Individual Pair (hand in one copy, please)

Name____KEY___________ Name___average 8.9______________

This quiz requires you to move about the lecture hall (periodic table, vitamin B12) and the science building. Please discuss your workly quietly and only with your partner. Problem 4 and useful information are on the back.

Chemistry 103--Fall 1999

Oct. 29, 1999 Quiz #8--Bonding

1. The Lewis structure for CO2 is O=C=O. CS2 is isoelectronic with CO2.

(The central atom is underlined)

Draw the Lewis structure, including any lone pairs, for CS2 and describe its molecular geometry.

S=C=S linear. Each S has 2 lone pairs.

 

 

2. Time for a short field trip to the "life-size" DNA molecule.

The yellow atoms represent phosphorus bound to red oxygen atoms.

Based on your observations, draw a three-dimensional representation of phosphorus and the atoms to which it is bound.

Most of you had great drawings! This is important.

The shape described by the atoms bound to P is __tetrahedral___.

 

  1. Examine the large, white cobalt atom in the middle of Vitamin B12 (at the front of the lecture hall). Describe and draw the geometry around cobalt.

The shape described by atoms bound to Co is __Octahdral________.

4. The structure of nitric oxide synthase, the enzyme responsible for synthesizing NO, was reported in Science two years ago. The article describes NO as the ideal cellular signal molecule because it is small and requires no receptor, it is lipophilic and can traverse non-polar membranes, and its lifetime is short because it self destructs.

  1. Draw a Lewis structure which shows all of the NO valence electrons.
  2. 5+6=11 electrons or 5.5 pairs. .N=O:

Neither N or O is allowed to have an expanded octet!!!! EVER. NEVER.

b. Calculate the formal charges for N and O. Please show your work.

For N 5 valence minus 3 -4/2 = 0

For O 6 valence minus 4 -4/2 = 0

 

c. Only non-polar molecules or those with very small dipoles would be expected to pass through membranes. Is NO polar or non-polar? Justify your answer.

O is slightly more electronegative than N so there is a small dipole DEN= 0.5. This small dipole means that NO may dissolve in non-polar solvents and lipids, the substance that forms membranes. A lipophilic substance is one that "loves lipids" (you didn't need to know this).

That O "has" more electrons or that N has fewer electrons are not directly relevant here. To say that NO crosses membranes so it must not be polar is an example of circular reasoning.

 

d. Why is NO self-destructive?

NO is a free radical because N has an unpaired electron and its octet is not satisfied. It will immediately react will something and will thus be destroyed.

 

 

Atom

Valence electrons

H

1

C

4

O

6

S

6

Co

9

P

5

N

5

Comments on Writing Quiz #6 (at last)

 

General grading scale

10

Excellent structure and writing. Good synthesis and integration of concepts. No grammatical or chemical mistakes. Discussion of the implications of the problem or answer.

8-9

Same as above, but with minor flaws

6-7

Writing flaws, perhaps lacking introduction of general chemical prinicples, or minor chemical errors. Lacking in structured paragraphs.
   

<5

Serious chemistry mistakes.