Teaching a Model for Writing Lewis Structures Juan Qullez Pardo Ext. lnstitut de Batxillerat, Santa Agueda sln. 46880 Bocairent, ValBncia, Espaiia The importance of using models and the difficulties in modeling have being pointed out by Martinand ( I ) . In this paper we present a didactic model whose objective is to improve the teachinghearning process in a specific point: the renresentation of Lewis structures. Writing Lewis structures is an ability that every student completing a course in general chemistry must acquire. Mastering this ability and the formal charge calculations for each atom in the Lewis structure, which represents a molecule or polyatomic ion, will help the student in predicting and understandine aualities such as molecular geometry, reactivity, etc., i d - w i l l give the student the basis for a better assimilation of concepts such as isomerism, resonance, etc. In spite of the importance of this aspect, many books on general chemistry do not give simple rules for the representation of Lewis structures, perhaps neglecting this aspect due to the excessive emphasis given to the introduction and develonment of the auantum mechanical model and the molecular orhital theory in basic or elementary courses in chemistrv. This lack of svstematization causes difficulties for manistudents when they try t o represent molecules and polyatomic ions using Lewis diagrams. In the past few years, various articles have been published (2-4) recognizing the importance of representing Lewis structures and the difficulty that this representation supposes for students due to the lack of simple, easy-to-apply rules. In these papers, the so called "6N 2" rule is used, whose application facilitates the representation of Lewis structures; its basis, however, can create unnecessary difficulties in understanding and can seem arbitrary to first-year students. Some texts give simple but limited procedures or rules (5-7) or procedures or rules that are too complicated and difficult to remember (8). Generally the ability of fust-year university students of chemistry at university to represent Lewis structures can be summarized as follows: ~
A~~~~~
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a way of representing Lewis strucIn a recent article (9), tures not hased on the octet rule is proposed. This particularity can lead t o teaching difficulties, especially when introducing the model. That is why, in our opinion, the didactic method must be based on the application of the octet rule and, a t a more advanced stage, i t will then be possible to perform the appropriate modifications. Representation of Lewis Structures: General Procedure1
This procedure is developed by its application to four examples: CCL, COz, So,'-, and SF4. Example 1: CCI. (a) Calculate the total number of valence electrons of the implicated atoms: (4 X 7) + 4 = 32 (Cl) (C) (Total)
~
+
(1) Most students are able to assien e correct Lewis structure to
smple molecules, in whieh there are no multiple bonds and the central atom has no expanded valence shell. (2) The first difficulties appear when there are multiple bonds. There are a great many cases in which a numher of bonds less than the correct number is written, and in many cases this causes some atomsto be surrounded by less than eight electrons. (3) The central atom is often chosen incorrectly. (4) Usually the number of electrons assigned to cations and anions is wrong, too. (5) The formal charge calculation is not carried out, and as a result we have representations of Lewis structures in which the high formal charges are not reduced through formation of multiple bonds. (6) Representation of molerules and ions in whieh the central emm expands the octet is not correct in mcet canes. T o correct the deficiencies just mentioned, a procedure for the representation of Lewis structures in p&posed. Thin procedure is bared on thedirect applicationof the octet ~ l e , in which the different steps t o be taken in order to achieve the final representation of the desired structure are rationally systematized.
456
Journal of Chemical Education
In the case of an anion, anumber of electronsequal to the charge will be added: OH-: 6
+
1 +1=8
(0) (HI In the case of a cation, a number of electrons equal to the charge will be subtracted:
+
NH,':
5 (4 X 1)- 1= 8 (N) (H) (b) The number of electronsneeded tmomplete theoctet with all the atoms is determined.
5 atoms X 8 electrons/atom = 40 electrons In case of existing hydrogen atoma, two electrons will be assigned to each of these atoms instead of eight:
+
CH,: 4 (4 X 2) = 12 (C) (H) (c) The number of shared electrans is calculated (b - a): as we need 40 electrons and we have only 32,40 - 32 = 8 electronsmust be shared. (d) Place each one of the elements around the central atom. (It is the less electronegativeatom that is usually the central atom in AB, species) c1
C1 C CI C1
(e) The number of bond regions is calculated. Each region corresponds to the area of union between two atoms. In our example, the carbon atom will be bonded to four chlorine atoms, so the number of regions will be four. (flThe number of "c electrons" is found (each pair corresponds to a single bond). It is two times the number of bond regions existing in themolecule, sincetwo electrons correspond toeach bond: 4 X 2 = 8 electrons.
'
Terminology assoclated with molecular orbital theory appears in Sectionsf and h. We are not attempting here to introduce the covalent bond concepts of the theory: we are simply anemptingto "differentiate" two types of shared electrons.
In this octet structure the fonnal charge of each oxygen is -1, and that of the sulfur is +2. Because sulfur can expand its valence shell, the Sod2ion can he renresented bv a Lewis dieram in which the sulfur &surrounded by12 electrons, reducing in-this way the high formal charges in the octet structure:
(g) Single bonds are drawn: CI
I I CI
(:I-c-CI
electrons" is determined.If such electrons (h) The number of exist, some of the single bonds written in section g must be multiple (double or triple). Their calculation is given by the difference between the total number of shared electrons and the number of "o" electrons (c - 0:8 - 8 = 0. (i) The total number of nonshared electrons (a - c) is calculated: 32 - 8 = 24. ti) The nonshared electrons are distributed so that each atom complies with the octet rule.
Lewis Structures for Compounds with Expanded Octets If the student tries to follow the procedure set out in the three previous examples t o write the Lewis structures of ions and molecules such as PCls, PFs-, SF4, SOF4, XeF2, etc., he or she will obtain absurd results. T h e algorithm used must be modified slightly when the central atom exhibits a n ex~ a n d e doctet. The student who has assimilated the eeneral procedure for representing Lewis structures will b e i b l e to detect the limitations and modifv them adeouatelv - (10). . . In general, the representation of these molecuies can be realized following these steps provided below (carried out for SF4).
Example 2: C02 4 (2 X 6) = 16 (a) 1m 10)
+
(e)
Two bond regions.
(0 Four o electrons, which correspond to two simple bonds. (9) O--C--O (h) 8 - 4 = 4 n electrons Possibilities: two double bonds or one triple bond: two double bonds: O=C=O one triple bond: O
