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MOLECULAR MOLECULAR GEOMETRYGEOMETRY

MOLECULAR MOLECULAR GEOMETRYGEOMETRY

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VSEPRVSEPR • VValence alence SShell hell EElectron lectron PPair air

RRepulsion theory.epulsion theory.

• Most important factor in Most important factor in determining geometry is determining geometry is relative relative repulsion between repulsion between electron pairs.electron pairs.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

MOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRY

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Some Common GeometriesSome Common Geometries

LinearLinear

Trigonal PlanarTrigonal Planar TetrahedralTetrahedral

4VSEPR chartsVSEPR charts

• Use the Lewis structure to determine the Use the Lewis structure to determine the geometry of the moleculegeometry of the molecule

• Electron arrangement establishes the bond Electron arrangement establishes the bond anglesangles

• Molecule takes the shape of that portion of the Molecule takes the shape of that portion of the electron arrangementelectron arrangement

• Charts look at the CENTRAL atom for all data!Charts look at the CENTRAL atom for all data!

• Think REGIONS OF ELECTRON DENSITY rather Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would than bonds (for instance, a double bond would only be 1 region)only be 1 region)

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Other VSEPR chartsOther VSEPR charts

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Structure Determination by Structure Determination by VSEPRVSEPR

Structure Determination by Structure Determination by VSEPRVSEPR

Water, HWater, H22OOThe electron pair The electron pair geometry is geometry is TETRAHEDRALTETRAHEDRAL

The electron pair The electron pair geometry is geometry is TETRAHEDRALTETRAHEDRAL

The molecular The molecular geometry is geometry is BENTBENT..

The molecular The molecular geometry is geometry is BENTBENT..

H O H••

••

H O H••

••

2 bond 2 bond pairspairs

2 lone 2 lone pairspairs

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Structure Structure Determination by Determination by

VSEPRVSEPR

Structure Structure Determination by Determination by

VSEPRVSEPRAmmonia, NHAmmonia, NH33

The electron pair geometry is tetrahedral.The electron pair geometry is tetrahedral.

H

H

H

lone pair of electronsin tetrahedral position

N

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

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Bond PolarityBond PolarityBond PolarityBond PolarityHCl is HCl is POLARPOLAR because it because it

has a positive end and a has a positive end and a negative end. (difference negative end. (difference in electronegativity)in electronegativity)

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has slight negative charge Cl has slight negative charge (-(-)) and H has and H has slight positive charge slight positive charge (+ (+ ))

H Cl••

••

+ -••H Cl

••

••

+ -••

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• This is why oil and water will not mix! Oil This is why oil and water will not mix! Oil is nonpolar, and water is polar.is nonpolar, and water is polar.

• The two will repel each other, and so you The two will repel each other, and so you can not dissolve one in the othercan not dissolve one in the other

Bond PolarityBond PolarityBond PolarityBond Polarity

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Bond PolarityBond PolarityBond PolarityBond Polarity

• ““Like Dissolves Like”Like Dissolves Like”

–Polar dissolves PolarPolar dissolves Polar

–Nonpolar dissolves Nonpolar dissolves NonpolarNonpolar

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Diatomic ElementsDiatomic Elements

• These elements do not exist as a single atom; they always appear as pairs

• When atoms turn into ions, this NO LONGER HAPPENS!– Hydrogen

– Nitrogen

– Oxygen

– Fluorine

– Chlorine

– Bromine

– Iodine

Remember: Remember: BrINClHOFBrINClHOF