MOLECULAR and ATOMIC WEIGHTS F. E. BROWN Iowa State
T
College, Arnes, Iowa
HE determination of molecular and atomic weights is often omitted from textbooks for beginners in chemistry and, when discussed, is usually treated as though the process were totally different from the determination of any other weight. A widely used text says: "It must always he remembered that the atomic weights are relative weights. That is, the atomic weight of copper is 63.57, not 63.57 grams or any other actual weight, but 63.57 as long as 16 is accepted as the standard atomic weight of oxygen." Almost all authors of chemistry texts stress the statement that molecular and atomic weights are relative weights made in terms of an arbitrarily chosen standard, thus implying that other weights are actual, real, or absolute weights made in terms of a unit. All such statements are misleading and many of them are absolutely false. The only possible significance of the term relatiere weights is that the bodies considered are all weighed in terms of the same unit. All weights are actuel and real if they are weights a t all. An absolute weight is one made in terms of a fundamental unit. The weights of molecules and atoms are: (a) relative, i. e., they are all evaluated in terms of the unit, one-sixteenth the weight of the oxygen atom; (b) real and actual (The total mass of the universe is only the sum of the weights of the atoms which are grouped in molecules and constitute all matter.); and (c) absolute (They are weighed in terms of a unit more fundamental than the pound, the grain or even the gram, and their weights might be converted t o and expressed in any other unit of mass or weight.). The choice of the unit for weighing atoms and molecules was the result of more consideration, is better suited to its purpose, and is less arbitrary than any other one of the more than 200 units of weight or mass listed in "International Critical Tables." Onesixteenth of the weight of an oxygen atom is a unit and should not be referred to as a standard unless all other units are included in a discussion of units and standards. This discussion should be carefully differentiated from the discussion of the process for finding molecular and atomic weights; for it has no more relevancy here than i t has when one discusses the weighing of gold or hay. The unit used in expressing the weights of molecules and atoms is called a standard and denied a place with the other fundamental units by nearly all authors of texts in general chemistry, though it is the only unit used for this purpose by any person in any country, while every weight labeled by them as real, actual, or absolute is expressed by means of a unit which might be changed by crossing a political boundary line.
There have always been three steps in determining and recording a weight: (1) choosing a suitable unit; (2) comparing the weight of the body to that of the unit directly, or, more frequently, indirectly; (3) recording the ratio, weight of body t o weight of unit. Since the weighing of molecules and atoms involves an indirect comparison, the process can be explained more clearly by separating i t further. The steps are: 1. The unit one-sixteenth the weight of the oxygen atom has been chosen. 2. The oxygen atom weighs sixteen units. It would he a great advantage to teachers of chemistry. if they would adopt a name for this unit. Miuacrith was suggested when the weight of an atom of hydrogen was the unit and it appears amongst the added words at the foot of the page in a Wehster's Unabridged Dictionary. Shortened to "mic" it is used by a small group yet. "Stas" has been suggested as a name for this unit. Schlesinger. "General Chemistry," Longman's Green & Co., 1930, p. 60.
3. The oxygen molecule contains two atoms and weighs thirty-two units. 4. Thirty-two grams of oxygen fill 22.4 liters. Then a 22.4-liter volume of oxygen contains the number of molecules which weigh as many grams as one molecule weighs in atomic weight units. 5. This number of molecules for any other gas is found by measuring out 22.4 liters of that gas a t standard conditions. Or, the weight of a single molecule of any gas is as many atomic weight units as there are grams in 22.4 liters of it in the gaseous state a t standard conditions. The essential requirement is the securing of the right number of moleculesAvogadro's number. Avogadro's number may be counted out not only by measuring 22.4 liters of gas but also by rise of boiling point, lowering of freezing point, or osmotic pressure. 6. An appropriate analysis determines the composition of any substance. The percentage composition of each molecule is the same as that of an aggregation of these molecules. 7. The weight of a molecule multiplied by the fraction by weight of an element present gives the weight of that element in that molecule. 8. All amounts of an element in molecules are integral multiples of the atomic weight of that element. Then, the greatest common divisor of the weights of an element found in the molecules which contain it is probably its atomic weight. The idea used in steps 4 and 5 is the only difficult idea used in the process. It depends on the relationship existing between the gram and the atomic weight unit.
Avogadro's number, 6.062 X loz3,is not primarily the number of molecules in a mol. but more fundamentallv it is the ratio of a gram to an atomic weight unit. As soon as the atomic weight unit was chosen equal to 1/6.062 X loz3gram it was inevitable that 6.062 X loza particles of any kind should constitute a mol. Analogies are common. A group of 2000 similar objects weighs exactly as many tons as a single object
weighs pounds. The weight in grams of a single object can be determined to one part in a thousand without calculation by weighing 454 of these objects in pounds. Any group of similar objects will weigh as many units of a larger magnitude as each individual of that group weighs in units of a smaller magnitude, when and only when the number of individuals is the same as the ratio of the larger unit to smaller unit.
