In estimating atomic weights, Dalton was confronted with certain grave difficulties. Since it is impossible to weigh single atoms, any system of atomic weights must be formulated on a comparative basis. The atom of some element must be arbitrarily selected as the reference weight. Dalton chose the hydrogen atom and assigned one as its weight. The atomic weight of oxygen could then be found by either (1) comparing the weights of equal numbers of oxygen and hydrogen atoms or (2) finding by analysis the combining weights of oxygen and hydrogen in water. Dalton considered the first approach but rejected it. Since to him, atoms in a gas were analogous to a pile of shot, and since he believed that atoms of different gases varied in diameter, therefore equal volumes of gases could not contain equal numbers of atoms. The second approach he considered valid. However, reflection will show that it is valid only when the ratio of atomic combination is known.[1]
[1] A. Ihde, The Development of Modern Chemistry, Harper & Row, 1964.
The breakthrough came when Avogadro devised an hypothesis to account for the observations of Gay-Lussac regarding the reacting volumes of gases. Gay-Lussac had noted that gases appeared to react in simple integer ratios.
The breakthrough came when Avogadro devised an hypothesis to account for the observations of Gay-Lussac regarding the reacting volumes of gases. Gay-Lussac had noted that gases appeared to react in simple integer ratios.
Avogadro’s paper of 1811, based on Gay-Lussac’s law and Dalton’s atomic theory, reconciled the … problems. Starting with the assumption that equal volumes of all gases contain equal numbers of molecules under similar conditions, Avogadro proceeded to analyze the facts of gaseous combination. Using the examples discussed by Gay-Lussac, he showed that the ambiguities disappeared if he assumed that the molecules involved in typical reactions might split into “half-molecules”; that is, he supposed the existence of molecules of elemental gases which contained more than a single atom. He did not use the term “atom”, but always used the term “half-molecule” as its equivalent. (Ihde)
Once we accept Avogadro’s Hypothesis, we can compare the mass of various gases and deduce the relative mass of the molecules. So, from the mass, can we determine the number of particles? Yes. In fact, the word chosen to represent the standard weighable amount of stuff, the mole, comes from the Latin: lump of stuff.
HW: Read the article "What is a Mole in Chemistry?"
