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What’s coming up???

What’s coming up???. Oct 25 The atmosphere, part 1 Ch. 8 Oct 27 Midterm … No lecture Oct 29 The atmosphere, part 2 Ch. 8 Nov 1 Light, blackbodies, Bohr Ch. 9 Nov 3,5 Postulates of QM, p-in-a-box Ch. 9 Nov 8,10 Hydrogen and multi – e atoms Ch. 9 Nov 12 Multi-electron atoms Ch.9,10

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What’s coming up???

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  1. What’s coming up??? • Oct 25 The atmosphere, part 1 Ch. 8 • Oct 27 Midterm … No lecture • Oct 29 The atmosphere, part 2 Ch. 8 • Nov 1 Light, blackbodies, Bohr Ch. 9 • Nov 3,5 Postulates of QM, p-in-a-box Ch. 9 • Nov 8,10 Hydrogen and multi – e atoms Ch. 9 • Nov 12 Multi-electron atoms Ch.9,10 • Nov 15 Periodic properties Ch. 10 • Nov 17 Periodic properties Ch. 10 • Nov 19 Valence-bond; Lewis structures Ch. 11 • Nov 22 VSEPR Ch. 11 • Nov 24 Hybrid orbitals; VSEPR Ch. 11, 12 • Nov 26 MO theory Ch. 12 • Nov 29 MO theory Ch. 12 • Dec 1 bonding wrapup Ch. 11,12 • Dec 2 Review for exam

  2. THE OCTET RULE Atoms combine to form compounds in an attempt to obtain a stable noble gas electron configuration with 8 electrons in the valence shell A stable electronic configuration can be attained in two ways……. ELECTRON TRANSFER IONIC BONDING COVALENT BONDING ELECTRON SHARING LEWIS MODEL OF BONDING

  3. IONIC BONDING Cl Cl Na Na electron transfer Cl 1s22s22p63s23p5 1s22s22p6 3s1 symbol for element Lewis Symbol and a dot for each valence electron  Na+ 1s22s22p63s23p6 1s22s22p6

  4. COVALENT BONDING F F F F electron sharing Atoms go as far as possible toward sharing electron pairs completing their octets by Consider F2 The electronic configuration of F is 1s22s22p5 Lewis Symbols….. These are combined to form F2

  5. COVALENT BONDING F F F F F F electron sharing Atoms go as far as possible toward sharing electron pairs completing their octets by F 1s22s22p5 + F 1s22s22p5 + or bonding pair of electrons non-bonding, or lone pair of electrons The simplest molecule

  6. IONIC OR COVALENT? The type of bond that forms depends on the electronegativity difference between the two atoms involved in the bond

  7. IONIC VERSUS COVALENT BONDS Compounds composed of elements with a large difference in ELECTRONEGATIVITY tend to have significant ionic character in their bonding B has a greater share A B

  8. H F H F HYDROGEN FLUORIDE Fluorine is more electronegative than hydrogen. + +

  9. LEWIS BONDING MODEL Lewis structures are based on a localized electron model Electrons are always localized in one of two ways: As lone pairs on a specific atom OR As bonding pairs between two specific atoms

  10. H O H Drawing Lewis Structures Lewis structures consider only valence electrons H2O Water bonding pair of electrons non-bonding, or lone pair of electrons How do we draw these?????

  11. Building Lewis (ELECTRON DOT) Structures of Molecules. HCN as an example... Count the total number of valence electrons Step 1. H has 1 Total of 10 C has 4 N has 5 Place one e-pair between each BONDED atom Step 2. H C N We have 6 e- left All atoms must have an octet or duet Add electrons to terminal atoms first Step 3. to get an octet or duet.

  12. Building Lewis (ELECTRON DOT) Structures of Molecules. Add remaining electrons to terminal atoms first Step 3. Add 6 electrons in pairs to give the N an octet. N H C Step 4. Add any electrons left over to central atom We have none left! Step 5. Check for an acceptable Lewis Structure Do all atoms have an octet? IN THIS CASE

  13. H H C C N N Building Lewis (ELECTRON DOT) Structures of Molecules. Step 5. Check for an acceptable Lewis Structure bring electron pairs from outer N atom to form shared pairs to give C its octet!!! H C N Still no octet on C Do it again!!!! three electron pairs between the C and N………...

  14. H N C H C N H C N Building Lewis (ELECTRON DOT) Structures of Molecules. Another possible structure is…. How can we choose? FORMAL CHARGE

  15. FORMAL CHARGE gives an indication of the extent to which atoms have gained or lost electrons in the process of covalent bond formation. Each atom is assigned all of its lone electrons and half of the electrons bonded to it. } _ { } Formal charge { _ #valence electrons #unshared electrons 1/2#shared electrons = Structures with the lowest formal charges are likely to have the lowest energy.

  16. } _ { } Formal charge { _ 1/2#shared electrons #valence electrons #unshared electrons = .. H C N H N C H C N All possible Lewis structures with stable electronic configurations for HCN and HNC. Calculate formal charge for this one - 1/2 (8) FC on C = 4 - 0 = 0 = 5 - 2 - 1/2 (6) = 0 FC on N Hydrogen is zero

  17. H N C H C N All possible Lewis structures with stable electronic configurations for HCN and HNC. 0 +1 -1 0 0 0 WE CHOOSE THE STRUCTURE WITH THE FORMAL CHARGES CLOSEST TO ZERO AND ANY NEGATIVE FORMAL CHARGES ON THE MOST ELECTRONEGATIVE ELEMENTS

  18. H C O H Example: H2CO Total number of valence electrons = 6+4+1+1=12 Put carbon in the middle….. Place electrons between atoms Now add remaining electrons to O Now O has an octet but C does not…. Share a pair between C and O Now we have a double bond between C and O

  19. H H C O C O H H H O C H Example: H2CO Total number of valence electrons = 6+4+1+1=12 Give C an octet We can write this….

  20. Let’s look at the nitrate anion NO3- Count up valence electrons N has five 1s22s22p3 O has six 1s22s22p4 Plus one extra for negative charge Valence electrons = 5 + 3 x 6 + 1 = 24

  21. O O O N Nitrate anion NO3- Put a pair between each atom nitrogen does not have noble gas structure!!! form a double bond by sharing a pair from one of the oxygen atoms……….

  22. - O O O O O O N N - - O O O N FORM A DOUBLE BOND BETWEEN O AND N Here is one Here is another! Here is another!

  23. O O O - O O O N Experiment shows all three bonds are the same. All bond lengths 128 pm N All bond angles 120 0 Any one of the structures suggests one is different! Double Bond Single Bond Should be different! So…….

  24. O O O O O N N N O O O O RESONANCE We use a double headed arrow between the structures.. The electrons involved are said to be DELOCALIZED over the structure. The blended structure is a RESONANCE HYBRID

  25. S S O O O O LEWIS STRUCTURE SO2 Experiment shows that both S-O bonds are equivalent. We say that the real SO2 molecule is a hybrid of the two resonance forms.

  26. EXCEPTIONS TO THE OCTET RULE……. Molecules with more than 8 electrons around central atom. Molecules with less than an octet around central atom Molecules with unpaired electrons. Lets do SF6……..

  27. F F F S F F F - I I I Elements in rows 3 and following can exceed the octet rule: When it is necessary to exceed the octet rule the extra electrons go on the central third row element. S … 12 SF6 CentralI … 10 I3-

  28. FREE RADICALS O N O O N O Molecules which have unpaired electrons. NO2 Is a free radical Total number of valence electrons = 5+6+6 = 17 O N O Form double bond to get N close to octet RESONANCE

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