Physical Chemistry : Molecular Geometry and VSEPR

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Molecular Geometry And Vsepr

Which of the following molecules has the smallest bond angle?

Possible Answers:

Methane

Carbon dioxide

Water

Ammonia

Correct answer:

Water

Explanation:

One of the biggest factors that can affect the bond angles in a molecule is the presence of lone pairs. A general rule is that each lone pair will decrease the predicted bond angle by about two degrees. Methane, for example, has no lone pairs on the central carbon, so its bond angles are 109.5 degrees. Ammonia, however, has one lone pair, which makes its angles closer to 107.5 degrees. Finally, water has 2 lone pairs, giving it a bond angle of about 105 degrees.

Example Question #2 : Molecular Geometry And Vsepr

Which molecule exhibits trigonal pyramidal geometry?

Possible Answers:

Methane

Carbon dioxide

Ammonia

Boron trifluoride

Correct answer:

Ammonia

Explanation:

A trigonal pyramidal geometry is witnessed when molecules have a central atom bonding to three additional atoms and one lone pair. The nitrogen atom in ammonia has three hydrogens attached, as well as a lone pair, in order to satisfy its octet. This gives ammonia a trigonal pyramidal geometry.

Methane has a tetrahedral geometry, carbon dioxide is linear, and boron trifluoride is trigonal planar.

Example Question #3 : Molecular Geometry And Vsepr

A metal with sp3d2 hybridized orbitals contains eight valence electrons. If four monodentate ligands are bound to this metal, what is the resulting molecular geometry?

Possible Answers:

seesaw

square pyramidal 

tetrahedral

T-shaped 

square planar

Correct answer:

square planar

Explanation:

An sp3d2 hybridized atom contains six degenerate orbitals. Hund's Rule states that all degenerate orbitals must be singly occupied before any orbital becomes doubly occupied. This molecule contains eight valence electrons, which results in two doubly occupied hybrid orbitals (two lone pairs) and four singly occupied hybrid orbitals to which the ligands bind. Thus, the molecule contains four bonds and two lone pairs, which corresponds to a square planar geometry.  

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