Molecular Shape

The Geometry of molecules

Molecular GeometryMolecular Geometry

The shape of a molecule is determined by where the nucleinuclei are located.

But the nucleinuclei go to certain locations because of the electronelectron pairs.

Goal: minimize electron-pair Goal: minimize electron-pair repulsions.repulsions.

Molecular ShapeMolecular Shape

Electron pairs repel each other. They want to be as far apart from each other as they can.

Nonbonding pairs take up a little more room than bonding pairs They will repel with a greater force.

To determine molecular geometry To determine molecular geometry start with the Lewis estart with the Lewis e-- dot Structure dot Structure

Lewis dot structures are 2-D, but Lewis dot structures are 2-D, but they can help you figure out the they can help you figure out the

3-D shape.3-D shape.

To help us predict the shape of To help us predict the shape of molecules we will use molecules we will use

the VSEPR Theory.the VSEPR Theory.

VSEPR Theory VSEPR Theory Valence Shell Electron Pair Valence Shell Electron Pair

RepulsionRepulsion1) Draw the Lewis structure.

2) Identify the regions of high electron density {Bonding and nonbonding Sites} on the central atom.

a) Each single, double, &/or triple bond counts as 1 region of bonded electron density.

b) Each nonbonding electron pair counts as 1 region of non-bonded electron density.

c) An unpaired electron counts as 1 region of non-bonded electron density.

3) Assign a VSEPR formula according to the # of regions.

Resonance structures will fluctuate their regions.

VSEPR Theory

The shape is always referenced around the central atom. Determine the formula using

A as the central atom

X as the bonded atoms or shared pairs around the central atom(A).

E as the unshared electron pairs.

Example H2O would be AX2E2

A = O ; X2 = H2 ; E2 = 2 unshared pairs of e-

.

Most Common VSEPR ShapesMost Common VSEPR Shapes

VSPER Formula

# bonding sites

Most stable arrangement

AX2 2 Linear

AX3 3 Trigonal Planar

AX4,

AX3E,

AX2E2

4

Tetrahedral,

Trigonal Pyramidal

or Bent

AX5 5 Trigonal bipyramidal

AX6 6 Octahedral

TWO Share Pairs. LINEARTWO Share Pairs. LINEAR

• 2 bonding regions. • 0 unshared pairs.• Linear. • Bond angle = 180.• Central atom &

regions of electron density arranged in a straight line.

:Cl:ClBeBeCl:Cl:

::::

::::

AX2

Three Shared pairs Three Shared pairs Trigonal PlanorTrigonal Planor

• Bond angle = 120.

BB

:F::F:

:F::F: :F::F:::

:: ::

• 3 bonding regions. 0 unshared pairs.3 bonding regions. 0 unshared pairs.

AX3

3 regions of high electron density3 regions of high electron density.

• NO2-

• Lewis Structure:

OONNO:O:

::::

::::

:: 1-1-

3 unequal regions of electron density 3 unequal regions of electron density Since only 2 are shared, the molecule Since only 2 are shared, the molecule will look bent. What is the bond angle?will look bent. What is the bond angle?

• 1 unshared pair.1 unshared pair.• 2 Shared pairs2 Shared pairs( one single or double bonds)( one single or double bonds). .

AXAX22EE

Bent slightly < 120°Bent slightly < 120°

AXAX22EE

Four shared pairsFour shared pairs

• 4 shared pairs• 0 unshared pairs• Lewis structure:

HH

HHCCHHHH

TetrahedralTetrahedral Bond angles = 109.5Bond angles = 109.5..

AX4

Trigonal PyramidalTrigonal Pyramidal

3 shared pair

1 unshared pair.

HHNNHH

::

HH

4 regions of electron density would be 109.5 4 regions of electron density would be 109.5 apart. But only 3 end in atoms, not all 4. The apart. But only 3 end in atoms, not all 4. The molecule will look like a squashed pyramid. molecule will look like a squashed pyramid. Trigonal pyramidal. Bond angles?Trigonal pyramidal. Bond angles?

107°AXAX33EE

4 regions of electron density.

2 Shared pairs & 2 unshared pairs

NH2- with 8 valence electrons:

4 regions of electron 4 regions of electron density but only 2 density but only 2 end in atoms. end in atoms. Molecule will look Molecule will look bent.bent.

AXAX22EE22

Bent 104.5Bent 104.5°°

5 regions of electron density5 regions of electron density

# of bonding regions

# of lone pairs Shape

5 0 Trigonal Bipyramid

4 1 “See-Saw”

3 2 T-Structure

2 3 Linear

Trigonal BipyramidalTrigonal Bipyramidal

“See-Saw”See-Saw”

T-shapedT-shaped

LinearLinear

6 regions of electron density6 regions of electron density

# of bonding # of bonding regionsregions

# of lone pairs# of lone pairs shapeshape

66 00 OctahedralOctahedral

55 11 Square PyramidSquare Pyramid

44 22 Square PlanarSquare Planar

OctahedralOctahedral

Square PyramidSquare Pyramid

Square PlanarSquare Planar