Prediction of Molecular Polarity by V.S.E.P.R. Theory Beginning chemistry students often have difficulty determioing whet,ller simple molecules are polar or nonpolar. The usual criteria for molecular polarity such as, "the noncoincidence of the centers of po.iit,ive and negative charge," or.. "polar bonds in nonsvmmetrieal molecules." sometimes are of little heln t,o a student havine diffieolties with t,he . concept of molecular polarity. A discussion of molecular shapes usually precedes or accompanies a discussion of polarity of molecules. Several recent texts make use of V.S.E.P.R. theory' to determine molecular shapes. According t o V.S.E.P.R. theory, molecular shapes are determined by the number of groups, X, and the number of lone elecbron pairs, E, smrmmding the central atom, A. Simple molecules can he placed into categories AXm E n where: m = the number of X groups attached to A and n = the number of lone electron pairs on A. With the exception of heteronuclear diatomic molecule.+ any simple molecule for which n # 0 will he a. polar molocule and any molecule for which n = 0 will be a nnnpolar molecule. Since students usually experience little diffieult,y in placing simple molecules in the proper AXm E n cat,egory, they are able t o use the principles of T'.S.E.P.R. theory to predict molecular polarity wit,h a high degree of success. An obvious objection t o the m e of this method is that a student can predict whether moleedes are polar or nonpolar and still not have a real understanding of the concept of molecular polarity. Therefore, this method is useful as a supplement to, rather than a5 a replacement for the usual diicussion of this topic. The advantage of the use of V.S.E.P.R. theory in this regard is that i t gives the student a simple mebhadwhich he can use with n high degree of success.
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GILLEBPIE, R. J., J. CHIIM. EDUC.,40,295, (1963). %Otherexceptions are the categories AX.E3 and AX,E. which include malecule~and ions such IC14-, andXeF,. These examples sro unlikely to be encountered in an introductory discmsian.
as
N. A.
IC12-, XeF2, I)AOGHI,:RTY
Volume 46, Number 5, May 1969
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