Intersection 4

9/26/06

Reading: 8.6 bond length p 348-351

8.10 exceptions p 363-366

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The Cutting Edge Science at the Extent of our Knowledge

Oct 9th 6:00 – 7:30 PMInformation Storage: DNA and Transistors

Attendance Optional

Outline

• Concept Questions

• Lewis Structures

• Bond Length

• Ethics

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Question 1

What is the approximate number of carbon atoms it would take placed next to each other to make a line that would cross this dot: •

a.4 b.200 c.30,000,000 d.6.02 x 1023

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Question 2

Following is a list of properties of a sample of solid sulfur: i. Brittle, crystalline solid. ii. Melting point of 113oC. iii. Density of 2.1 g/cm3. iv. Combines with oxygen to form sulfur dioxide

Which, if any, of these properties would be the same for one single atom of sulfur obtained from the sample?

a. i and ii only. b. iii and iv only. c. iv only.

d. All of these properties would be the same. e. None of these properties would be the same.

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Find the Molecular Weight of:

Methane CH4

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C 12.011 g/mol (*1) H 1.007 g/mol (*4) CH4 16.039 g/mol

Cholesterol C27H46O

Question 3

Which of the following are true about 18.013 grams of water H2O :

a) Contains 2 hydrogen atoms and 1 oxygen atom

b) Contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms

c) Contains 1 mole of water molecules

d) Is 66.7% hydrogen by weight

e) Contains 6.02 x1023 water molecules

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Question 4

Which of the following samples could be methane, CH4?  A sample that contains:

(a) 25% hydrogen by weight; 75% carbon by weight

(b) 4.0 g of H atoms and 1.0 g of C atoms

(c) 0.40 mole of H atoms and 1.0 x 1023 C atoms

(d) 0.40 mole of H2 molecules and 0.20 moles of C atoms  

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Question 5

True or False? When a match burns, some matter is destroyed.

a. True

b. False

What is the reason for your answer to question 3?

a. This chemical reaction destroys matter.

b. Matter is consumed by the flame.

c. The mass of ash is less than the match it came from.

d. The atoms are not destroyed, they are only rearranged.

e. The match weighs less after burning.

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Lewis Structures

Count valence electrons: H 3 * (1) = 3

O 1 * (6) = 6

- 1

8 valence e-

O

H

H H

H3O+

8e- - 6 bonding e- = 2e-

O

H

H H

O

H

H H

+1

Formal charge = # valence electrons -# assigned electrons

Assigned electrons:

Nonbonding electrons + 1 e- from each bond

Sum of formal charges = charge of molecule

SCN-

16 valence e-

S C N

Note: Usually central atom is least electronegative

S C N 20e-

S C N S C N18 e-

S C N S C N S C N

16 e-S C N

+1 -2

S C N

-1

S C N

-1

N 3.0; S 2.5

Rules for Lewis Structures

• Count valence electrons• Select central atom; draw in bonds to connect

atoms• Fill octets with remaining electrons• If octets cannot be filled, increase bond order• Repeat octet filling and bond order increase until

satisfied• Minimize formal charges• Double check

Draw the Lewis Structure for Cyanate NCO-

N C O N C O N C O-1 -1 -2 +1

Formal charge rules• The smaller the better• Negative formal charges should reside on the more

electronegative atom• Avoid like charges on adjacent atoms

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B C N O F Ne

Al Si P S Cl Ar

Bond Order

O N O

-1

O N O

-1

1.5

O

N

OO O

N

O

O

O

N

O

O

-1 -1

-1

-1

-1

-1

1.33

Is there more than one Lewis structure for SO4

-2?

O

SO

O

O

-1

-1

+ 5 resonance structures

A

O

SO

O

O

-1

-1

-1

-1 +2

Draw the Lewis Structures for the Following Molecules

• BH3

• SF6

• PCl5

• NO

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Group 3

B

H

H

H

A

Expanded Octet

F

S

F

Cl

PCl

Cl Cl

ClF

F

F

F

An expanded octet can be achieved with atoms that have d-orbitals in their valence shell.

A

You must have an atomic orbital in the valence shell for every bond that you make.

You may not use all valence atomic orbitals to make a bond (ex, borane, water, sulfur hexafluoride.)

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H

H

HF

S

F

F

F

F

FH

O

H

A

Radicals

N O

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Exam 1

• Tuesday, October 3

• 8-10pm CHEM 1400– Material through Friday 9/29– Non-programmable calculator– Practice problems posted on-line

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