VSEPRVSEPRValence Shell Electron Pair Repulsion Valence Shell Electron Pair Repulsion

TheoryTheory

Vocabulary:Vocabulary:““domain”domain” = any electron pair, or any double or triple = any electron pair, or any double or triple bond is considered one domain.bond is considered one domain.

““lone pair” = “non-bonding pair” = lone pair” = “non-bonding pair” = “unshared pair”“unshared pair”= any electron pair that is not involved in bonding= any electron pair that is not involved in bonding

““bonding pair” = “shared pair”bonding pair” = “shared pair”= any electron = any electron pairpair that is involved in bonding that is involved in bonding

2 domains on central atom2 domains on central atom

LINEARLINEAR

2 domains2 domains both are bonding both are bonding

pairspairs

They push each other They push each other to opposite sides of to opposite sides of center (180center (180 apart). apart).

BeCl2

3 domains on central atom3 domains on central atom

TRIGONAL TRIGONAL PLANARPLANAR

3 domains3 domains all are bonding pairsall are bonding pairs

They push each They push each other apart equally other apart equally at 120at 120 degrees. degrees.

GaF3

3 domains on central atom3 domains on central atom

BENTBENT

3 domains:3 domains: 2 are bonding pairs2 are bonding pairs 1 is a lone pair1 is a lone pair

The 2 bonding pairs The 2 bonding pairs are pushed apart by are pushed apart by 33rdrd pair (not seen) pair (not seen)

SnF2

NOTE:NOTE:

The The geometry geometry around the central around the central atomatom is is trigonal trigonal planar.planar.

The The molecular shapemolecular shape is is bentbent..

SnF2

4 domains on central atom4 domains on central atom

TETRAHEDRALTETRAHEDRAL

4 domains4 domains

Each repels the Each repels the other equally - other equally - 109.5109.5 - - notnot the the expected 90expected 90..

Think in 3D.Think in 3D.CH4

4 e4 e-- pairs on central atom pairs on central atom

TRIGONAL TRIGONAL PYRAMIDALPYRAMIDAL

4 domains4 domains 3 bonding pairs3 bonding pairs 1 lone pair1 lone pair

The thicker, lone The thicker, lone pair forces the pair forces the others a little bit others a little bit closer together closer together (~107.3(~107.3))

NH3

Tetrahedral Tetrahedral vs.vs. Trigonal Trigonal pyramidalpyramidal

Tetrahedral geometry around the central

atom

Tetrahedral Molecular Shape

Tetrahedral geometry around the central

atom

Trigonal Pyramidal Molecular Shape

Tetrahedral Tetrahedral vs.vs. Trigonal Trigonal pyramidalpyramidal

On the right, the 4th lone pair, is not seen as part of the actual molecule, yet affects

shape.If another one of the bonding pairs on “trigonal pyramidal” were a lone pair, what

is the result?

4 domains on central atom, 4 domains on central atom, con’tcon’t

BENTBENT

4 domains4 domains 2 bonding pairs2 bonding pairs 2 lone pairs2 lone pairs

The bonds are The bonds are forced together still forced together still closer (104.5closer (104.5) by ) by the 2 thick unshared the 2 thick unshared pairs.pairs. H2O

Comparing the 2 “bents”…Comparing the 2 “bents”…

Both bent molecules are affected by unshared pairs – 1 pair on the left, 2 on the

right.

Other Molecular Other Molecular GeometryGeometry

Just for fun, let’s look at some Just for fun, let’s look at some others that we will not study others that we will not study

in detail in this course…in detail in this course…

Note that if there are more than five domains around the Note that if there are more than five domains around the central atom, it must be an exception to the octet rule!central atom, it must be an exception to the octet rule!

5 e5 e-- pairs on central atom pairs on central atom

TRIGONAL TRIGONAL BIPYRAMIDALBIPYRAMIDAL

5 shared pairs5 shared pairs

Three pairs are found Three pairs are found in one plane in one plane (“equator”) 120(“equator”) 120 apart; apart; the other two pairs are the other two pairs are at the “poles,” 180at the “poles,” 180 apart, 90apart, 90 from the from the “equator.”“equator.” PCl5

5 e5 e-- pairs on central atom pairs on central atom

SEE-SAWSEE-SAW

4 shared pairs & 4 shared pairs & 1 unshared pair1 unshared pair

One of the equator One of the equator pairs is unshared & pairs is unshared & pushes the other 2 pushes the other 2 together.together.

