Complete History of the Avogadro Number

The script begins with an exploration of the Avogadro number's history, starting from the ancient Greek philosopher Democritus, who first proposed the idea of indivisible particles. It then moves through the scientific method's introduction by Francis Bacon, leading to the foundational work of Antoine Lavoisier, Henry Cavendish, and Robert Boyle. The paragraph also touches on the early experiments in chemistry that measured weight changes during reactions, leading to Joseph Proust's law of definite proportions, which marked the distinction between compounds and mixtures.

This section delves into John Dalton's 1803 atomic theory, which posited that elements are composed of atoms of different weights. Dalton's work established a scale for atomic weights, using hydrogen as the reference element with a weight of one. The paragraph also discusses how Dalton's scale, although flawed due to the unknown diatomic nature of hydrogen, laid the groundwork for understanding the relationships between the weights of different elements.

The script then introduces Amedeo Avogadro's hypothesis from 1811, which stated that equal volumes of gases at the same temperature and pressure contain an equal number of molecules. This hypothesis was revolutionary, providing a method for counting molecules through volume measurements. The paragraph also covers the work of Jöns Jacob Berzelius, who refined the measurements of relative atomic weights and proposed using oxygen as the reference element with a weight of 100, despite the issues that arose from the existence of hydrogen isotopes.

The fourth paragraph discusses the development of the ideal gas law by Benoit Clapperion, which combined Avogadro's hypothesis with Boyle's law and Charles's law. This law allowed chemists to compare the number of molecules in samples without needing to know the exact count. The script also touches on the confusion in the mid-1800s regarding the various scales used for comparing atomic weights and the unification efforts at the first international chemistry conference.

The script highlights Stanislau Cannizzaro's pivotal role in the 1850s, who clarified the understanding of atoms and molecules in chemical reactions. Cannizzaro's work on gases led to the establishment of the concept of gram molecules, which related directly back to Avogadro's hypothesis. His contributions are considered the birth of modern chemistry, emphasizing the importance of Avogadro's hypothesis in accurately measuring molecular involvement in reactions.

This section describes the introduction of the term 'mole' by Wilhelm Ostwald in 1900, which replaced the term 'gram molecule' to avoid confusion about the existence of molecules. The script also discusses the work of Jean Perrin, who used Einstein's laws of Brownian motion to estimate Avogadro's constant, the number of molecules in one mole of gas, and how this number became a fundamental constant in chemistry.

The script explains the discrepancies between physicists and chemists in defining atomic weights, leading to the 1971 decision to base the mole on the number of nucleons in 12 grams of carbon-12. This decision aimed to reconcile the differences and establish a common standard. The paragraph also discusses the challenges in defining the mole and the constant changes in Avogadro's number due to technological advancements.

The final paragraph concludes the script by detailing the 2019 decision by the International Bureau of Weights and Measures to fix the Avogadro number at 6.02214076 × 10^23, based on the most accurate measurement of a pure sample of silicon-28. This decision resolved the issue of the constantly changing Avogadro number and provided a definitive value for all time. The script summarizes the historical journey from Avogadro's hypothesis to the modern definition of the mole and Avogadro's number.