Chapter 1: Understanding Critical Mass

Many people might recoil at the mention of “chemistry,” recalling an undergraduate course that faded from memory after the final exam. However, my aim with this series is to shift that perception, making chemistry accessible and enjoyable for everyone.

In the year 1858, we find ourselves at a crucial juncture in chemical understanding. About fifty years earlier, John Dalton introduced the concept of atoms as the fundamental, indivisible units of matter. This revolutionary idea led to the establishment of molecular formulas based on equivalent masses. Yet, the arbitrary assignment of atomic weights left many in the field puzzled rather than enlightened. Despite this confusion, remarkable organic analysis was achievable thanks to the efforts of brilliant minds like Justus von Liebig and his students, who managed to analyze up to four molecules daily.

While dualistic theory was effective for binary inorganic compounds, it fell short in the realm of organic chemistry. Pioneers like Humphry Davy and Michael Faraday demonstrated that electrostatic interactions—complementary positive and negative charges—played a role in chemical bonding, but this principle was primarily relevant to inorganic substances. Jöns Jacob Berzelius attempted to adapt this theory for organic compounds, giving rise to the flawed "radical theory." This theory faced challenges from contradictory evidence, including the existence of isomers, leading to the emergence of an alternative concept known as "type theory."

The complexities of organic chemistry were eventually clarified by August Kekulé, who illustrated the tetravalence of carbon and introduced the structural representation of molecules. Yet, despite these significant advancements, the field still grappled with unresolved questions. Chemists struggled to predict chemical outcomes due to the inconsistencies in atomic weights, meaning they could not determine the mass of an unknown compound based solely on known substances. Moreover, the notion that tiny, invisible particles were responsible for chemical reactions was difficult for many to accept. What was needed was a standard system of equivalents, and that solution came from Stanislao Cannizzaro (1826–1910).

Stanislao Cannizzaro: The Unifier of Chemistry

As you might guess, Cannizzaro was Italian, so imagine reading this with a playful Italian accent. Initially aspiring to become a physician, he got involved in political activities that forced him to leave Italy temporarily. During his time in France, he delved into chemistry, and upon returning home, he began teaching. It was during this period that Cannizzaro recognized the challenges in the field and sought to address them.

He revisited the foundational work of his fellow Italian, Amadeo Avogadro, who had proposed a groundbreaking hypothesis: under identical conditions, equal volumes of gas contain the same number of particles. This hypothesis effectively bridged the gap between volumetric and gravimetric data. To clarify, "volumetric" refers to the volume occupied by a substance, while "gravimetric" pertains to its weight. This allowed chemists to compare atomic weights directly. For instance, if one liter of hydrogen weighed one unit, then a liter of oxygen, at the same temperature and pressure, would weigh sixteen times that amount.

In 1858, Cannizzaro published a paper that articulated how Avogadro’s hypothesis aligned with existing knowledge, which he integrated into his teachings. He demonstrated that employing vapor densities could yield accurate atomic weights, facilitating reliable predictions. Unfortunately, his work initially went unnoticed.

Fortuitously, Cannizzaro’s ideas gained traction when, inspired by Kekulé, around 140 young chemists gathered in Karlsruhe to discuss pressing issues in chemistry. In 1860, Cannizzaro presented Avogadro’s hypothesis at the Karlsruhe Congress, where it was largely embraced as a basis for establishing atomic weights.

It's important to clarify that Avogadro himself did not establish what is now known as "Avogadro's Number" (6.022 x 10²³). This term was first introduced by Jean Perrin in 1924. The modern approach to calculate Avogadro's Number involves dividing Faraday's Constant by the charge of an electron, but the intricacies of the math aren't essential here.

Indeed, Avogadro's Number is often regarded as the most recognized figure in chemistry.

Works Consulted:

Brock, William H. The Chemical Tree: A History of Chemistry. New York: Norton and Co, 2000.

Ihde, Aaron J. The Development of Modern Chemistry. New York: Dover Publications, 1984.

Chapter 2: Influential Chemists and Their Contributions

This video highlights the top ten greatest chemists in history, showcasing their pivotal contributions to the field of chemistry.

In this informative video, learn about atomic structure, ions, isotopes, valence electrons, and the periodic table, essential concepts for understanding modern chemistry.