Part A: Lewis Structures (How do I draw a legitimate Lewis structure?)

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1 hemactivity 2 Lewis Structures 1 hemactivity 2 Part A: Lewis Structures (ow do I draw a legitimate Lewis structure?) Dot and line-bond representations which follow certain rules will be called legitimate Lewis structures. Model 1: G.. Lewis Method for Predicting Molecular Properties In the early part of this century, a chemist named Gilbert. Lewis devised a system for diagramming atoms and molecules. The system is still used today because predictions made from these simple diagrams often match experimentally determined ion charges, bond angles, molecular shapes, bond orders and other chemical properties. Lewis proposed the following representations for the first ten elements with their valence electrons. e Li Be B e f these 12 elements, only e and e are found in nature as single neutral atoms (as shown). The other elements are found as ions or as parts of molecules. Lewis proposed that any atom, ion or molecule that can be represented by a legitimate Lewis structure should exist in nature or be possible to make in the laboratory. A legitimate Lewis structure is a dot or line bond representation in which: 1. The sum of the electrons around a hydrogen is two (a bonding pair). 2. The sum of the electrons (in bonding pairs and lone pairs) around a carbon, nitrogen, oxygen or fluorine atom is eight an octet. (The octet rule. ) or example: 2 = "lone pair" "bonding pair" 2 = we will usually use this line-bond notation

2 hemactivity 2 Lewis Structures 2 igure 1: Shell and Lewis Representations of Selected ompounds +1 Shell Lewis ritical Thinking Questions 1. The valence shell of an atom in a legitimate Lewis structure (e.g.,, or, above) has what in common with the valence shells of e, e and all elements in the last column of the periodic table? They all have a completely filled outer shell (valence shell). 2. Draw a shell representation and Lewis structure for the ion of fluorine that you predict is most likely to be stable. Explain your reasoning. (overall -1 charge) (overall -1 charge) +9 with an extra electron has a filled valence shell. 3. Draw a Lewis structure of a neutral molecule that is a naturally occurring combination of fluorine atoms and one carbon atom. is equivalent to What is the shape of the molecule in TQ 3? tetrahedral 5. Make a checklist that can be used to determine if a Lewis structure for a molecule is legitimate. i. Make sure the total number of valence electrons is correct ii. Make sure there are 8 electrons around each,, or. iii. Make sure there are 2 electrons around each.

3 hemactivity 2 Lewis Structures 3 6. a) ow many valence electrons does one nitrogen atom have? 5 b) ow many valence electrons does one hydrogen atom have? 1 c) ypothetically, how many valence electrons would a (neutral) 4 molecule have if it could exist? 9 d) + ow many valence electrons does one 4 ion have? 8 e) + Draw the Lewis structure for 4 (overall +1 charge) 7. Describe how to calculate the total number of valence electrons in an ion (+ or -)? ount the number of valence electrons you would expect for a neutral molecule and then add one if the ion has a charge, or subtract one if the ion has a + charge. Model 2: Multiple Bonds Table 2a: Bond Information for Second Row Diatomic Molecules ormula Lewis Structure Bond rder Type of Bond umber of Bonding Domains 2 1 single bond double bond triple bond 1 ritical Thinking Questions 8. Does the Lewis structure of 2 satisfy the checklist you made in TQ 5? yes 9. ow many electrons are involved in a triple bond? onsider the molecule 2. a) Draw the Lewis structure. b) ow many bonding domains are there around carbon, the central atom? 3 c) What is the shape of this molecule? Trigonal planar d) Are your answers above consistent with a bond angle very close to 120 o? Yes

4 hemactivity 2 Lewis Structures 4 Part B: ormal harge (ow can you choose between two possible Lewis structures?) Model 3: Two Lewis Structures for 2. I II Experimentally, we find that both carbon-oxygen bonds in 2 are identical. ritical Thinking Questions 11. Are both structures in Model 3 legitimate Lewis structures? Yes. 12. Which Lewis structure in Model 3 best fits the experimental data? I best fits the experimental data because it predicts equivalent carbon-oxygen bonds. Model 4: ormal harge. or many molecules we can draw more than one legitimate Lewis structure. In these situations it can be helpful to calculate each atom s formal charge. This will help you determine hot spots of + or charge on a structure. or each atom formal charge = core charge* number of assigned electrons Rules for assigning electrons: 1. on-bonding electrons are assigned to the attached atom. 2. Shared electrons are evenly divided between the bonded atoms. (*core charge = nuclear charge number of core electrons) Example: Electron accounting for a Lewis structure of ammonia, 3 1 electron to 1 electron to 2 nonbonded electrons to 1 electron to 1 electron to 1 electron to 1 electron to ormal charge on the nitrogen = +5 (core charge) 5 (assigned electrons) = 0 ormal charge on each hydrogen = +1 (core charge) 1 (assigned electron) = 0 Zero formal charges are not written on Lewis structures so we do not need to add any formal charges to the Lewis structure of ammonia shown above.

