JOURNAL OF CHEMICAL EDUCATION
To the Editor: I n a recent review article on "The Concept of Electronegativity" Pritchard and Skinner' point out that Mulliken's measure of electronegativity of an atom "is better and more precise than that of Pauling's." Mulliken's relation is: z(A) = I(A)
+ EA(A) 2
(1)
where %(A)is the electronegativity, I(A) the ionization potential and EA(A) the electron affinity of A. Mulliken has given a quantum mechanical argument for the use of this definition for electronegativity. Nevertheless, it may not be obvious that this quantity should be a measure of the "power of an atom in a molecule to attract electrons to itself."' The following is a simplified method for demonstrating that the electronegativity of an atom should be proportional to the sum of the ionization ~otentialand electron affinity of the atom. Consider the problem of deciding whether R will attract electrons more strongly to itself than A (i. e., whether B is more electroneczative than A) when the two atoms form a compound AB. The two extreme cases that may occur when A and B are brought together are the pure ionic forms, either A+B- or A-B+. If B is the more electronegative, A+B- would be more stable than A-B+ and so the change in energy on forming the former compound would be more negative than the latter, i. e., .
static term and a repulsion term. If one assumes the molecular systems are essentially two point charges a t the same internuclear distance and the repulsion terms are the same, then the energy terms, [ I , would be equal. Thus by substituting equations (5) and (6) into the inequality of ( Z ) , the condition necessary for B to be more electr~negat~ive than A is obtained; which can be rewritten: I(A)
+ EA(A) < I ( B ) + EA(B)
(8)
It is thus seen that a number proportional to the sum of the ionization potential and electron affinity can be assigned to each element and the greater this number the greater the electronegativity.
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To the Editor: While I agree with almost everything Dr. Abernethy has to say in his interesting article, "Some difficulties and common errors related to the designation of sugar configurations," (THIS JOURNAL, 33, 88 (1956)), I should like to amplify the discussion of the proper configurational prefixes for the tartaric arids. The common, dextrorotatory variety of tartaric acid AE(AtB-) < AE(A-Bt) (2) was assumed by Fisher t o have structure I, an assumption which has now been verified experimentally. The change in energy on forming each of the ionic According to the Fisher convention for representing the forms can be evaluated by considering their formation tetrahedral valences of carbon in a plane, the structure by the following steps, (a) the formation of the positive is also represented by projection formula 11. ion and negative ion, and (b) bringing the two ions toD-Glyceraldehyde has stmrture 111,and L-glyceraldegether to form the molecule. hyde structure IV. From this, it can he seen that the A+B-.A++B--A+B(3) COOH group of (+)-tartaric acid corresponds in position to the CHO group of D-glyceraldehyde or to the and CH20H'group of L-glyceraldehyde, ~vhilethe C2Ha03 (-CHOH.COOH) group in tartaric acid corresponds to the CH,OH group in D-glyceraldehydeor to the CHO The change in energy for formation of A+B-is: group in kglyceraldehyde. AE(ACB-) = I ( A ) - E A ( B ) + U ( A + B - ) (5) The assignment of D- and L- symbols to tartaric acid and for A-B+ it is: (and to any other asymmetric molecule containing more than one group different from those in glyceraldehyde) AE(A-Bt) = I ( B ) - EA ( A ) + U ( A - B + ) (6) therefore requires specification of which group "corThe energy term U contains an attractive electro- responds" to the groups in glyceraldehyde; without 'PRITCHARD, H. O., AND H. H. SKINNER, Chem. Revs.. 55, such specification, stated or implied, the assignment is meaningless. Thus, the Freudenberg assignment of a 745 (1956).
ILLINOIS ASSOCIATION OF CHEMISTRY TEACHERS OCTOBER 19, 1956, is the date of the fall meeting of the Illinois Association of Chemistry Teachers to be held on the campus of the University of Illinois at Urbana. A session of papers and discussions is being planned hy President Carl Weatherbee and Professor B. Roger Ray of the university chemistry faculty.
