Molecular shapes

A lot of balls... and sticks

Learning objectives

Describe underlying

principles that govern

theories of molecular

shapes

Use Lewis dot

diagrams to predict

shapes of molecules

using VSEPR

Valence shell electron pair repulsion

In order to understand properties like

polarity, we need to predict molecular

shapes

Lewis dot structure provides 2D sketch of

the distribution of the valence electrons

among bonds between atoms and lone

pairs; it provides no information about the

shape of the molecule

A hierarchy of models

VSEPR

Consider the problem in terms of electrostatic repulsion

between groups of electrons (charge clouds, domains)

Valence bond theory

Acknowledges the role of orbitals in covalent bonding

Molecular orbital (MO) theory (the “real” thing)

Accommodates delocalization of electrons - explains

optical and magnetic properties

Counting sheep clouds

Valence shell electron pair repulsion (VSEPR)

Identify all groups of charge on central atom only:

• Non-bonding pairs count 1

• All bonded atoms count 1 (singles/doubles/triples)

Distribute them about central atom to minimize potential

energy (equals maximum separation)

This specifies electronic geometry (also known as

electron domain geometry or sometimes, confusingly,

as molecular geometry)

Electronic geometry and molecular

shape

Electronic geometry includes all

atoms and lone pairs on central

atom

H2O has tetrahedral electronic

geometry (2 +2 = 4)

Molecular shape (geometry)

ignores lone pairs

H2O is bent (2 atoms)

Must know electronic geometry

to obtain correct molecular

shape

With no lone pairs: molecular

shape = electronic geometry

Blob counting: Choices are limited

Groups (domains) of charge range from 2 – 6

Only one electronic geometry for each number

Note: more than one molecular shape follows from electronic geometry depending on number of lone pairs

One surprise: lone pairs occupy more space than the bonded atoms (with very few exceptions) Manifested in bond angles (examples follow)

Molecular shape selection (particularly in trigonal bipyramid – the tricky one)

Two groups: linear

Except for BeX2 (Be violates octet rule), all cases

with two groups involve multiple bonds

Electronic geometry = molecular shape = linear

Three groups: trigonal planar

Two possibilities for central atoms with complete octets: Trigonal planar (H2CO)

Bent (SO2)

BCl3 provides example of trigonal planar with three single bonds B is satisfied with 6

electrons – violates octet rule

Four groups: tetrahedral

Three possibilities: No lone pairs (CH4) -

tetrahedral

One lone pair (NH3) – trigonal pyramid

Two lone pairs (H2O) – bent

Lone pairs need space: • H-N-H angle 107°

• H-O-H angle 104.5°

• Tetrahedral angle 109.5°

Representations of the tetrahedron

Five groups of charge: trigonal

bipyramid – most variations Two different positions:

Three equatorial

Two axial

Equatorial positions are lower energy: Lone pairs need more space

Lone pairs require occupy equatorial sites preferentially

Five bonds, no lone pairs

Four bonds, one lone pair

Lone pair dictates geometry: equatorial position

has lower energy than axial

Three bonds, two lone pairs

Both lone pairs occupy equatorial positions –

lower energy than in axial

Two bonds, three lone pairs

The trend continues: all equatorial positions filled –

lowest energy

Octahedron has six identical

positions and high symmetry

No lone pairs

High symmetry

One lone pair All positions are equally probable

Symmetry reduced

Two lone pairs

Minimum energy has axial symmetry, lone pairs lie

along straight line

VSEPRBasicMoleclrConfigs.html

Molecules with multiple centers

A central atom is any atom with more than one atom

bonded to it

Perform exercise individually for each atom

Electronic geometry and molecular shape will refer only to

the atoms/lone pairs immediately attached to that atom

Taking it to the next level:

acknowledging orbitals

VSEPR is quite successful in predicting

molecular shapes based on the simplistic

Lewis dot approach

But our understanding of the atom has the

electrons occupying atomic orbitals

How do we reconcile the observed shapes

of molecules with the atomic orbital picture

of atoms