The 2 poles are The 2 poles are pushed slightly pushed slightly together.together. SF4

5 e5 e-- pairs on central atom pairs on central atom

T-SHAPEDT-SHAPED

3 shared & 2 3 shared & 2 unshared pairsunshared pairs

2 of the 3 equator 2 of the 3 equator pairs are unshared.pairs are unshared.

All 3 remaining pairs All 3 remaining pairs are pushed together.are pushed together.

ClF3

5 e5 e-- pairs on central atom pairs on central atom

LINEARLINEAR

2 shared & 3 2 shared & 3 unshared pairsunshared pairs

All 3 equator pairs All 3 equator pairs are unshared. The 2 are unshared. The 2 remaining pairs are remaining pairs are forced to the poles.forced to the poles.

XeF2

5 e5 e-- pairs on central pairs on central atomatom

5 shared, 0 unshared5 shared, 0 unshared 4 shared, 1 unshared4 shared, 1 unshared

3 shared, 2 unshared3 shared, 2 unshared 2 shared, 3 unshared2 shared, 3 unshared

6 e6 e-- pairs on central atom pairs on central atom

OCTAHEDRALOCTAHEDRAL

6 shared pairs6 shared pairs

Each pair repels the Each pair repels the others equally.others equally.

All angles = 90All angles = 90

Now, if one of these Now, if one of these pairs was unshared pairs was unshared ……

SF6

6 e6 e-- pairs on central atom pairs on central atom

SQUARE SQUARE PYRAMIDALPYRAMIDAL

5 shared pairs & 5 shared pairs & 1 unshared pair 1 unshared pair

4 shared pairs in one 4 shared pairs in one plane; the 5plane; the 5thth pair at pair at the pyramid’s top. the pyramid’s top.

If the pair at the top If the pair at the top was unshared …was unshared … IF5

6 e6 e-- pairs on central atom pairs on central atom

SQUARE PLANARSQUARE PLANAR

4 shared & 2 4 shared & 2 unshared pairsunshared pairs

The 4 shared pairs The 4 shared pairs are in the same are in the same plane; the 2 plane; the 2 unshared pairs are unshared pairs are 9090 from them. from them.

XeF4

6 e6 e-- pairs on central pairs on central atomatom

6 shared, 0 unshared6 shared, 0 unshared 5 shared, 1 unshared5 shared, 1 unshared

4 shared, 2 unshared4 shared, 2 unshared

Steps for using VSEPR:Steps for using VSEPR:

1.1. Draw a Lewis Dot Structure.Draw a Lewis Dot Structure.

2.2. Predict the geometry around the Predict the geometry around the central atom.central atom.

3.3. Predict the molecular shape.Predict the molecular shape.

… … also, we can try and predict the angles between atoms.also, we can try and predict the angles between atoms.

AllAll e e-- pairs push each pairs push each other as far apart as other as far apart as possible.possible.

Shared (bonding) pairs are Shared (bonding) pairs are “stretched” between two atoms “stretched” between two atoms that want them.that want them. ““Longer & Thinner”Longer & Thinner”

Unshared (non-bonding) pairs are Unshared (non-bonding) pairs are not “stretched.”not “stretched.” ““Shorter & Thicker”Shorter & Thicker”

Electron Pair RepulsionElectron Pair Repulsion

2 lone pairs require the most space 2 lone pairs require the most space & & repel each other the most, repel each other the most, resulting in the greatest distance resulting in the greatest distance (angle).(angle).

1 lone pair (thick) & 1 bonding pair 1 lone pair (thick) & 1 bonding pair (thin) require less space(thin) require less space

2 bonding pairs (both thin) require 2 bonding pairs (both thin) require the least space & the least space & repel each other repel each other the smallest distance (angle).the smallest distance (angle).