U6L4 - Lecture Notes - Molecular Shapes

​Lesson 4
Molecular Shape

Essential Questions

• What is the VSEPR bonding theory?

• How can you use the VSEPR model to predict the shape of, and the bond
angles in, a molecule?

• What is hybridization?

Molecular Shapes
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VSEPR Model

• The shape of a molecule determines many of its physical and chemical properties.

• Often, shapes of reactant molecules determine whether or not they can get close
enough to react.

• Molecular geometry (shape) can be determined with the Valence Shell Electron
Pair Repulsion model, or VSEPR model which minimizes the repulsion of shared
and unshared atoms around the central atom.

• Electron pairs repel each other and cause molecules to be in fixed positions
relative to each other.

• The angle formed by two terminal atoms and the central atoms is a bond angle.

• Unshared electron pairs also determine the shape of a molecule.

• Electron pairs are located in a molecule as far apart as they can be.

Molecular Shapes
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Multiple Choice

Which is the model used to determine molecular shape that is based on an arrangement that minimizes the repulsion of shared and unshared electron pairs around the central atom?

• molecular formula

• resonance model

• Lewis structure

• VSEPR model

Multiple Select

Which can be determined by the shape of a molecule?

Multiple select question.

• physical properties

• chemical properties

• reactivity

• number of valence electrons

Hybridization

• Hybridization is a process in which atomic
orbitals mix and form new, identical hybrid
orbitals.

• Carbon often undergoes hybridization, which
forms an sp³ orbital formed from one s orbital
and three p orbitals.

• The number of atomic orbitals that mix and form the hybrid orbital equals the total number of
pairs of electrons.

• Lone pairs also occupy hybrid orbitals.

• Single, double, and triple bonds occupy only
one hybrid orbital (CO2 with two double bonds
forms an sp hybrid orbital).

Molecular Shapes
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Multiple Choice

The four hybrid orbitals in a carbon atom in methane form from one s orbital and three p orbitals. Therefore, they are called ___ orbitals.

• sp³

• s³p

• 1s3p

• (sp)⁴

Molecular Shapes
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FIND THE SHAPE OF A MOLECULE

Use with Example Problem 7.
Problem
Phosphorus trihydride, a colorless gas, is
produced when organic materials, such
as fish flesh, rot. What is the shape of a
phosphorus trihydride molecule? Predict
the bond angle and identify hybrid
orbitals.

Response
ANALYZE THE PROBLEM
A phosphorus trihydride molecule has three hydrogen atoms bonded to a central
phosphorus atom.

SOLVE FOR THE UNKNOWN

Find the total number of valence electrons
and the number of electron pairs.

1 P atom × 5 valence electrons

1 P atom

× 1 valence electron = 8 valence electrons

• Determine the total number of bonding pairs.

8 electrons

2 electrons/pair

​ ---------------- + 3 H atoms

​1 H atom

​= 4 pairs

Molecular Shapes
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Find the Shape of a Molecule

EVALUATE THE ANSWER

All electron pairs are used and each atom
has a stable electron configuration.

SOLVE FOR THE UNKNOWN

• Draw the Lewis structure, using one pair
of electrons to bond each H atom to the
central P atom and assigning the lone
pair to the P atom.

The molecular shape is trigonal pyramidal
with a predicted 107° bond angle and sp3
hybrid orbitals.