Avogadro’s law
Avogadro's law is a fundamental principle in chemistry that states that equal volumes of different gases, when measured at the same temperature and pressure, contain an equal number of molecules. This law, proposed by Italian scientist Amedeo Avogadro in 1811, emerged during a time when the nature of atoms and molecules was still poorly understood. Avogadro distinguished between atoms and molecules, resolving conflicts between the ideas of contemporaries like Joseph-Louis Gay-Lussac, who studied gas combination ratios, and John Dalton, who viewed atoms as indivisible. While Avogadro's initial hypothesis was largely overlooked during his lifetime due to misunderstandings and a lack of empirical support, its significance was later recognized by his peers, especially after being championed by the scientist Stanislao Cannizzaro in the 1860s. The acceptance of Avogadro's law marked a pivotal moment in the evolution of chemical science, clarifying misconceptions surrounding gaseous substances and paving the way for advances in understanding molecular weights and densities. Avogadro’s contributions ultimately transformed the landscape of chemistry, emphasizing the importance of molecular theory in the study of gases.
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Avogadro’s law
Avogadro's law is a crucial law of chemistry, which states that equal volumes of different gases contain an equal number of molecules when held under the same temperature and pressure conditions. First proposed by Italian scientist Amedeo Avogadro in 1811, this breakthrough observation initially earned little recognition from Europe's elite scientific community, mostly because of Avogadro's own obscurity and some notable irregularities with his work. When the law was rediscovered decades later, however, it quickly came to play a significant role in the development of modern chemistry.
Background
In the late eighteenth and early nineteenth centuries, scientists struggled to develop an accurate understanding of the atom, one of the most fundamental concepts in chemistry. Two of the men at the forefront of the effort to unlock the secrets of the atom were Joseph-Louis Gay-Lussac and John Dalton. In his studies, Gay-Lussac determined that under identical temperature and pressure conditions gases always combine in simple ratios of whole numbers. Dalton, firmly believing that atoms were indivisible, could not accept Gay-Lussac's premise. In his mind, atoms always tried to combine in a one atom–to–one atom ratio. If they were able to combine in other ratios, then his theory about the indivisibility of atoms would be disproven. The main reason why the two were unable to reconcile their different thought processes was that neither understood the difference between atoms and molecules. An atom is the smallest unit of a substance that can exist by itself. Dalton believed that all gaseous elements existed naturally as single atoms. What he and others failed to realize, however, is that some gases exist naturally as molecules, which are combinations of atoms. For example, nitrogen (N) exists naturally as a molecule that contains two nitrogen atoms (N2).
Amedeo Avogadro, an Italian scientist born in 1776, recognized the difference between atoms and molecules and proposed an explanation to satisfy both Gay-Lussac's observations on the combination ratios of gases and Dalton's theories on the indivisibility of atoms. Previously, Dalton had studied water (H2O), the molecules of which are made from a combination of hydrogen (H) and oxygen (O) atoms. Dalton mistakenly believed the formula for water was HO, based on his idea that atoms always try to combine in a one atom–to–one atom ratio. When studying water, however, he could not understand how the oxygen atom seemed to divide itself. What he did not realize was that both hydrogen and oxygen exist naturally as molecules containing two atoms each: H2 and O2. When Avogadro came to this realization, he concluded that when oxygen combined with hydrogen to form water, it was an oxygen molecule that split, not an oxygen atom. The breakthrough observation led Avogadro to the discovery of the important law of chemistry that bears his name.
The Law and Its Impact
Using what he learned about the true nature of atoms and molecules, Avogadro began to further investigate gases and their molecules. In 1811, he arrived at an important hypothesis: equal volumes of different gases contain an equal number of molecules when held under the same temperature and pressure conditions. The discovery also led to his conclusion that the relative molecular weights of any two gases are equal to the gases' densities when temperature and pressure conditions are the same. Together with his explanation of the difference between atoms and molecules, Avogadro's new law represented a major breakthrough in humankind's understanding of chemistry. Unfortunately, few of Avogadro's peers recognized the significance of his work at the time.
When Avogadro first published his hypothesis, his contemporaries largely ignored it for various reasons. First and foremost, many other scientists simply could not understand it due to both a lack of clarity in Avogadro's writing and the ongoing confusion regarding the interchangeable use of the words atom and molecule. Further, Avogadro failed to support his claim with a sufficient amount of experimental data. Equally problematic was that, at the time, most scientists were more concerned with organic chemistry, a branch of chemistry in which analysis is based primarily on weight rather than volume. Finally, because Avogadro worked from his hometown of Turin, Italy, he was isolated from what was then the center of scientific thought in Europe on the other side of the Alps. Because of this, Avogadro was a relative unknown among the scientific elite and, therefore, was dismissed easily. Though Avogadro floundered in obscurity for the remainder of his life, the importance of his work was recognized eventually.
In 1860, just four years after Avogadro died, another Italian scientist, Stanislao Cannizzaro, rediscovered Avogadro's work and recognized its significance. Hoping to earn the late innovator the recognition he never attained in life, Cannizzaro presented and strongly defended Avogadro's hypothesis to the scientific community. With the help of Cannizzaro's spirited argument, Avogadro's work finally gained acceptance, and his hypothesis that equal volumes of different gases contain an equal number of molecules when held under the same temperature and pressure conditions became known as Avogadro's law. Upon its acceptance, Avogadro's law ended decades of misinformation about atoms and molecules and set the stage for the development of chemical science as the world knows it today. Moreover, thanks to Cannizzaro, the much maligned Avogadro was vindicated at last.
Bibliography
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Balchin, Jon. "Amedeo Avogadro 1776–1856." Science: 100 Scientists Who Changed the World. New York: Enchanted Lion Books, 2003, 100–101. Print.
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