5 hemactivity 2 Lewis Structures 5 rom now on, a Lewis structure is not complete unless the non-zero formal charges are indicated. (Place a circled + or next to the atom holding that formal charge, as in TQ 18, structure II, on the next page.) ewer non-zero formal charges = a better Lewis structure for making predictions. ormal charges greater than ±1 are never found in good Lewis structures. ritical Thinking Questions 13. alculate the formal charge on each atom of the following Lewis structures, and write in these formal charges where appropriate. 14. Add formal charges to each Lewis structure of 2 in Model 3. a) Based on the concept of formal charge, which is the better Lewis structure for 2 (in Model 3) I or II? Explain your reasoning. I has no non-zero formal charges. II has a +1 formal charge and a -1 formal charge. This means that I is a better Lewis structure. b) Is your choice consistent with the experimental data? Yes. I fits the experimental data and has fewer non-zero formal charges than II. 15. Draw a Lewis structure (including any non-zero formal charges) of carbon monoxide (). int: there is no Lewis structure for with all zero formal charges. 16. T or : The sum of the formal charges on a Lewis structure is always equal to the total charge on the molecule or ion. True 17. T or : If the net charge on a molecule is zero, the formal charge on every atom in the molecule must equal zero. alse

6 hemactivity 2 Lewis Structures 6

7 hemactivity 2 Lewis Structures Two Lewis structures for glycine (an amino acid) are given below. I II a) Predict the bond angle based on the Lewis structure on the left o b) Predict the bond angle based on the Lewis structure on the right. 120 o c) Which prediction do you expect to be more accurate? Explain why. The bond angle will likely be close to o since this is the prediction based on the better Lewis structure (the one with fewer overall formal charges). 19. Make a checklist that can be used to determine if a Lewis structure is correct and that it is the best Lewis structure. i. Make sure the total number of valence electrons is correct, adding or subtracting electrons if it is an ion. ii. Make sure there are 8 electrons around each,, or. iii. Make sure there are 2 electrons around each. iv. Make sure the total number of formal charges is at a minimum. + Exercises for Part A 1. Draw the Lewis structure of a neutral molecule that is a naturally occurring combination of hydrogen atoms and one sulfur atom. What is the shape of this molecule? 2. Draw legitimate Lewis structures of the following species and predict the geometry about the central atom (shape). a) 3 d) + 2 b) l 4 e) 2 3 c) 2 f) 2 (try with or at center) 3. or each element, predict (and draw a Lewis structure of) the most commonly occurring ion: a) sulfur c) magnesium b) iodine d) oxygen 4. Predict which of the following species is T commonly found in nature. 2 +

8 hemactivity 2 Lewis Structures 8 5. The molecules B 3 and S 6, and the ion S 4 2- are found in nature. Is this consistent with Lewis predictions? Explain why or why not. 6. The following are T legitimate Lewis structures. Show (as in the example) how a single pair of electrons can be moved to make the Lewis structure legitimate. (curved arrow shows where the electron pair was moved from and to) legitimate Lewis structure 7. Read the assigned pages in your text, and do the assigned problems. Exercises for Part B 8. Some of the following Lewis structures are missing formal charges. ill in the formal charges (other than zero) where needed. Then use your list of factors (from TQ 19) to verify that each completed Lewis structure is correct. a) b) c) d) S S e) f) S or a-f in Exercise 8, predict the shape of the molecule or the geometry about each central atom, as appropriate. 10. The following Lewis structure for has no formal charges. Explain why this is not a valid Lewis structure.

9 hemactivity 2 Lewis Structures omplete each row on the following table by drawing an example of a molecule containing the atom listed with +1, 0 and 1 formal charge and the correct number of hydrogens and lone pairs. (ote: entry for n + is an exception to the octet rule.) Exception to octet rule (uncommon) 12. Each on the table above can be replaced with any atom and the formal charges will stay the same. This means, for example, that any oxygen with three bonds will have a +1 formal charge. a) What is the formal charge on any with 4 bonds to it? b) What is the formal charge on any with one bond to it? c) What is the formal charge on any with two single bonds and one double bond to it (as in 2 )? d) What is the formal charge on any with one single bond and one double bond to it?

10 hemactivity 2 Lewis Structures 10 e) ow many total bonds (a double bond counts as two) must a neutral atom have going to it? f) Draw an example of a molecule containing a neutral with a double bond to that. 13. arbon is a little strange in that it does not always follow the octet rule. We will learn about this later in the course. or now know that: with three bonds and a lone pair must have a 1 formal charge. with three bonds and no lone pair must have a +1 formal charge. a) A molecule containing a with a 1 charge must have how many bonds and lone pairs to that? b) A molecule containing a with a +1 charge must have how many bonds and lone pairs to that? 14. Read the assigned pages in your text, and do the assigned problems.

11 hemactivity 2 Lewis Structures 11

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