Dalton, Gay-Lussac, and Avogadro Contributions to Atomic Mass

Connecting Mass to Moles  One of the greatest challenges early chemists faced was trying to find a way to connect the mass of a substance to the number of particles in the sample.  It was determined that “elementary particles” combined in fixed ratios by weight.

Connecting Mass to Moles  This led Dalton to the “atomic model” of matter  Example: The mass ratio of oxygen to hydrogen in water is 8:1  This does not tell us how many atoms of each element are involved  It could tell us this if we knew the relative mass of each kind of atom

Relative Mass  To assign relative masses to elements it is necessary to know that the samples being compared have the same number of particles  If particles are all the same size, the same size sample would have the same number of particles  If particles are different sizes, the same size sample would have different numbers of particles

Relative Mass  At the time, chemists did not know which was true and tended to think the latter was more likely  Example: A bucket of golf balls has more balls that an identical bucket of baseballs  If this is true in the macroscopic world, why wouldn’t it be true in the sub-microscopic one?

Relative Mass  Consider earlier this year when we studied density: was iron more dense than aluminum because iron had more particles per given volume than aluminum or because iron’s individual particles were more massive than aluminum’s? Could it be some combination of both?

Relative Mass  The truth is, based on the experiments we conducted earlier in the year, we couldn’t say which was true.  Dalton did not know what was true during his time either.  Since the mass of individual atoms could not be determined, a system of atomic masses had to be determined by comparison.

Relative Mass  To determine a system of masses by comparison, one element would have to be chosen as the basis of comparison for all others  Dalton chose hydrogen and assigned it a mass of 1.

Relative Mass  To find the mass of another element like oxygen:  Compare the masses of equal number of oxygen and hydrogen atoms OR  Find the combining masses of oxygen and hydrogen in water

Relative Mass  Dalton thought that the former approach was invalid because he thought identical volumes of hydrogen and oxygen gases would have different numbers of particles  He thought the latter was valid but did not take into account that it is valid ONLY if the ratio of atomic combination is known

Reactions of Gases  Research conducted by Gay-Lussac suggested that equal volumes of gases, at the same temperature and pressure, contain equal numbers of particles

Reactions of Gases  Gay-Lussac noted that gases appear to react in simple integer ratios  Example: Two volumes of hydrogen reacted with one volume of oxygen to produce two volumes of water  These findings appeared to contradict the idea that equal volumes of gases have equal numbers of particles

Reactions of Gases  Why? Well, if water is was H 2 O, then two volumes of hydrogen and one volume of oxygen should make one volume of water + + = + + =

Reactions of Gases  Gay-Lussac’s law that equal volumes of gases have equal numbers of particles and Dalton’s atomic theory did not seem to be compatible  In 1811, Avogadro wrote a paper that reconciled the two

Avogadro’s Hypothesis  Avogadro assumed  Equal volumes of gases have equal numbers of molecules  These molecules can be split into half- molecules during chemical reactions  That molecules of elemental gases could contain more than a single atom

Avogadro’s Hypothesis  Two volumes of hydrogen react with one volume of oxygen to produce two volumes of water when hydrogen and oxygen can be split into half-molecules! + + = = +

Avogadro’s Hypothesis  If we accept Avogadro’s Hypothesis, we can compare the mass of various gases and deduce the relative mass of the molecules  To do this, we pick a weighable amount of the lightest element (how about 1.0 g?) then use mass ratios to assign atomic masses to the other elements

Implications  If two volumes of hydrogen combine with one volume of oxygen gas, it is reasonable to assume that two molecules of hydrogen are reacting with each molecule of oxygen  The word chosen to represent the standard weighable amount of stuff, the mole, comes from the Latin “mole cula” or little lump