Lewis diagrams and Molecular Shape

Four electron pairs define the tetrahedral shape family, as in SiCl4

There is a direct link between Lewis diagrams and molecular shape

The Valence Shell Electron Pair Repulsion theory states a molecule’s shape is determined by the electron pairs that surround central atoms

These electron pairs (EP) include both bond pairs (BP) and lone pairs (LP)

We define five shape families based on the need to accommodate mutually repelling electron pairs around a central atom

The balloons illustrate these “natural” shapes

The 5 Shape Families

The five shape families that electron pairs develop are illustrated above

Linear – Trigonal-planar – Tetrahedral – Trigonal-bipyramidal – Octahedral

In this case each molecule has all its electron pairs as bond pairs around the central atom

Not all the shape-determining electrons pairs need be bond pairs

General Shape Family Scheme (1)

0 LP 2 BP

1 LP 1 BP X X..

X....2 EP’s 3 EP’s

0 LP 3 BP 1 LP 2 BP 2 LP 1 BP

X

..X

Lone pairs and shape: 4 Electron Pairs

Linear

Hydrogen Fluoride, HF1 bond pair3 lone pairs

When there are four electron pairs, the shape family is tetrahedral

Four possibilities exist: with 0, 1, 2 or 3 lone pairs

0 LP – tetrahedral shape

1 LP – trigonal-pyramidal shape

2 LP – bent shape

3 LP – linear shape

General Shape Family Scheme

X X: X:......X:

4 BP 0 LP 3 BP 1 LP 2 BP 2 LP 1 BP 3 LP

General Shape Family Scheme

5 BP 0 LP 3 BP 2 LP 2 BP 3 LP 1 BP 4 LP

X X..

4 BP 1 LP

X.. .. ..

X .... X

..

..

....

Special features of 5 EP shapesIn the trigonal bipyramidal geometry, alone among the 5 shape families, the sites are not identical

There are three equivalent sites which are identical to those in trigonal-planar, with bond angles of 120 . These are called the equatorial sites

Axial Sites - above and below the plane in a mutually linear relationship.

Note - the angles are different

Note - if there are LP’s, these will always first occupy equatorial sites

General Shape Family Scheme

X..X X

.... ..

X.. ..

..X

........X

......

..

6 BP 0 LP 5 BP 1 LP 4 BP 2 LP 3 BP 3 LP 2 BP 4 LP 1 BP 5 LP

Some special features of VSEPRmultiple bonds - occupy almost the same volume of space

- considered as single BP of electrons

Ex) NO2

LP are considered to be larger than BP because constrained by only a single positively charged nucleus as opposed to two for a BP

For 5 EP since neighboring LP’s at 120 experience much less repulsion than at 90o they tend to cluster on the equatorial sites of the trigonal-bipyramidal geometry

Bent shape

For 6 EP case LP prefer to be opposite since otherwise at 90o they would experience strong repulsive forces

N has 4 EP’s

1 LP & 3 BP’s

One double bond

Exercise Predict the shape of NO3-

4 EP’s

4 BP’s

3 Bonds

Triangular planar

SO42-

S

O

•• ••

• •

O• •

••O ••

••

• •

O

•• ••

-1

-1

6 EP’s

6 BP’s

4 Bonds

Tetrahedral

N

O

O

O

-1

SO

OO

O

-1

-1

N

O

•• ••

• •O• •

• •O• •

••

]-•

•

POF2-

P

O

•• ••

• •

F• •

• •

•• ••

• •

F• •

-15 EP’s

4 BP’s

1 LP

Trigonal pyramid

PF

OF

• •-1

Geometry of large moleculesVSEPR method extends to larger molecules by treating them as a chains central atoms

The geometry at the S, O and N are similar to those in water and ammonia, i.e. bent and trigonal pyramidal

The geometry at the C1 and C2 atoms are tetrahedral, while at C3 it is trigonal planar

The complete molecule simply moves parts around by twisting around any single bond to relieve congestion.

Shape and polarity

Recall that most covalent bonds are polar

Symmetry in a molecule can cause bond dipole vectors to cancel each other making the molecule non-polar even though its individual bonds are very polar

Therefore molecules must be polar Not always!!!

Permanent Dipole MomentsMolecules that have permanent dipole moment are called polar

The unit of dipole moment is the Debye

Note that for molecules like H2 and CCl4, there is no permanent dipole moment!

Such molecules are non-polar.