The VSEPR Theory

The VSEPR TheoryAdvanced ChemistryMs. Grobsky

Determining Molecular Geometries•In order to predict molecular shape, we use the

Valence Shell Electron Pair Repulsion (VSEPR) theory

•This theory proposes that the geometric arrangement of groups of atoms about a central atom in a covalent compound is determined solely by the repulsions between electron pairs present in the valence shell of the central atom▫The molecule adopts whichever 3-D geometry minimizes the repulsion between valence electrons

Determining Molecular Geometries• To determine the shape of a molecule, we distinguish

between:▫ Lone pairs (non-bonding pairs)▫ Bonding pairs (those found between two atoms)

Multiple bonds are considered as ONE bonding pair even though in reality, they have multiple pairs of electrons

• All electrons are considered when determining 3-D shapeAXmEn

A - central atomX – surrounding atom

E – non-bonding valence electron groupm and n - integers

Electron Group Repulsions and the Five Basic Molecular Shapes

Factors Affecting Electron Repulsion(And therefore, Bond Angles!)• Two factors that affect the amount of electron repulsion

around an atom:▫ Multiple bonds

Exert a greater repulsive force on adjacent electron pairs than do single bonds

Result of higher electron density Distorts basic geometry!

▫ Non-bonding (lone) pairs Lone pairs repel bonding pairs more strongly than bonding

pairs repel each other

The Effect of Non-Bonding Electrons on Bond Angles

• Remember, electron pairs of bonding atoms are shared by two atoms, whereas the nonbonding electron pairs (lone pairs) are attracted to a single nucleus▫ As a result, lone pairs can

be thought of as having a somewhat larger electron cloud near the parent atom

• This “crowds” the bonding pairs and the geometry is distorted!▫ Bond angles change!

Factors Affecting Bond AnglesDouble Bonds Non-Bonding (Lone) Pairs

The Single Molecular Shape of Linear Electron-Group Arrangement• AX2• Examples

▫ CS2, HCN, BeF2

A XX

The 2 Molecular Shapes of Trigonal Planar Electron-Group Arrangement Trigonal Planar Bent• AX3

• Examples▫ SO3, BF3

• AX2E• Examples

▫ SO2

AX

X

X X

X AE

The 3 Molecular Shapes of the Tetrahedral Electron-Group ArrangementTetrahedral• AX4

• Examples▫ CH4, SiCl4,

SO42-, ClO4

-

• AX3E• Examples

▫ NH3, PF3, ClO3, H3O+

Trigonal Pyramidal Bent

• AX2E2

• Examples▫ H2O, OF2,

SCl2

A

X

XX

XA

X X X

E

A

E

EXX

The 4 Molecular Shapes of the Trigonal Bipyramidal Electron-Group Arrangement

• AX5

• Examples▫ PCl5, PF5,

AsF5, SOF4

Trigonal Bipyramidal See-Saw T-Shaped Linear

• AX4E• Examples

▫ SF4, XeO2F2, IF4+, IO2F2-

• AX3E2

• Examples▫ ClF3, BrF3

• AX2E3

• Examples▫ XeF2, I3-,

IF2-

The 3 Molecular Shapes of the Octahedral Electron-Group ArrangementOctahedral• AX6

• Examples▫ SF6, IOF5

• AX5E• Examples

▫ BrF5, XeOF4, TeF5-

Square Pyramidal Square Planar

• AX4E2

• Examples▫ XeF4, ICl4-

What You Need to Know From All of This• Five BASIC geometries of covalent compounds and their

bond angles (ideal bond angles)▫ Linear (AX2)▫ Trigonal planar (AX3)▫ Tetrahedral (AX4)▫ Trigonal bipyramidal (AX5)▫ Octahedral (AX6)

• The following “special” geometries of covalent compounds with lone pairs▫ AX2E▫ AX3E▫ AX2E2

Steps in Determining a Molecular Shape•Refer to front of Page 237!

• Electronegativities determine polarity since it measures a nucleus’ attraction or “pull” on the bonded electron pair

▫ When two nuclei are the same, sharing is equal Non-polar

▫ When 2 nuclei are different, the electrons are not shared equally Polar

▫ When electrons are shared unequally to a greater extent, IONIC• Bonds can be polar while the entire molecule is not

▫ Determined by geometry! More on this later!

• Dipole moment▫ Separation of the charge in a molecule (slightly positive/slightly

negative poles)▫ IF octet rule is obeyed AND all the surrounding bonds are the

same (even if they’re very polar), then the molecule is NONPOLAR

Example: CCl4

Electronegativity

VSEPR and Polarity• Knowing the geometry of a molecule allows one to predict

whether it is polar or nonpolar▫ A bond between unlike atoms is usually polar with a positive

end and a negative end• The symmetry of the molecule determines polarity

▫ A diatomic molecule containing two different atoms is polar HF, CO

▫ A diatomic molecule containing the same two atoms is nonpolar N2, O2

▫ A polyatomic molecule may be nonpolar even if it contains polar bonds because, in such cases, the polar bonds are counteracting each other CO2, CH4 = nonpolar

VSEPR Symmetry and Molecular Polarity

VSEPR Symmetry and Molecular Polarity