$Molecular Geometry

Molecular geometry as determined by the Valence Shell Electron Pair Repulsion Model (VSEPR model)

Molecular Geometry = three-dimensional arrangement of atoms in a molecule/ion, including bond angles, (bond angles in degrees)

Valence electrons = outer electrons, bonding electrons and lone pair electrons

VSEPR model/theory – molecular geometry is that which minimizes repulsion of valence electrons pairs by having electron pairs as far away from each other as geometrically possible. (Gillespie/Nyholm theory)

a. Double/triple bonds between 2 atoms treated as one location b. VSEPR structure for individual resonance structures

A. Central atom has only bonding pairs/ no lone pairs on central atom: AB2 – AB6 (AB7 has several structures)

Electron pairs arrangement Molecular Geometry

Examples

2 linearBeCl2, HgCl2

3

Trigonal planar

BF3

4

tetrahedral

CH4, NH4+, BF4

5

Trigonal bipyramid

PF5

Equatorial

axail

6SF6, PF6

Cis

trans

octahedralRelative strength of electron-pair repulsion:

Lone pair vs lone pair > lone pair vs bonding pair > bonding pair vs bonding pair. (Lone pair “need more space”; bonding pairs contained between atoms)

ABxEy, A = central atom, B = bonded atoms, E = lone pairs on A:Overall geometry of electron pairs = x + y positionsGeometry of molecule/ion = X positions only; only atoms are measurable.

B. Central atom with Bonding Pairs (B) and Lone pairs (E):

Class, ABxEy Number of elec. pairs

Bonding pairs

Lone pairs

Overall geometry

Molecular Geometry

AB2E

SO2, O2

3 2 1Trigonal planar

Bent molecule

AB3E

NH3

4 3 1Tetrahedral

Trigonal pyramidAB2E2

H2O4 2 2

Tetrahedral

Bent moleculeAB4ESF4, IF4

+, XeO2F2

5 4 1Trigonal bipyramidBond angles120o and 180o

See-saw moleculeAB3E2

ClF3

5 3 2Trigonal bipyramid

T-shaped moleculeAB2E3

XeF2, I3

5 2 3Trigonal bipyramid

Linear

AB5E

BrF5, XeOF4

6 5 1Octahedral

Square pyramidAB4E2

XeF4, ICl4 6 4 2Octahedral

Square planar

Examples:

NH3

AB3E, Overall = tetrahedral; Molecular = trigonal pyramidRepulsion i > ii, so the HNH angle shorter than that normally expected for a tetrahedral structure (HNH = 107.3o)

H2O AB2E2, overall geometry = tetrahedral; molecular

geometry = bent molecule Electron repulsion, I > ii > iiiActual bond angle HOH is 104.5o

SF4 AB4E, 5 pairs trigonal bipyramid FSF bond angle, expected is 120o, but actual one is 116

SO2 or both AB2E. Trigonal planar/bent molecule.

Actual bond angle OSO is 119.5o

Review Procedure

1. Write Lewis structure

2. Count bonding pairs (B) and lone pairs (E) around central atom (A), treating double/triple bonds as 1 bonding pair, to obtain ABxEy formula.

3. Predict overall geometry of electron pairs from x + y.

4. Predict geometry of molecule by B positions in ABxEy, remembering that one pair repulsion stronger than bonding pair repulsion.

ICl4 5 x 7 + 1 = 36 electrons/18 pairsAB4E2 overall = octahedral

Molecular = square planar

Polar (covalent) Bonds/ Dipole Moments

Polar covalent bonds result when electron-pairs are shared between atoms of different electeronegativities (A), and have a partial separation of charge (because of this unequal sharing)

dipole molecule align in an electrical field

Dipole moment, = Q x r|charge| in coulombs, distance in mDebye unit (D), 1D = 3.33 x 10-30 C.m

{electron-positron 100 pm separated = 4.80 D}

Covalent bond between atoms of same element is non polar, because of equally-shared electron pairs: H-H, F-F etc.Homo-nuclear diatomic molecules, = 0Hetero-nuclear diatomic molecules 0 (have dipole moment)

Polyatomic molecules have dipole moment, 0, if:1. has polar covalent bonds;

2. Individual bond moments do NOT cancel in vector addition

CO2, AB2, linear molecule, , , = 0

Non-polar molecule with polar bonds

SO2, AB2E bent molecule, , 0, has dipole moment, is polar molecule

NH3, AB3E, trigonal pyramid, , 0

NF3, , 0, has dipole moment.

