High School Chemistry : Help with VSEPR Theory and Geometry

Study concepts, example questions & explanations for High School Chemistry

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Example Questions

Example Question #1 : Help With Vsepr Theory And Geometry

What is the molecular geometry of an ammonia molecule with the formula ?

Possible Answers:

Trigonal pyramidal

Tetrahedral

Trigonal bipyramidal

Trigonal planar

Correct answer:

Trigonal pyramidal

Explanation:

In an ammonia molecule, the nitrogen is bonded to three hydrogen atoms and also has a lone electron pair. This lone pair will repel the three hydrogens out of a planar orientation, which results in a trigonal pyramidal geometry.

Compounds with the general formula AX3 and one lone pair will be trigonal pyramidal.

Compounds with the general formula AX3 and no lone pairs will be trigonal planar.

Compounds with the general formula AX4 and no lone pairs will be tetrahedral.

Compounds with the general formula AX5 and no lone pairs will be trigonal bipyramidal.

Example Question #1 : Help With Vsepr Theory And Geometry

What is the bond angle found in a methane molecule with the formula ?

Possible Answers:

Correct answer:

Explanation:

Methane has a carbon atom attached to four hydrogen atoms. In order to be as far as possible from one another, the hydrogen atoms will orient themselves around the carbon in a tetrahedral geometry. Tetrahedral geometries have bond angles of  between each constituent.

Example Question #3 : Help With Vsepr Theory And Geometry

Which of the following choices correctly describes the VSEPR shape of the water molecule, ?

Possible Answers:

Trigonal bipyramidal

Linear

Tetrahedral

None of the answer choices correctly describe the VSEPR shape of the water molecule

Trigonal planar

Correct answer:

Tetrahedral

Explanation:

In the water molecule, there are four electron pairs. Two of them are bonded and two of them are lone pairs. This causes the water molecule to have a tetrahedral shape (it is important to note that it is a bent tetrahedral shape due to the two lone pairs).

Example Question #3 : Help With Vsepr Theory And Geometry

Which of the following compounds has a bent shape according to VSEPR theory?

Possible Answers:

Correct answer:

Explanation:

Generally, a central atom bound to two peripheral atoms will result in a linear shape, as exemplified by carbon dioxide. Exceptions come into play, however, with the introduction of lone pairs of electrons. These lone pairs carry a negative charge, pushing other atoms (and their negatively-charged electrons) farther away. In water, the central oxygen atom is bound to two hydrogen atoms and carries two lone pairs of electrons. As a result, the lone pairs propel the hydrogen atoms away from the linear structure, "bending" the molecule. The result is known as a bent molecular geometry, according to VSEPR theory. Any molecule in which the central atom is bound to two atoms and carries two lone pairs will result in a bent shape.

Carbon dioxide and cyanide are both linear. Ammonia is trigonal pyramidal. Methane is tetrahedral.

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