VValencealence

SShellhell

EElectronlectron

PPairair

RRepulsioepulsionn

TheoryTheory

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

bonding pair = shared pairbonding pair = shared pair= any electron = any electron pairpair that is involved in bonding that is involved in bonding

domaindomain= any electron pair, or any single, double, or triple bond is considered one domain= any electron pair, or any single, double, or triple bond is considered one domain

VSEPR theory assumes that the shape of a VSEPR theory assumes that the shape of a molecule is determined by the repulsion of molecule is determined by the repulsion of

electron pairs.electron pairs.

These are the These are the electron-domain electron-domain

geometries for two geometries for two through six electron through six electron domains around a domains around a

central atom. central atom.

» The Electron Geometry is often The Electron Geometry is often NOTNOT the shape of the shape of the molecule, however.the molecule, however.

» The Molecular Geometry is defined by the The Molecular Geometry is defined by the positions of positions of only the atoms/bonds only the atoms/bonds in the in the molecules, molecules, NOTNOT the nonbonding pairs. the nonbonding pairs.

» Electron Geometries: Count EVERYTHING

»Molecular Geometries: ONLY count the atoms/bonds

o</:)---/-----<

LINEAR LINEAR

» 2 domains2 domains» both are bonding pairsboth are bonding pairs» They push each other to They push each other to

opposite sides of center opposite sides of center (180(180 apart). apart).

BeCl2

TRIGONAL PLANAR TRIGONAL PLANAR

» 3 domains3 domains» all are bonding pairsall are bonding pairs» They push each other They push each other

apart equally at 120apart equally at 120 degrees.degrees.

AlF3

BENT BENT

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

» The 2 bonding pairs are The 2 bonding pairs are pushed apart by 3pushed apart by 3rdrd pair pair (not seen)(not seen)

SnF2

» The The Electron Geometry Electron Geometry around the central atom around the central atom is Trigonal Planar.is Trigonal Planar.

» The The Molecular GeometryMolecular Geometry is Bent.is Bent.

SnF2

TETRAHEDRAL TETRAHEDRAL

» 4 domains4 domains» Each repels the other Each repels the other

equally - 109.5equally - 109.5 - not the - not the expected 90expected 90..

»Think in 3D.Think in 3D.CH4

TRIGONAL PYRAMIDALTRIGONAL PYRAMIDAL

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

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

NH3

Tetrahedral Tetrahedral ElectronElectron Geometry Geometry around the around the

central atomcentral atom

Tetrahedral Tetrahedral Molecular Geometry Molecular Geometry

Tetrahedral Tetrahedral ElectronElectron Geometry Geometry around the around the

central atomcentral atom

Trigonal Pyramidal Trigonal Pyramidal Molecular Geometry Molecular Geometry

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

If another one of the bonding pairs on trigonal If another one of the bonding pairs on trigonal pyramidal were a lone pair, what is the result?pyramidal were a lone pair, what is the result?

BENT BENT

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

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

H2O

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

TRIGONAL BIPYRAMIDAL TRIGONAL BIPYRAMIDAL

» 5 shared pairs5 shared pairs

» Three pairs are found in Three pairs are found in one plane (“equator”) one plane (“equator”) 120120 apart; the other two apart; the other two pairs are at the “poles,” pairs are at the “poles,” 180180 apart, 90 apart, 90 from the from the “equator.”“equator.”

PCl5

OCTAHEDRAL OCTAHEDRAL

» 6 shared pairs6 shared pairs

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

» All angles = 90All angles = 90

SF6

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

detail in this course…detail in this course…

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

rule!rule!

5 e- pairs on central atom

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 e- pairs on central atom

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

4 shared, 2 unshared4 shared, 2 unshared

1.) Draw a Lewis Dot Structure.1.) Draw a Lewis Dot Structure.2.) Find the electronic geometry 2.) Find the electronic geometry around the central atom.around the central atom.3.) Find the molecular shape.3.) Find the molecular shape.

… … also, we can try and predict the angles between atoms. also, we can try and predict the angles between atoms. USE THE USE THE ELECTRONIC GEOMETRY ELECTRONIC GEOMETRY

» Stop Notes Here!\

o</:)----/------<

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

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

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

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

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

SEE-SAW

» 4 shared pairs & 1 unshared pair

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

» The 2 poles are pushed slightly together.

SF4

T-SHAPED

» 3 shared & 2 unshared pairs

» 2 of the 3 equator pairs are unshared.

» All 3 remaining pairs are pushed together.

ClF3

LINEAR

» 2 shared & 3 unshared pairs

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

XeF2

SQUARE PYRAMIDAL

» 5 shared pairs & 1 unshared pair

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

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

SQUARE PLANAR

» 4 shared & 2 unshared pairs

» The 4 shared pairs are in the same plane; the 2 unshared pairs are 90 from them.

XeF4