (early elements).

4

Homonuclear Diatomic MO Diagram

6u

2g

5g2px 2py 2pz 2pz 2py 2px

No longer named after

AO. Numbered and

symmetry (u or g) given.

1u

2s 2s

g

4u

3g

MO Diagram: Li2, Be2, B2, C2, N2

6u

2g

5gLi


1u

2s 2s

g

4u

3g

Li2

bond order

Li Li


6u

2g

5gB


1u

2s 2s

4u

3g

Be2

bond order

Be Be


6u

2g

5gB


1u

2s 2s

g

4u

3g

B2

bond order

B B


6u

2g

5gC


1u

2s 2s

g

4u

3g

C2

bond order

C C


6u

2g

5gN


1u

2s 2s

g

4u

3g

N2

bond order

N N

5

MO Diagram: O2, F2, Ne2

*2p

*2p

2pO


2s 2s

2p

2p

*2s

2s

O2

bond order

O O


*2p

*2p

2pF


2s 2s

2p

2p

*2s

2s

F2

bond order

F F


*2p

*2p

2pN


2s 2s

2p

2p

*2s

2s

Ne2

bond order

Ne Ne

Bond Length 1/Bond Order

*2p

*2p

2p

Superoxide dismutase (SOD)

O +

order length, pm

2p

2p

*2s

2s

O O

O2+

O2

O2–

O22–

x2–y2 z2xzyzxy z2 xz yz x2–y2xy

Cr2: Bonds

bond order*z2

*xz, yz

*s

*x2–y2, xy

Cr2

y yy y yy

4s3d

4s3d

z2

xz, yz

s

x2–y2, xy

z2

xz, yz

x2–y2, xy

Cr Cr

2s

nb

2p

2p

Carbon Monoxide MO Diagram

2s

2s

nbC O

6

b1*

a1*

b22p

A1

B1

H2O

O

HH

O

HH

O

2a1

2s

b2

b1

1a1

p

O 2 H

A1

A1 B1 B2

O

HH

O

HH

HH

O

HH(~98% O)

CH4 SALC

A1= H1 1s + H2 1s + H3 1s + H4 1s

T = H1 1s – H2 1s + H3 1s – H4 1s

T = H1 1s – H2 1s – H3 1s + H4 1s

T = H1 1s + H2 1s – H3 1s – H4 1s

bonding orbitals

A1 T

BH

H

H

BH

H

H H: A1g= 1s

B SALC: A1g= sp3 + sp3

B2u= sp3 – sp3

D2h

Diborane, B2H6: 3-Center, 2 e– Bond

H

B, B

A1g

B2unb

A1g

3 0

3.5

4.0

4.5

y BrCl

F

Electronegativity: Periodic Property

Fr

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 60 80 100Atomic Number

Ele

ctro

neg

ativ

ity

At

Cs

I

Rb

Br

KNaLi

70

95

120

y (M

J/m

ole)

NeFO

Cl

Total Energy

–5

20

45

–1 0 1 2 3

Oxidation State

Tot

al E

ner

gy

E = q + q2

E = IE or EA

q = ionic charge

where

6

8

10

12

14

tal E

ner

gy

Cl

N

Mulliken-Jaffe Electronegativity

Cl larger than Na

–4

–2

2

4

0 1–1

Charge

Tot Na

7

6

8

10

12

14

tal E

ner

gy

Electronegativity Equalization

E = 11.0q + 5.7q2

E 2 8 2 3 2

Cl

N

Na0.51+Cl0.51–

–4

–2

2

4

0 1–1

Charge

Tot E = 2.8q + 2.3q2Na

A B

A B

=

+ δa a

b b

11.0 2.8 = 0.51

2(5.7) + 2(2.3)

drawing good lewis structures molecular shapechemistry.bd.psu.edu/jircitano/413ch5.pdf ·...

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