Another Procedure for Writing Lewis Structures T h o m a s J. Clark Humboldt State University, Arcata, CA 95521 Many students have difficulty in learning to write Lewis structures for molecules and polyatomic ions. Several procedures for writing them are presented in general chemistry texts. I recently devised a scheme which has two uncommon features: (1) whether multiple bond& or expanded valence shells are present is first determined using the method described by Lever'; (2) the dots representing electrons are entered before any hydrogen atoms are placed on the structure. T h e verv simnle orocedure for writine a correct. or a t least reasonable, Lewis structure for a molecule or ion containing onlv s-block and D-block elements can be summarized in a series of steps.
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1) Identifv anv monatomic cations. The rules far writine Lewis qtmrttrka which follow are not needed for and do nat&lv .~~ . . . to rnonarmnlc cxiun*. Croup I and Gnmp I1 elrrnrntsarr almoqf ~
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nlusy~present ascations. Exceptions arc He in moct nmp,unds and the other elements of these two groups in organometallic compounds. Determine the number of dots to be shown in the structure of a molecule or polyatomic ion. There is one for each s and p electron in the valence shell of each constituent atom; there is one more for each neeative charee on an anion and one less for " each positive charge on a cation. Evaluate 6y + 2 where y is the number of atoms other than hydrogen in the molecule or ion. Compare the number of dots to be shown with 6y + 2. a) If the two are equal, all atoms in the aggregate obey the octet rule, and there are no multiple bonds. Examnles: P O P . CIHR.CCla. Br?. NHnC. C10sb) If the number of d o c tobe shiwnk lessthan 6; + 2, either of the following holds. i) There are multiple hands in the structure, a deficiency four Of two indicating a bond and a indicating either a triple bond or two double bonds. ii) An atom a Group I, 11, or 111element has less than an octet of electrons. Examnles: CHIO. GHa, NI, COz, CO, BeH2, LiCHz C) If the number of dog to be shown is greater than 6y 2, the central atom has an exoanded valence shell. Examoles: SF&. SeC4, XeFz, SbFsArrange the non-hydrogens in a likely fashion using the following principles asguides. Atoms of elements in GroupsK 111, likelyto be central atoms. If just One atom of some IV, and element and several atoms of some other element are present, the unique atom is likely to he central. Enter the dots making use of the decision made in step (4). Start with the dots far any multiple bond, placing them between the central atom and one of its neighboring atoms. Then complete the octets for the atoms attached to the central atom. Finally enter dots for any pairs on the central atom.
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Journal of Chemical Education
I ) Place the hydrogen atomson the formula in a way whichminimizes the number of atoms carrying formal charge and minimizes the magnitude of formal charges. Characteristics of atoms having zero formal charge when the octet rule is obeyed are given in the table. IV V VI VII Group Number of Shared Pairs 4 3 2 1 Number of Unshared Pairs 0 1 2 3 The formal charge is mare positive by one for each additional shared pair and more negative by one for each additional unshared paw.
Development of the Lewis structure of formic acid illustrates the procedure. Using step (2) one finds that the formula CHzOz requires 18 dots to be shown. Steps (3) and (4) lead to the conclusion t h a t one double bond is present. Carbon, a Group IV element, is central, and placing the dots on the formula as directed in step ( 6 ) gives the partial structbre shown on the left below. A glance shows that the 0 on the right having twoshared and twiunshared pairsalready has formal charee of zero. T h e other 0 and the C can have formal charce of zero if each shares a pair with H, and entering the H S completes the structure.
T h e procedure is not flawless, of course. T h e simple rules would have t o be modified to account for n i c k structures, to allow correct placement of the single H of formate ion, and to deal with odd-electron structures. T h e rules do not lead t o a atomis central in thiosulfate ion, and decision they do lead to a n incorrect structure for phosphorous acid. T h e most serious flaw is t h a t a n incorrect choice of central atom (e.g., 0 in CzH40)can occasionally cause the student to spend time developing a structure which cannot be properly completed. These defects do not seriouslv imoair the usefulness of this procedure. One should make s i r e i h a t a t the start students encounter only examples which are dealt with correctly by the simple rules. Students quickly become aware of the features possessed by correct Lewis structures, and one can then make modifications of the rules t o extend the range of applicability the procedure.
Lever, A.B.P., J. CHEM.EDUC.,49,819 (1972).
