Introductory Chemistry

Quiz: Molecular shape (VSEPR); Lewis structures.

Cracolice 2/e Ch 11-12.

Answer key
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Quiz is "closed book" except for PT & calculator (although the calculator really isn't needed for this particular quiz). If a question requests an explanation, there may be no credit unless you have provided an explanation.


In questions #1-2, letters such as Q, R etc refer to imaginary elements. This lets us work out some of the ideas of molecular shape without being biased by knowledge of specific elements. However, H is really hydrogen. For simplicity in #1-2, use only single bonds.

1. In general, the shape of a triatomic molecule is either "linear" or "bent".
a. Draw Lewis formulas for QH2, a linear molecule, and for RH2, a bent molecule. For R, assume that it satisfies the octet rule.
b. What is the "electron geometry" of each of these?
c. What is the key difference that determines whether a triatomic molecule is linear or bent?
d. What would be different in any of the above parts if H were replaced by a halogen, such as Cl?
e. Give an example of a real element that would behave like Q. like R. Explain.
f. Give an example of real molecules QH2 and RH2.

2. The general idea of this question is to follow the same issues as in #1, but now with tetratomic molecules -- with three atoms attached to a central atom. That is, these are TH3.
a. How many shapes are possible for molecules of the general type TH3? (Do not exceed an octet around the central atom.) For each...
- Draw the Lewis structure.
- Give the electron geometry.
- Give the molecular shape.
- Give an example of a real element that would behave like the central atom you show.
b. What is the key difference between your various structures, in how various tetratomic molecules have different shape?

3. [The answer page shows a single table for parts a and b of this question, all together. The two parts, and their sub-parts, will guide you through this a little at a time.]

a. Consider the nitronium ion, NO2+.
- How many valence electrons does this ion have? (Check yourself on this key point before continuing.)
- Draw the Lewis structure.
- Give the electron geometry.
- Give the molecular shape.

b. Now consider NO2 (the nitrogen dioxide molecule) and NO2- (the nitrite ion). Go through the same questions asked in part a. For one of these you will be unable to satisfy the octet rule, for an obvious reason; do the best you can in this case.

c. This part is somewhat off topic, but presents a logical follow-up to an issue raised above. What do you think would happen if two molecules of nitrogen dioxide got very near each other? Show a Lewis formula for the product.


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Last update: August 15, 2011