Avogadro's Hypothesis By: Ryan Ouimette and John Mindek


Background Information

Amedeo Avogadro was a very successful Italian lawyer, and despite his popularity and good fortune, he turned to philosophy due to an interest in that particular field as well as a need to better the world with his ideas. He became a professor of philosophy at a very popular college in Italy, University of Turin. He was a professor of natural philosophy and eventually became the first chair of mathematical physics at the University. He stayed here until 1850 until retirement.

During the middle years when Avogadro was at his peak ability in the fields of Math and Science, he closely studied Dalton and Gay-Lussac. Among these two scientists theories, Avogadro found flaws in the concepts of atoms and molecules, and worked to correct them. After creating the constant surrounding the atom-molecule concept, Avogadro developed the famous Avogadro principle:

"equal volumes of all gases at the same temperature and pressure contain the same number of molecules."

Source 6

Brief Explanation

Avogadro's Hypothesis

Avogadro's Hypothesis was based off the work done by Joseph Gay-Lussac. Gay-Lussac had determined that the with a given increase in temperature, all gases expand to the same extent. Avogadro furthered this work when he hypothesized that the equal volumes of different gases had the same number of molecules, not the same number of atoms. Through this he was able to show when Hydrogen is mixed with Chlorine, the result is a volume of Hydrogen and a volume of Chlorine, not a volume of Hydrochloric Acid, which was what was believed at the time (Source 5, 281). Avogadro first coined the term molecule, which are groups of atoms, that he called "elementary molecules." He discovered this concept by looking at steam. When two volumes of Hydrogen and one of Oxygen combined together to make steam, it was a lesser volume than when the two volumes were added previously.

Mole and Weight Relationships of Water and Its Parts

2 moles H


1 mole O


1 mole water



water molecule-small
water molecule-small

2 * 1.01 g


16.00 g


18.02 g


This table explains how mass is calculated by the guidelines of Avogadro's Hypothesis

Concept of the Mole

Therefore, though the same number of molecules existed, they were combined differently in the steam and because of that had a smaller volume. This meant that equal volumes of gases didnt have the same number of atoms, but instead the same number of molecules (Source 2, 439). Therefore a mole of each element, measured by the molecular mass of the particular element or compound, will always contain the same number of molcules, regardless of whether the molecular mass is different or not. When Avogadro presented his hypothesis in 1811 though, its reasoning was rejected by leading scientists John Dalton and Jons Jacob Berzelius on the grounds that they thought atoms would repel one another in the situations provided by Avogadro (Source 4, 9). It wasn't until the Karlsruhe Conference in 1860 that Avogadro's work was finally accepted. Stanislao Cannizaro, who had published a paper in support of Avogadro's findings in 1858, made a compelling speech in front of 160 scientists at the conference, and Avogadro's hypothesis became the acceptable hypothesis to determine the molecular weight of gases (Source 4, 22). Said Avogadro, "It must be admitted that very simple relations exist between the volumes of gaseous substances and the number of simple or compound molecules which form them," (Source 3, 69).

This video offers a simple overview of Avogadro's hypothesis and some of the basic ideas it generates.



Discovery of Avogadro's Number

Though Avogadro never actually caluculated the constant that is named after him today, because he discovered the concept of the mole, the number was rightfully named in recogniztion of his work. The first attempt to value the number which Avogadro had hypothesized was done by the Austrian, Josef Loschmidt. He used the Kinetic Molecular Theory in his attmept to calculate how many molecules of gas were in a cubic centimeter, and his work yield an approximation of 2.6 *10^19. Avogadro's name was first attatched to the number by Jean Baptiste Jean Perrin in a paper in 1909. Perrin's research was focused around molecular dynamics,and through his Nobel Prize winning work he proved that molecules do exist, giving a scientific explanation to the work done mathematically by Einstein. At thistime though there was no accuracte estimation of the constant. In 1933 S.E. Virgo, published a paper title "Loschmidt's Constant," on this topic. In it, he distin-guished that Loschmidt's constant referred to the number in a cubic centimeter, while Avogadro's number when it was determined would be the number of mole-cules per gram. Once the two were distinguished, many scientists have worked to discover the true value of Avogadro's constant. Using atoms of Titanium, they have made the folowing calculations,


There are 2 Titanium atoms in one unit cell, the length of the edge of the unit cell is 330.6 picometers, the molecular mass of Titanium is 47.88 grams, and the density of Titanium is 4.401 g/cm​³. In this calculation, using the lengths of the Titanium atom, the volume in cubic centimeters is determined. From there, cubic centimeters in converted to grams, which provides the conversion factor into grams of the the molecule. Then, using the density of Titanium, the number of atoms per grams of the molecule can be calculated. This results in the number 6.02214179*10^23, which we now approximate as 6.02*10^23 atoms per grams of the molecule. Avogadro's number can thus be used to determine molecular weight as well as the number of molec
Nobel Prize winning physicist Jean Baptiste Jean Perrin
ules in a given volume of a substance.
Source 1

Importance of Discovery

Avogadro's number changed the wide world of science forever. Up until this point in history, a number to show how many molecules of a particular atom are in one mole was not only un-exact, but most likely sloppily estimated. Avogadro gave scientists a way to numerically perform experiments and significantly decrease their margin of error. This helped to develop new theories that were once looked over, as well as give scientists a new form of motivation. If this number could be found, imagine what other breakthroughs could be made.

Amedeo Avogadro has changed the way experiments, classes, and ideas are made in the classroom and in the lab. he has given young minds a greater chance for success and has made another giant step in the right direction for science. He gave science a way to show how small an atom is, and how scientists should respect the work others have done to reach that point.


1. Furtsch, T.A. "Some Notes on Avogadro's Number, 6.022 X 1023." Tennessee Technological University. Web. 2 June 2010. <http://iweb.tntech.edu/chem281-tf/avogadro.htm>.
2. Ronan, Colin A. Science, Its History and Development Among the World's Cultures. New York: Facts on File Publications, 1982. Print.
3. Sharlin, Harold I. The Convergent Century, The Unification of Science in the Nineteeth Century. New York: Abelard-Schuman, 1966. Print.
4. Spangenburg, Ray, and Diane K. Moser. The History of Science in the 19th Century. New York: Facts on File, 1994. Print.
5. Taton, Rene, ed. History of Science, Science in the Nineteenth Century. New York: Basic, 1965. Print.

6. "Home page of Avogadro." Blot on the net. N.p., n.d. Web. 4 June 2010. <http://www.bulldog.u-net.com/avogadro/avoga.html>.