CHEM 1411

Chapters 8 Notes

VSEPR Theory

· Predicts the spatial arrangement of atoms in molecules and polyatomic ions

· Accounts for the geometric arrangements of electron pairs around a central atom in terms of the repulsion between electron pairs

Regions of high electron density (RHED)

· The repulsion between electrons, is caused by two types of electron pairs:

Bonding Electrons

· Electrons which are shared between 2 atoms

Lone Pairs

· Electrons not involved in a bond

· Double and triple bonds are treated as single in terms of geometry

· However, multiple bonds exert more repulsive force than single bonds

· If resonance structures exist, VSEPR model applies equally to each of them

· We will see in this theory that valence electrons arrange about the central atom so that electron-electron repulsion is minimized;

the RHED will be as far apart as possible

· These repulsions are ranked greatest repulsion to least: LP vs. LP > LP vs. BP > BP vs. BP

Electronic geometry (EG)

· Refers to the arrangement of RHED about the central atom

· Includes BP’s and LP’s

Molecular geometry (MG)

· Refers to the shape of the molecule which is determined only by bonding type RHED

· This is the arrangement of atoms about the central atom

· Includes BP’s only; however the LP’s do affect the MG

Dipole Moment

· This is actually a quantity used to express the extent of polarity on a molecule as a whole

· Just because a molecule contains bond dipoles, has polar covalent bonds, it is not necessarily polar “overall”

· Sometimes this is the case, but other times the overall molecule is nonpolar

· This is because dipole moments are vector sums; they have both direction and magnitude

· Symmetry can help in recognizing Dipole Moments

Nonpolar: CO₂ CH₄

· In these cases the dipoles cancel each other out and their is no dipole moment

Polar: H₂O CH₂Cl₂

· In these cases the dipoles do not cancel each other out and a dipole moment is generated

Electronic Geometries

1. Linear EG (AB₂)

AB₂

· Central atoms primarily from Group IIA

Examples: CO₂, CS₂, HCN, CdX₂, HgX₂, BeX₂ X = Cl, Br, or I

· MG – linear

· 180^o bond angle

Nonpolar: If the Bs are the same, no dipole moment

Polar: If the Bs are different, there is a dipole moment

2. Trigonal Planar EG (AB₃ and AB₂U)

AB₃

· Central atoms primarily from Group IIIA

· Boron forms several molecules with this EG

Examples: COCl₂, NO₃^-, CO₃^2-, BBr₃, BCl₃, BF₃

· MG-trigonal planer

· Trigonal means 3 vertices, 3 corners

· 120^o bond angles

Nonpolar: If the Bs are the same, no dipole moment

Polar: If the Bs are different, there is a dipole moment

· Greater electron density around the double bond causes some changes in the bond angles

· This also results from LP vs. BP repulsion

AB₂U

· Often molecules with resonance structures

Examples: NO₂^-, SO₂ O₃, PbCl₂, SnBr₂

· MG - bent or angular

· All molecules are polar due to the lone pair on the central atom

· Again, greater electron density around the double bond causes some changes in the bond angles

· This also results from LP vs. BP repulsion

3. Tetrahedral EG (AB₄, AB₃U and AB₂U₂)

AB₄

· Central atoms often from Group IVA

Examples: CH₄, SiCl₄, SiF₄, NH₄⁺, SO₄^2-, CH₂Cl₂, ClO₄^-

· MG - tetrahedral

· Tetrahedron means 4-faces, 4-vertices

· 109.5^o bond angles

· We will begin to look at 3-D representations here

Nonpolar: If the Bs are the same, no dipole moment

Polar: If the Bs are different, there is a dipole moment

AB₃U

· Central atoms often from Group VA

Examples: NH₃, PCl₃, NF₃, SO₃^2-, ClO₃, H₃O⁺

· MG-trigonal pyramidal

· The base of the molecule is an equilateral triangle

· All molecules are polar due to the lone pair on the central atom

· LP vs. BP repulsion causes changes in the bond angles

AB₂U₂

· Central atoms often from Group VIA

Examples: H₂O, H₂S, OF₂, SCl₂, NH₂^-

· MG- bent, angular, or v-shaped

· All molecules are polar due to the lone pairs on the central atom

· LP vs. BP and LP vs. LP repulsions cause a change in the bond angle

4. Trigonal Bipyramidal EG (AB₅, AB₄U, AB₃U₂ and AB₂U₃)

· Central atoms often heavier elements from Group VA

AB₅

Examples: PCl₅, PF₅, PCl₄F, AsF₅, SOF₄

· MG-trigonal bypyramidal

· These central atoms are from the 3^rd period and beyond, so they can have expanded octets

· There are 5 vertices and 6 faces

· There are two types of bond angles: Axial/equatorial (90^o) and Equatorial/ equatorial (120^o)

Nonpolar: If all the Bs are the same

Polar: If all the Bs are not the same

AB₄U

Examples: SF₄, XeO₂F₂, IF₄⁺, IO₂F₂^-

· MG-see-saw, square or distorted tetrahedron

· All molecules are polar due to lone pair on the central atom

· The LP’s always go in the equatorial position in this EG, b/c there is more space

AB₃U₂

Examples: BrF₃, ClF₃, ICl₃

· MG-T-shaped

· All molecules are polar due to lone pairs on the central atom

AB₂U₃

Examples: I₃^-, IF₂^-, XeF₂, BrIF^-

· MG- linear

· Due to the larger bond angles, the lone pairs reside in the equatorial positions

Nonpolar: All Bs are the same and the lone pairs are at equatorial positions on

the central atom

Polar: All Bs are not the same

5. Octahedral EG (AB₆, AB₅U and AB₄U₂)

· Central atoms often heavier elements from Group VIA

AB₆

Examples: SF₆, IOF₅

· MG-octahedral

· All positions in this EG are equivalent

· Octahedron means 8-faces

· There are 8 faces and 6 vertices

· Each face is an equilateral triangle

Nonpolar: All Bs are the same

Polar: All Bs are not the same

AB₅U

· MG- square pyramidal

Examples: PF₆^-, BrF₅, TeF₅^-, XeOF₄, BrF₅, IF₅,

· All molecules are polar due to lone pair on the central atom

· All positions are the same in terms of the placement of the lone pair

AB₄U₂

· MG-square planer

Examples: IF₄^-, ICl₄^-, XeF₄

· These molecules are nonpolar because the lone pairs are opposite each other

Valence Bond Theory

· Describes how atomic orbitals overlap to form hybrid atomic orbitals, and thus chemical bonds

· Atoms position themselves to achieve the maximum amount of overlap b/c this is the most stable situation,

lowest potential energy

· The bond is formed when 2 spin-paired electrons are shared by 2 overlapping atomic orbitals

· One orbital comes from each of the atoms involved in the bond

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Last Updated: 01/12/06