ClHC=CHCl, 1, 2, dichloroehtylene (planar molecule)

, 0

= 0, no dipole moment

When all the B atoms the same, the following ABxEy formulas

= 0 for AB2, AB3, AB4, AB5, AB6, AB2E3, AB4E2

0 for AB2E, AB3E, AB2E2, AB4E, AB3E2, AB5E

BF3(AB3), = 0; NF3(AB3E) 0

Care should be taken when considering different B atoms; can have 0.

BF3, BBr3 = 0

But, BFBr2, 0,

Valence Bond Theory

Overlap of atomic orbitals illustrates covalent bonds; H-H

-bondH 1s-1s covalent bond

- bonds – end-on overlap of atomic orbitals along bond axis:

F-F, [He]2s2 2px2 2py2 2pz1 , 2pz-2pz, overlap of same sign

H-F 1s-2pz

Multiple bonds [He] 2s2 2px1 2py

1 2pz1

1 – bond2 - bonds

- bond – parallel or side-by-side overlap of orbitals producing an electron cloud above and below bonding axis but a nodal plane through the axis.

Single bond = – bond, H-H, F-F, H-F etcDouble bond = 1 -bond + 1 -bond, O=C=O etc

Triple bond = 1 -bond + 2 -bonds, -bonds usually slightly weaker than -bonds

Hybridization of Atomic Orbitals

Simple atomic orbitals often suggest wrong number and geometry of bondse.g C [He]2s2 2px1 2py1 – suggests 2 covalent bonds at 90o angle- but CH4, in general CX4, tetrahedral – is observed

Hybridization is a possible explanation

1. Promotion (excitation) of valence electron:[He]2s2 2p2 [He] 2s1 2px

1 2py1 2pz

1 – an allowed excited state that would give 4 covalent bonds, but wrong angles and unequal bonding

2. Hybridization (mixing or linear combination) of valence atomic orbitals [He] 2s1 2px

1 2py1 2pz

1 [He] four (sp3 hybrid orbitals)4

Shape of each sp3 hybrid orbital:

4 equivalent sp3 hybrid orbitals directed to vertices of tetrahedronCH4

Four (Csp3 – H1s) bondsHybridization of 4 atomic orbitals (S, px, py, pz) produces 4 hybrid orbitals (sp3) having correct geometry (tetrahedral) for covalent bonds on central atom with equivalent bonding.

n atomic orbitals n hybrid orbitals

NH3 – use 4 sp3 hybrid orbitals

N – [He] 4 (sp3)5 bond angle – 107.3o

H2O – use 4 sp3 hybrid orbitals: O – [He] 4 (sp3)6 – two covalent bonds + 2 lone pairs of electrons

Sp hybridizationBCl2(g) – linear molecule

Be – [He] 2s2 – promotion [He] 2s1 2pz1

sp linear combination

Two sp hybrid orbitals – orbitals at 180o angle

Sp 2 hybridization

BF3 – trigonal planar

B – [He] 2s2 2px1 [He] 2s1 2px1 2py1 [He] three sp2 hybrid orbitals

Three – bonds (B sp2 – F2pz)

Three sp2 hybrid orbitals directd to points of triangle (120o angles)

Sp3d hybridization

PF5 trigonal bipyramid

P – [Ne] 3s2 3p3 [Ne] 3s1 3px1 3py1 3pz1 3d1 – [Ne] five (sp3d)5

Five sp3d hybrid orbitals directed to vertices of a trigonal bipyramid ; 5 s bonds (Psp3d – F2pz)

Expanded octet/expanded valence shell by atoms having available d orbitals (3rd, 4th period): P, As, Sb, (NOT N), Cl, Br, I (not F)

Sp3d2 hybridization – SF6 octahedral

S: [Ne] 3s2 3p4 [Ne] 3s1 3px1 3py1 3pz1 3x2-y2d1 3dz21

Six covalent bonds (Ssp3d2 – F2pz)

Summary of Hybridization schemes:

Number of Overall geometry Hybridization Valence electrons valence electron pair scheme

2 lnear sp3 trigonal planar sp2

4 tetrahedral sp3

5 trigonal bipyramid sp3d6 octahedral sp3d2

Hybridization procedure

Draw Lewis structures /determine geometry by ABxEy classMatch geometry to hybridization schemeHybridize atomic orbitals on central atom in molecules, never isolated atomsN atomic orbitals used – n hybrid orbitals formed

Energy needed for promotion recovered in bond formationCovalent bonds explained as overlap of orbitals