Issue Date: March 21, 2011
Pondering The Atom’s Weight Versus Mass
I was surprised that C&EN would devote two columns to the letter by Harvey Carroll on the matter of atomic weight versus atomic mass (C&EN, Jan. 24, page 4). I realize the majority of the informed scientific community is thoroughly acquainted with what I am about to share, but for the benefit of Carroll and others who are apparently otherwise informed, I present the following.
The “CRC Handbook of Chemistry and Physics” prides itself on accuracy and being updated on all data entries. According to the 86th edition of the handbook, weight is defined as that force which, when applied to a body, gives it acceleration equal to the acceleration due to gravity; mass is that quantity of matter which resists acceleration. Hence, the term weight is defined relative to the force of gravity tending to accelerate an object of finite mass, and mass is defined as that quantity of matter that resists acceleration because of gravity. So within Earth’s environment of gravity, the concepts of weight and mass are interchangeable, as is also true of their units.
Furthermore, in the Table of the Isotopes in the same CRC handbook (page 11-50), it is stated: “Atomic Mass or Atomic Weight [emphasis added]—Atomic mass relative to 12C = 12. Atomic weight is given on the same scale.” So it is clear that atomic weight and atomic mass are both given in the same conventional units relative to 12C. The conventional units of mass—the kilogram or gram—are the same units for weight. Thus, in the case of 12C, the atomic mass (or weight) is 12.0000 g (per mole), which means a mole of 12C atoms will weigh or have a mass of 12.0000 g.
Similarly, for 13C its mole mass (or weight) is 13.00355 g. But the natural abundance of 12C is 98.90% while that of 13C is only 1.10%. Consequently, the weighted average mass (or weight) listed in the periodic table for carbon is 12.011 (rounded off to five significant figures). The atomic weights (or masses) of all other elements are derived relative to 12C in the same way. Consequently, if an element has only one naturally abundant isotope, or if it is radioactive and only the most stable isotope is accepted, there will be no weighted average atomic weight (or mass).
The point is that not only was Carroll’s letter unnecessary, but it threatens to undermine the intelligence of the scientific community at large. The best that can be said about this letter is that it is nitpicking over semantics but has nothing to do with what is actually correct or incorrect.
Edward A. Boudreaux
I wish to thank Carroll for his letter discussing atomic mass and atomic weight. I include myself among the many chemists, other scientists, and engineers who regularly make errors in scientific discourse, as well as language in general. Most of my own errors are a result of simple ignorance and I do make every effort to correct such errors when identified.
Many people do not recognize the ramifications of tolerating language error, or they must consider such errors too trivial to concern them and make no effort to identify or correct errors when confronted. Among the more minor annoyances that result from this apathy for language precision and accuracy are mass media reports that speak of vehicles “traveling at a high rate of speed” and compounds and elements that do not exist, e.g., “toyulene” and “florine.” Is it any wonder the general public is unaware of what constitutes a chemical, much less what is organic and what is inorganic?
Where we tacitly approve of the misuse of language through our failure to correct its misuse, we invite abuses. Crafted terminology such as “collateral damage” has become a routine propaganda tool, and one need not search far afield to find examples where language abuse is both costly and dangerous. Imprecise and incorrect use of language is a fundamental tool for special interests who strive to discredit scientific data. As scientists, we cannot rationally expect to educate the populace in the scientific bases affecting policy decisions where we do not share a common understanding of the language.
I urge all of my fellow scientists, and fellow citizens, to cease tolerance of language misuse and strive for personal perfection as a first step to improving communication throughout our society. I urge all chemists to join and support IUPAC. I also urge chemistry educators to avoid teaching multiple systems of chemical nomenclature—IUPAC’s should suffice—and urge consumer chemical companies to label products for the 21st century. Why must I adjust the pH of my pool with “muriatic acid”?
With respect to Carroll’s letter, the American Heritage Dictionary (online) defines atomic weight as, “The average mass of an atom of an element, usually expressed relative to the mass of carbon 12, which is assigned 12 atomic mass units.” Such erroneous references must be corrected. In contrast, editorially open Wikipedia gets it just about right.
As a teacher of general chemistry, I found Carroll’s letter interesting and informative. It is probably futile for me, not an IUPAC guru, to make any suggestions on this topic of atomic weight versus atomic mass; however, I will do so. Much of the problem is the repetition of the vague term “atomic.”
What would be far more useful is to use the term “elemental weight” rather than “atomic weight.” The (unit-less) value of 10.811 for boron is the terrestrial weight of that element, namely the combination of all of the isotopes of the element boron in their crustal abundance—this would be the “elemental weight.” The term “isotopic mass” then refers to the mass (in unified atomic mass units) of a given isotope of whatever element, such as boron-11 at 11.009305 u.
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