Shapes of Small Molecules
Objectives:
• 1. Define VESPR theory and explain its relationship to
the shape of molecules.
• 2. Name and describe 5 basic shapes of molecules.
• 3. Define the term hybrid orbital and explain how it
differs from atomic orbital.
• 4. Explain two important trends in bond length.
Key Terms:
• ball-&-stick model, valence-shell electron pair
repulsion (VSEPR) theory, bond angle, hybrid orbital
Molecular Geometry
• Ball-and-stick models let us see the molecules in three
dimensions which aids in our understanding of the
characteristics of the molecules.
– There are 5 basic shapes that can be easily displayed by this model
system.
– Shapes are determined by the VSEPR (valence shell electron pair
repulsion theory) which states that in a small molecule, valence
electrons pairs will configure themselves as far apart from each other
as possible.
• This configuration represents the lowest amount of energy possible
required to maintain the shape of the molecule.
Hybrid Bonds
• Bonding of atoms can be further explained by the
concept of hybrid orbitals. A hybrid orbital describes the
interaction of the valence electrons an atom as they
approach another atom.
• Sigma bonds:
– are the 1st bond made and are Linear
• Pi bonds
– overlap the sigma bonds in double and triple bonds
Bond Hybridization
• Bond Hybridization can be determined by counting the
directions from the central atom.
– s (Sigma) is 1st
– p (Pi) bonds up to three
– d bonds are added for exceptions to octet greater than 8
Molecular Shapes - tetrahedral
• Methane (CH4)
– Ball -n- stick
– Hybridization and bond angle
• 4 sigma bonds
Molecular Shapes - pyramidal
• Ammonia (NH3)
– Ball –n- stick
– Hybridization & bond angle
• 3 sigma bonds & an unshared pair
Molecular Shapes - Bent
• Water (H2O)
– Ball –n- stick
– Hybridization & bond angle
• 2 sigma bonds & two unshared pairs
Molecular Shapes – Trigonal Planar
• Boron Trifluoride(BF3)
– Ball –n- stick
– Hybridization & bond angle
• 3 sigma bonds
Molecular Shapes – linear
• Carbon dioxide (CO2)
– Ball –n- stick
– Hybridization & bond angle
• 2 sigma bond & 2 pi bonds around carbon