LECTURE 21- Shapes of Molecules & the Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

lewis structures are 2-D

gives no indication of 3D arrangement

molecular geometry

3D shape/ affects properties & behavior

diatomic molecules

linear

3 or more molecules

angles between the bonds joining atoms together in the molecule or ion

bond angle

angle in degrees between 2 bonds sharing a common atom

methane (example)

• 6 bond angles

• H-C-H angle between 2 C-H bonds

• all 6 are identical & have a value of 109.5 degrees

• tetrahedral geometric shape

• bond angles & lengths make shape and size

• C-H lengths are 110 pm

VSEPR theory

bonding pairs of electrons & the lone pairs of electrons around the central atom in a molecule/ polyatomic ion will adopt a spatial arrangement that minimizes electrostatic repulsions between them around that atom

steric number (SN)

number of ligands and lone pairs

coordination number (CN)

number of ligands

furthest away electrons are

lowest energy & more favorable

SN: 2

• linear

• 180

SN: 3

• trigonal planar

• 120

SN: 4

• tetrahedral

• 109.5

SN: 5

• trigonal bipyramidal

• 120 and 90

SN: 6

• octahedral

• 90

linear is

most favorable

step-by-step procedure

• draw lewis structure

• determine SN & CN

• use SN to predict electron geometry (ligand and & lone pair electrons that puts them as far apart as possible)

• from electron geometry (& CN), determine positions of ligand atoms only & predict molecular geometry & bond angles

effect of lone pairs of electrons

presence of lone pairs carries slight distortions of bond angles from ideal geometries

lone pair is less confined & more spread out

bonding pair is opposite

lone pair tales up more space

& exerts greater repulsive force on the neighboring electron pairs

most repulsive to least repulsive

lone pair- lone pair repulsions> line pair-bond pair repulsions> bond pair-bond pair repulsions