Johann Josef Loschmidt And Avogadro’s Number

Johann Josef Loschmidt is a name that might not ring many bells, yet everyone who took chemistry in junior college had surely come across Loschmidt’s groundbreaking contribution to science. Loschmidt calculated the exact number of elementary units (atoms or molecules) that one mole of a substance contains—a number that bears the name of an Italian scientist, which is unfortunate, because even this celebrated Italian scientist didn’t know its value. That number is Avogadro's number, or Avogadro's constant.

Avogadro's Number. Photo: Ronnie Pitman/Flickr

Josef Loschmidt was born into a poor Bohemian peasant family in 1821, in a small village in what is now Czech Republic. With the help of the village priest and the Catholic Church, Loschmidt managed to gain an education in a monastery, before moving to Prague where he became a student at the Charles University studying philosophy and mathematics. At the age of twenty, Loschmidt moved to Vienna and took admission at the Polytechnic Institute in physics and chemistry. He graduated in 1846 with the equivalent of a bachelor’s degree, but was unable to get an academic position. Dejected, Loschmidt briefly considered migrating to the United States to become a settler in the new state of Texas. Instead, he went to work in a paper factory and then started a company near Vienna to produce potassium nitrate. But the company went bankrupt.

Johann Josef Loschmidt

In 1856, Loschmidt became a high-school teacher in Vienna. During his free time, Loschmidt conducted experiments in a small laboratory that the school allowed him to keep. Five years later he published his first important treatise on chemistry—a 47-page booklet depicting the molecular structures of more than 270 organic compounds. It included compounds that were yet to be made, such as cyclopropane, whose structure he correctly predicted more than two decades before it was discovered. Furthermore, the style which Loschmidt adopted for graphically depicting these molecular structures is remarkable similar to the style used by modern chemists—double lines for double bonds and triple lines for triple bonds.

Figures of molecular structure, as they appeared in Loschmidt’s 1861 booklet “Chemische Studien I”. Photo: Alfred Bader

In this same book, Loschmidt discussed benzene, a compound that baffled scientists. Although scientists knew how many carbon and hydrogen atoms a molecule of benzene contains, they couldn’t figure out how they were arranged in the molecule. Loschmidt was the first who saw that the atoms had to be arranged in a ring. Unfortunately, Loschmidt got the arrangement wrong, which was corrected by German chemist August Kekulé four years later. Kekulé insisted that the structure came to him in a dream. Kekulé received all the glory while Loschmidt’s contribution was pretty much forgotten. Ironically, Kekulé originally dismissed Loschmidt’s work, stating that one cannot deduce the molecular structure of organic compounds solely from chemical reactions. Today, he is regarded as the principal founder of the theory of chemical structure.

Proposed structure of benzene. The correct one is right-bottom. Image : Jü/Wikimedia Commons

A great puzzle of the time was the size of molecules. Many scientists tried to measure it indirectly from other physical properties, such as surface tension or tensile strength of liquids, but the problem remained unsolved until 1865, when Loschmidt deduced a relationship between the size of gas molecules to the distance they travel between collisions. Loschmidt came up with a surprisingly good estimate—one nanometer, which was only twice as large as the modern accepted value.

While working on the problem of the size of a molecule, Loschmidt realized that it’s possible to determine the number of molecules in a certain volume of a gas at standard temperature and pressure. The Italian scientist Avogadro had suggested, half a century ago, that if two different gasses at the same temperature and pressure occupy the same volume, they must have the same number of molecules. Avogadro did not know how many molecules that was, but now Loschmidt had an answer—approximately 2.686 ×10²⁵ per cubic meter at zero degree centigrade and one atmosphere of pressure. That number is now known as Loschmidt constant. Thanks to Loschmidt’s work, other scientists were able to prove Avogadro’s hypothesis. They determined “Avogadro’s number”, which is defined as the number of molecules contained in one mole of the substance. The Loschmidt constant is barely used, but Avogadro's number (6.022×10²³) finds wide application in physics and chemistry. The only subtle recognition Loschmidt received for his work is the symbol L, which is sometimes used to denote the Avogadro constant.

Lorenzo Romano Amedeo Carlo Avogadro

Loschmidt also contributed to Maxwell's and Boltzman's work on thermodynamics. His experimental investigations in electromagnetism now bear the names of people who completed them — like Edwin Hall, John Kerr, and Heinrich Hertz. He once joked to his long-time friend Boltzmann that he should start a negative scientific journal to publish his failed experiments.

When Loschmidt died in 1895, Boltzmann spoke of his good friend, “His work forms a mighty cornerstone that will be visible as long as science exists … Loschmidt’s excessive modesty prevented his being appreciated as much as he could and should have been.”

References:
# John H. Lienhard, Johann Josef Loschmidt, The Engines of Our Ingenuity
# Alfred Bader, Joseph Loschmidt, Physicist and Chemist, Physics Today