Vsepr-shapes-of-molecules -illustrated Graphic- .pdf

VSEPR & THE SHAPES OF MOLECULES Using Valence Shell Electron Pair Repulsion Theory

Shape

VSEPR is a model used to predict shapes of molecules. Electron pairs repel each other, and adopt an arrangement that minimises repulsion. To find the shape, a Lewis structure can be drawn, or use the following method (assumes single bonds only):

pa irs

o

fb o nd

in g & lo

LINE

LE A NG

SHAPE & BOND OF

No.

KEY

NAM E

A SUMMARY OF THE MOLECULE SHAPES PREDICTED BY VALENCE SHELL ELECTRON PAIR REPULSION THEORY

ne

AR - 180˚

2

electron pairs

2 b.p.

TRI

PL ANAR -

Find the number of electrons the central atom normally has in its valence shell.

2

Add one electron for every atom that the central atom is bonded to.

3

Add or subtract electrons to account for charges if the molecule is charged.

4

Divide the number arrived at by two to find the number of electron pairs.

5

Subtract no. of atoms bonded to the central atom to find no. of lone pairs.

6

Arrange electron pairs in the correct shape.

Bonding Pairs & Lone Pairs

BENt ~119˚

1 20˚

3

NAL GO

1

Lone pairs lie closer to the central atom, and hence repel more than a bonded pair. The order of strengths of repulsion is:

electron pairs

2 b.

NA

Lone Pair Repulsion

T - ~104.5˚ BEN

L PYRAMIDAL -

TRIG

O

HEDRAL - 109

LONE PAIR/LONE PAIR > BONDED PAIR/ LONE PAIR > BONDED PAIR/BONDED PAIR

p. , 1 l .p.

7˚ ~10

TR A

.5˚

4

TE

3 b.p.

Each lone pair reduces the bond angle by

APPROXIMATELY 2.5 DEGREES

electron pairs

3 b.

TRIGON

SA

IPYRAMIDAL -

(if there are 4+ electron pairs arranged around the central atom, ignore repulsions at >90˚ angles)

p. , 2 l .p.

APED - 90˚ T-SH

ORSE - <90˚/<1 H W

LINE

AR - 180˚

20˚

LB

120˚ 90˚/

5

A

4 b.p.

2 b.

p. , 1 l .p.

Isoelectronic species

electron pairs

PYRAMIDAL -

E UAR

P L A N AR -

4 b.

p. , 2 l .p.

SQ

SQU A

RE

2 b.

p. , 2 l .p.

9

p. , 3 l .p.

Transition metal compounds

APED - 90˚ T-SH

LINE

3 b.

2 b.

AR - 180˚

0˚

DRA TAHE L - 90˚

3 b.

˚ ~90

6

OC

p. , 1 l .p.

4 b.

5 b.p.

FAILS FOR:

electron pairs

6 b.p.

5 b.

p. , 1 l .p.

p. , 3 l .p.

R.K. MALIK'S NEWTON CLASSES , RANCHI NEWTONCLASSES.NET

p. , 4 l .p.