Lewis Structures and the “Real” 3D-Shape of Molecules

Sulfanilamide

A. Molecular Shape or Geometry - The way in which atoms of a molecule are arranged in space. It refers to the actual position of the nuclei.

B. What influences molecular shape ? The major factor is the tendency of valence electrons whether they are involved in bonding or not to repel each other.

C. The theory which describes molecular shape is called

VALENCE SHELL ELECTRON PAIR REPULSION

Because of their negative charge valence electrons will arrange themselves about a central atom in a way that minimizes repulsion between electrons.

D. Why is molecular shape important?

1. Molecular shape influences molecular polarity (the degree of separation of plus and minus charges in a molecule)

2. Molecular polarity influences such physical properties as melting point, boiling point, and solubility.

Shapes and Polarities of Molecules

Common molecular shapes include:

1. Linear 2. Bent (angular) 3. Triangular

4. Tetrahedral 5. Pyramidal

H Cl

Hydrogen Chloride, HCl, a Linear Molecule

HOH

Water, H2O, a Bent or Angular Molecule

Ammonia, NH3, a Pyramidal Molecule

HNHH

Methane, CH4, a Tetrahedral Molecule

CH

HH

H

VSEPR Arrangements of Groups Around an Atom

Electron Groups are Single Bonds, Multiple Bonds, or Unshared Pairs of Electrons

H O H O O O

VSPER Molecular Shape Table

HC O

H

Formaldehyde, CH2O, a Planar, Triangular Molecule

H BH

H

Borane, BH3, a Planar, Triangular Molecule

O C O

Carbon Dioxide, CO2, a Linear Molecule

H C C H

H C N

Acetylene and Hydrogen Cyanide, Linear Molecules

Covalent Bonds Exist in a Continuum

When describing the polarity of a molecule, we must consider bond polarities as

VECTOR QUANTITIESquantities with magnitude and direction.

Some molecules are inherently polar because of the atoms which they contain and the arrangement of these atoms in space.

H2O NH3 CH2O HCl

δ− δ+ A crude representation of a polar molecule

Other molecules are considered nonpolar

CH4 BH3 C2H2 CO2

Nonpolarizedelectronclouds