How were atomic weights originally measured?

According to the Science Encyclopedia,

Because atoms were much too small to be seen or measured by any common methods, absolute weights of atoms could not be determined. Rather, these first measurements were made by comparing weights of various atoms to hydrogen.

How, though, in the early 1800's, were atomic weights compared to hydrogen? For example, given a bottle of hydrogen gas, how can we use this to determine the atomic weight of gold?

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• You react for instance two grams of hydrogen with roughly 16 grams of oxygen to get 18 grams of water. If you chose a different ratio, there will be either some hydrogen or some oxygen left. These relative atomic weights are not natural numbers, though, but they can greatly differ because of isotope ratios. Having said that, this is more of a history of chemistry question, especially for elements for which chemical reactions may not have been available back then. – CuriousOne Mar 7 '16 at 23:53
• OK... after some digging I found a citation to "H. Moissan, Compt. rend., 1889, 109, 807", which mentions the synthesis of gold fluoride, which would have allowed a relative atomic mass measurement with chemical methods. Not sure about the chlorides... which seem to produce different size nanoparticles, depending on conditions. – CuriousOne Mar 8 '16 at 0:01
• The question is already answered here hsm.stackexchange.com/questions/3398/… – Conifold Mar 9 '16 at 22:57

1 Answer

Do you know about Lomonosov's and Gay Lussac's contribution in understanding of chemical reactions and Avogadro's contribution?

These three chemists contributed to a great extent to the understanding of absolute formulas of substances. Lomonosov earlier was the first to come up to explain four types of chemical reactions in mid of 1700s and was the one to explain law of mass conservation during chemical reactions. Gay-Lussac later in early 1800s mentioned that gases react with each other in certain proportions. "For example, at the same temperature and pressure, two volumes of hydrogen react with one volume of oxygen and form two volumes of water."[1]. "Later, Amedeo Avogadro proposed that equal volumes of gases have the same number of particles if measured at the same temperature and pressure. The difficulty of explaining how one volume of oxygen could form two volumes of water without violating the current theory that atoms were indivisible was not resolved until the 1850s when Avogadro's explanation that molecules of gases, such as hydrogen and oxygen, existed as diatomic molecules (molecules with two atoms joined together) was finally accepted. If each oxygen molecule was composed of two oxygen atoms, then it was the molecule and not the atom that split apart to form two volumes of water."

"J. J. Berzelius did a great deal of experimental work in establishing atomic weights and he published his list of the weights of 54 elements along Unlike Dalton's atomic weights, the weights published by Berzelius match quite well the atomic weights used today." [1]

Mendeleev later came up with periodic classification based on his systematic understanding and analysis of the then known elements and classified them in the periodic table. This later led to coming up with the long form of periodic table which is currently in use.

• Don't ignore the contributions of Lavoisier in this early understanding of the ratios in which elements reacted. – Floris Mar 8 '16 at 2:22
• Most of this answer was directly plagiarized from science.jrank.org/pages/634/Atomic-Weight-History.html (paragraph 4), with one or two minor words changed here and there. Please edit your answer to clearly identify what works you have copied and what their sources were. That page specifically requests citations and has details near the bottom of the article. – Jason C Mar 8 '16 at 2:30
• Jason! Should have cited! I confess! But I also should confess that this was all taught in the high school though! I will take care of citations properly form the next time on! Thanks – user118008 Mar 8 '16 at 2:41
• It doesn't matter when the information was taught. You copied the text word for word rather than describing the information in your own words. You will take care of the citations properly now, by using the edit button. You should know better than to directly copy text without citing its source. – Jason C Mar 8 '16 at 2:52
• You should mark the quotes explicitly as well as citing using >  at the beginning of line will blockquote the following material. – dmckee Mar 8 '16 at 3:43