Molecular Shape The Geometry of molecules

Molecular Geometry nuclei The shape of a molecule is determined by where the nuclei are located. nuclei electron But the nuclei go to certain locations because of the electron pairs. Goal: minimize electron-pair repulsions.

Molecular Shape Electron pairs repel each other. They want to be as far apart from each other as they can. Nonbonding pairs take up a little more room than bonding pairs They will repel with a greater force.

To determine molecular geometry start with the Lewis e - dot Structure Lewis dot structures are 2-D, but they can help you figure out the 3-D shape. To help us predict the shape of molecules we will use the VSEPR Theory.

VSEPR Theory Valence Shell Electron Pair Repulsion 1) Draw the Lewis structure. 2) Identify the regions of high electron density {Bonding and nonbonding Sites} on the central atom. a) Each single, double, &/or triple bond counts as 1 region of bonded electron density. b) Each nonbonding electron pair counts as 1 region of non-bonded electron density. c) An unpaired electron counts as 1 region of non-bonded electron density. 3) Assign a VSEPR formula according to the # of regions. Resonance structures will fluctuate their regions.

VSEPR Theory The shape is always referenced around the central atom. Determine the formula using A as the central atom X as the bonded atoms or shared pairs around the central atom(A). E as the unshared electron pairs. Example H 2 O would be AX 2 E 2  A = O ; X 2 = H 2 ; E 2 = 2 unshared pairs of e-.

Most Common VSEPR Shapes VSPER Formula # bonding sites Most stable arrangement AX 2 2Linear AX 3 3Trigonal Planar AX 4, AX 3 E, AX 2 E 2 4 Tetrahedral, Trigonal Pyramidal or Bent AX 5 5Trigonal bipyramidal AX 6 6Octahedral

TWO Share Pairs. LINEAR 2 bonding regions. 0 unshared pairs. Linear. Bond angle = 180 . Central atom & regions of electron density arranged in a straight line. :Cl  Be  Cl: : : : : AX 2

Three Shared pairs Trigonal Planor Bond angle = 120 . B :F:    :F: :F: : :: 3 bonding regions. 0 unshared pairs. 3 bonding regions. 0 unshared pairs. AX 3

3 regions of high electron density 3 regions of high electron density. NO 2 - Lewis Structure: O  N  O: : : : : : 1- 3 unequal regions of electron density Since only 2 are shared, the molecule will look bent. What is the bond angle? 1 unshared pair. 1 unshared pair. 2 Shared pairs ( one single or double bonds). 2 Shared pairs ( one single or double bonds). AX 2 E

Bent slightly < 120° AX 2 E

Four shared pairs 4 shared pairs 0 unshared pairs Lewis structure:H HCHHCHHCHHCH H   Tetrahedral Bond angles = . Bond angles = . AX 4

Trigonal Pyramidal 3 shared pair 1 unshared pair. HNHHNHHNHHNH:H  4 regions of electron density would be apart. But only 3 end in atoms, not all 4. The molecule will look like a squashed pyramid. Trigonal pyramidal. Bond angles? 107° AX 3 E

4 regions of electron density. 2 Shared pairs & 2 unshared pairs NH 2 - with 8 valence electrons: 4 regions of electron density but only 2 end in atoms. Molecule will look bent. AX 2 E 2 Bent °

5 regions of electron density # of bonding regions # of lone pairsShape 50Trigonal Bipyramid 41“See-Saw” 32T-Structure 23Linear

Trigonal Bipyramidal

See-Saw” “See-Saw”

T-shaped

Linear

6 regions of electron density # of bonding regions # of lone pairs shape 60Octahedral 51 Square Pyramid 42 Square Planar

Octahedral

Square Pyramid

Square Planar