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Unit 8

Unit 8. Lewis Structures; Resonance; Exceptions to the Octet; Molecular Geometry; Sigma and Pi bonds. Lewis Structures: general rules to follow.

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Unit 8

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  1. Unit 8 Lewis Structures; Resonance; Exceptions to the Octet; Molecular Geometry; Sigma and Pi bonds

  2. Lewis Structures: general rules to follow • Many molecular (covalent) compounds follow the octet rule which assumes that when an atom is correctly bonded, it will have 8 electrons either shared or otherwise around it. .. .. • :Cl : Cl: Cl2 .. .. • Electrons not shared are called lone pairs and these have electron repulsion that affect the shape of the molecule. • When the electron pairs exactly offset each other, they have the largest bond angle , 180 degrees

  3. Calculate the number of bonds you need • Consider the compound, CH2Cl2, Counting up valence electrons you get C= 4; H=1 x 2=2; and Cl=2 x 7=14 with a total of 20 valence electrons • Subtract from this the number of electrons needed for all atoms to fulfill the octet (C=8; H=2 x 2=4; Cl=8 x 2=16 total needed is 28; divide the result by 2 to get the # of bonds • 28-20/2 = 8/2= 4 • 4 = the number of bonds on the central atom • You should assume carbon is always the central atom because it usually has the lowest electron-negativity, you should also assume that hydrogen is a “terminal” atom (only bonds to one other atom) not a central atom.

  4. Draw the Lewis Structure • Draw these structures, for each single bond sharing one electron from each atom, add in the dots to complete the valence for each atom (remember hydrogen doesn’t get ANY EXTRA) • a. HF d. CH4 • b. N2e. CF4 • c. NH3f. NO+ • Can you predict if there will be a dipole and if so in what direction?

  5. Lewis Structures: general rules to follow • Many molecular (covalent) compounds follow the octet rule which assumes that when an atom is correctly bonded, it will have 8 electrons either shared or otherwise around it. .. .. • :Cl : Cl: Cl2 .. .. • Electrons not shared are called lone pairs and these have electron repulsion that affect the shape of the molecule. • When the electron pairs exactly offset each other, they have the largest bond angle , 180 degrees

  6. Is the molecule polar or non-polar? ( we are not talking about the bonds here) IF octet rule is obeyed AND all the surrounding bonds are the same [even if very polar] then the molecule is NONpolar since all the dipole moments cancel each other out.

  7. What about multiple bonds? • If all you have is a single bond this is called a sigma bond(σ) • If there is double bond, then the first bond is a sigma bond, σ, the second bond is called a pi bond,π. • If there is a triple bond then there is one sigma bond and two pi bonds, π. • There are only certain atoms that can make multiple bonds, C, N, O, P, S, remember it as “C-NOPS”

  8. Comments about the Octet Rule • Second row elements, like C, N, O, P, S should always be assumed to obey the octet rule. • Second row elements Be and B often can be found to have fewer than eight electrons • The second row electrons never violate the octet rule they have 2s and 2p and can accommodate no more than eight electrons • Third period and heavier elements often follow the octet rule but they can exceed it by using their empty valence d orbitals • PROCEDURE: When writing the Lewis Structure for a molecule, satisfy the octet for each element first, if there are still electrons remaining, then place them on the central element which should have available d orbitals (3d or higher)

  9. How do you tell if there are multiple bonds? Once all the electrons have been placed Around each atom, if the central atom is not yet surrounded by four electron pairs, convert one or more terminal atom lone pairs to pi bond pairs. Try Oxygen gas, O2, and carbon dioixde and sulfur dioxide

  10. VSEPR determines a molecule's shape:Valence Shell Electron Pair Repulsion Theory • Molecular shape changes with the numbers of σ bonds plus lone pairs about the central atom.

  11. What do they look like? 180° linear [and of course planar] 120° trigonalplanar

  12. 120° & 90° trigonalbipyramidal • 109.5° tetrahedral

  13. 90° octahedral

  14. Try another, and another… and another and so forth Drawing Lewis Structures (predicting molecular shape) Give the Lewis structure for each of the following, is there a dipole? What about polarity of the molecule?: a. HF d. CF4 b. CO2e. ICl2+ c. NH3f. HCN

  15. Solution • H-F with the remaining electrons around the F the noble gas, octets are fulfilled. A linear shape is made but there is a dipole from H to F and this makes the molecule polar. • O=C=O when remaining valence electrons are placed around the oxygens the octets are fulfilled, a linear shape is made and there is no dipole because they are cancelled out

  16. Solution . . • H – N – H the extra electrons on the N bend the H H downward, making a trigonal pyramidal shape, the dipole is generally downward thus making the molecule polar. d. F F- C –F The valence electrons fill the octet F around each F making a tetrahedral shape, there is no dipole due to canceling, and thus it is a non-polar molecule

  17. Solution • [Cl – I – Cl ]+ When extra valence are arranged around the Cl’s and the remaining are placed on the central atom (don’t forget the one less electron due to the 1+ charge), this makes a “bent or V-shaped” molecule; the dipole is generally downward and the molecule is polar. • H- C=N: putting extra valence on the N gives a linear shaped molecule but the dipole towards the N means it is a polar molecule.

  18. Exceptions to the octet: usually occur in the central atom of the 3rd and higher periods; • Place lone pairs about each terminal atom (EXCEPT H) to satisfy the octet rule. Left over pairs are assigned to the central atom. • If the central atom is from the 3rd or higher period, it can accommodate more than four electron pairs. • Calculate the number of bonds for phosphorus penta-chloride: P= 5 Cl=7 x 5= 35; 5 + 35= 40 • Valence if all atoms have fulfilled their octet= P=8 and Cl=5 x 8= 40 so 48. • 48- 40/ 2= 4 bonds but that’s not enough for the five Cl in the formula this means it is an EXCEPTION!!! • Write the Lewis structure for PCl5.

  19. Solution See this as two axial (north and south) and three equatorial chlorines, this represents the farthest apart electron configuration possible; What Shape is this?

  20. Other Exceptions to the octet • when there are an odd number of valence electrons • Be and B are always “electron deficient”, not enough to fill the octet. • Sometimes there is uneven sharing and only the central atom has to share but they still end up following the octet rule

  21. Try these problems Write the Lewis structure for each molecule or ion. a. ClF3 b. XeO3 c. RnCl2 d. BeCl2 e. ICl4-

  22. Solutions • The chlorine atom (third row) accepts the extra electrons. Shape? Dipole? Polarity? b. All atoms obey the octet rule. Sometimes there is uneven sharing, when the central atom shares but the others don’t. Shape? Dipole? Polarity?

  23. Solutions • Randon, a noble gas, accepts the extra electrons and bonds with the Cl. Shape? Dipole? Polarity? d. Beryllium is electron deficient. Shape? Dipole? Polarity?

  24. Solutions e. Don’t’ forget the extra electron from the ion when you calculate the # of bonds. Iodine exceeds the octet rule. Brackets are used to show the ion charge.

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