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Avogadro’s number is the amount of atoms in 12 grams of 12C. Amedeo Avogadro was the first to state that the volume of a gas is proportional to how many atoms it has. His law states that equal volumes of gases contain the same number of particles. The constant can be expressed as (p1)(V1)/(T1)(n1) = (p2)(V2)/(T2)(n2) = constant. Josef Loschmidt calculated the number of molecules in a single cubic centimeter of gas. The name “Avogadro’s number” was first used in 1909 by Jean Baptiste Jean Perrin. The value of the constant was controversial until it was based on carbon-12 in 1960.
Avogadro’s number, also known as Avogadro’s constant, is defined as the amount of atoms in exactly 12 grams of 12C. The designation is an acknowledgment of Amedeo Avogadro, the first to state that the volume of a gas is proportional to how many atoms it has. This number is given as 6.02214179 x 1023 mol-1.
Amadeo Avogadro lived in the early 19th century and was an Italian scholar known for his role in many different scientific disciplines. His most famous statement is known as Avogadro’s Law, and it is a hypothesis which states: “Equal volumes of ideal or perfect gases, at the same temperature and pressure, contain the same number of particles or molecules”.
This is an intriguing hypothesis, because it says that quite different elements, like nitrogen and hydrogen, still have the same number of molecules in the same volume as an ideal gas. While in real-world settings this isn’t strictly true, it is statistically close enough, and thus the ideal model still holds great value.
The constant can be expressed as (p1)(V1)/(T1)(n1) = (p2)(V2)/(T2)(n2) = constant; where p is the pressure at which the gas is located, T is the temperature at which it is located, V is the volume of the gas and n is the number of molar units.
Part of Avogadro’s genius, and even though this issue was named after him, is that he was able to see this fundamental relationship long before experimental evidence was available to validate it. His innate understanding of the nature of ideal gases was astounding, and it was only decades later that experimental evidence finally confirmed his hypothesis.
In 1860, more than 50 years after Avogadro first formulated his hypothesis, Austrian high school teacher Josef Loschmidt calculated how many molecules there were in a single cubic centimeter of a gas at typical pressure and temperature. He determined that it was about 2.6X1019 molecules, a number now known as Loschmidt’s constant, and that it has since been expanded to 2.68677725X1025 m-3.
During the early 1920s a search was undertaken to discover the precise value of Avogadro’s number. Molecules were still largely theoretical entities for many scientists until the early part of the 20th century, and thus their value could not be determined through experiment. Once that became feasible, however, it quickly became apparent that the value did matter, reflecting the fundamental nature of ideal gases.
The name “Avogadro’s number” was first used in a 1909 paper, by scientist Jean Baptiste Jean Perrin, who later won the Nobel Prize in Physics in 1926. He stated in the paper that “The invariable number N is a universal constant, which may appropriately be designated as ‘Avogadro’s constant’”.
For years up until the 1960s, there was some controversy about the actual value of this number. Some factions used oxygen-16 to base their calculations, while others used a natural isotope of oxygen, leading to slightly different values. In 1960, the constant was changed to be based on carbon-12, making the number much more regular.
