CHEMICAL CHEMICAL BONDINGBONDING

Set 4Set 4

CHEMICAL CHEMICAL BONDINGBONDING

Set 4Set 4

Cocaine

SAVE PAPER AND INK!!! When you print out the notes on PowerPoint,

print "Handouts" instead of "Slides" in the print setup. Also, turn off the

backgrounds (Tools>Options>Print>UNcheck

"Background Printing")!

Credits

• Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.com

• Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Violations of the Octet RuleViolations of the Octet RuleViolations of the Octet RuleViolations of the Octet RuleUsually occurs with B and elements of Usually occurs with B and elements of

higher periods. Common exceptions higher periods. Common exceptions are: Be, B, P, S, and Xe. are: Be, B, P, S, and Xe.

BF3BF3

SF4SF4

Be: 4Be: 4

B: 6B: 6

P: 8 OR 10P: 8 OR 10

S: 8, 10, OR 12S: 8, 10, OR 12

Xe: 8, 10, OR 12Xe: 8, 10, OR 12

Lewis Structures: Comments about the Octet Rule• The second-row elements C, N, O and F should always be assumed to

obey the octet rule.• The second-row elements B and Be often have fewer than eight

electrons around them in their compounds. These electron-deficient compounds are very reactive.

• The second-row elements never exceed the octet rule, since their valence orbitals (2s and 2p) can accommodate only eight electrons.

• Third-row and heavier elements often satisfy the octet rule but can exceed the octet rule by using their empty valence d orbitals.

• When writing the Lewis structure for a molecule, satisfy the octet rule for the atoms first. If electrons remain after the octet rule has been satisfied, then place them on the elements having available d orbitals (elements in Period 3 or beyond).

Resonance

• Resonance occurs when more than one valid Lewis structure can be written for a particular molecule.

• Example: Nitrate Ion

Resonance Explained

• If any one of these structures accurately represented the bonding in the nitrate ion, there should be two types of N-O bonds observed in the molecule: one shorter bond (the double bond) and two identical longer ones (the two single bonds).

• Experiments clearly show that the nitrate ion exhibits only ONE bond length and bond strength between those expected for a double and single bond.

• Resonance structures are the depictions of all possible structures, but in reality, the molecule’s bonding is the average of all three.

Structural Isomers

http://xsiborganic1011.wikispaces.com/TangYaoFlo

• Examine the three models of the three alkanes above to determine how they are similar and how they are different.•All three have 5 carbon atoms and 12 hydrogen atoms, so they all have the molecular formula C5H12.•BUT, you can see that the arrangements of the atoms results in three different compounds: pentane, 2-methyl butane, and 2,2 –dimethyl propane

Isomer Definitions• Isomers are two or more compounds that have the same

molecular formula, but their atoms are bonded in different arrangements.

• Structural isomers have different chemical and physical properties despite having the same chemical formula.

• The b.p. of each of the three isomers shown in the previous slide are as follows (in order as shown): 36°C, 28°C, 9°C– This observation supports a main principle of chemistry:

structure determines properties.– How does the trend in boiling points of C5H12 isomers

relate to their structures?

Number of Possible Isomers

• As the number of carbons in a hydrocarbon increases, the number of possible structural isomers increases.– There are nine alkanes with the molecular formula

C7H16

– There are more than 300,000 structural isomers with the formula C20H42

Odd-Electron Molecules

• Relatively few molecules formed from nonmetals contain odd numbers of electrons.

• Two common examples are NO and NO2 (try drawing them)

• Since the Lewis structure model is based on electron pairs, it does not handle odd-electron cases in a natural way. We usually just tack on the odd electron onto the central atom.

Homework Questions

• 1) Which has the greater bond lengths: NO2- or

NO3- ? Explain. (HINT: You’ll have to draw the

Lewis structures first)• 2) The following ions are best described with

resonance structures. Draw the resonance structures: a) NCO- b) CNO-

• 3) Explain the terms resonance and its connection to delocalized electrons. Use additional resources if necessary.

• 4) Explain the difference between a resonance structure and an isomer.