lpetrich
Contributor
Now to the mole. I don't have in mind a small burrowing animal or metaphorical extensions like a tunneling machine and a spy who hides in an organization for a long time. Or a dark spot on one's skin. Or a wall in a body of water for protecting a harbor. I mean a gram molecular weight.
Take how many atomic mass units a material's molecules weigh and use that as its number of grams. Chemists like that quantity, because substances react molecule by molecule.
To get from atomic mass units to grams, one uses Avogadro's number, about 6.02*1023, also called Avogadro's constant. One gram = (Avogadro's number) * (one atomic mass unit). It was named after a certain Amedeo Avogadro, because in 1811, he proposed that every gas at some pressure, volume, and temperature has the same number of component parts (atoms or molecules).
The story starts with the discovery of the law of definite proportions. It states that some mixtures of elements occur in well-defined proportions, while others do not. Though noted by Joseph Priestley and Antoine Lavoisier before him, Joseph Proust stated it explicitly in 1794, noting experiments on rusting metals. Oxygen will dissolve in water in varying proportions, but it makes iron rust in constant proportions.
John Dalton expanded on this distinction, coming up with the first rigorous atomic theory in 1804. He proposed that compounds of elements consist of a small number of atoms of each element, and he proposed a table of relative atom masses, atomic weights. In his honor, the atomic mass unit is sometimes called the dalton, especially by biochemists. Hemoglobin, for instance, has four parts that each weigh 16,000 daltons, to make a total of 64,000 daltons.
In his table, hydrogen was 1 and the others were integer multiples. But his table contained some mistakes, like saying that oxygen is 8 rather than 16, because he believed that water is HO rather than H2O. This led to Prout's hypothesis, proposed by William Prout in 1815 and 1816, that hydrogen atoms are a fundamental constituent of matter. But some atomic weights turned out to be rather far from integers, like chlorine's at 35.45, and hydrogen itself made elements like carbon and oxygen seem a bit lighter than integer values.
In 1903, Wilhelm Ostwald proposed setting oxygen = 16. Hydrogen was 1.008 and carbon very close to 12. But in 1932, oxygen was discovered to be composed of isotopes, leading to a split between the physical standard of oxygen-16 = 16 and the slightly heavier chemical standard of (natural mix of oxygen isotopes) = 16. This discrepancy was officially resolved in 1961, by making carbon-12 exactly equal 12.
Prout's hypothesis itself was half-vindicated by the discovery of electrons, nuclei, protons, and neutrons, and discrepancies in measured masses were successfully explained by isotopic composition, binding energies of nuclei, and the nucleons themselves having slightly unequal masses.
As I write this, Avogadro's number was officially fixed today, making the mass of the carbon-12 atom in amu's a measured quantity again.
Take how many atomic mass units a material's molecules weigh and use that as its number of grams. Chemists like that quantity, because substances react molecule by molecule.
To get from atomic mass units to grams, one uses Avogadro's number, about 6.02*1023, also called Avogadro's constant. One gram = (Avogadro's number) * (one atomic mass unit). It was named after a certain Amedeo Avogadro, because in 1811, he proposed that every gas at some pressure, volume, and temperature has the same number of component parts (atoms or molecules).
The story starts with the discovery of the law of definite proportions. It states that some mixtures of elements occur in well-defined proportions, while others do not. Though noted by Joseph Priestley and Antoine Lavoisier before him, Joseph Proust stated it explicitly in 1794, noting experiments on rusting metals. Oxygen will dissolve in water in varying proportions, but it makes iron rust in constant proportions.
John Dalton expanded on this distinction, coming up with the first rigorous atomic theory in 1804. He proposed that compounds of elements consist of a small number of atoms of each element, and he proposed a table of relative atom masses, atomic weights. In his honor, the atomic mass unit is sometimes called the dalton, especially by biochemists. Hemoglobin, for instance, has four parts that each weigh 16,000 daltons, to make a total of 64,000 daltons.
In his table, hydrogen was 1 and the others were integer multiples. But his table contained some mistakes, like saying that oxygen is 8 rather than 16, because he believed that water is HO rather than H2O. This led to Prout's hypothesis, proposed by William Prout in 1815 and 1816, that hydrogen atoms are a fundamental constituent of matter. But some atomic weights turned out to be rather far from integers, like chlorine's at 35.45, and hydrogen itself made elements like carbon and oxygen seem a bit lighter than integer values.
In 1903, Wilhelm Ostwald proposed setting oxygen = 16. Hydrogen was 1.008 and carbon very close to 12. But in 1932, oxygen was discovered to be composed of isotopes, leading to a split between the physical standard of oxygen-16 = 16 and the slightly heavier chemical standard of (natural mix of oxygen isotopes) = 16. This discrepancy was officially resolved in 1961, by making carbon-12 exactly equal 12.
Prout's hypothesis itself was half-vindicated by the discovery of electrons, nuclei, protons, and neutrons, and discrepancies in measured masses were successfully explained by isotopic composition, binding energies of nuclei, and the nucleons themselves having slightly unequal masses.
As I write this, Avogadro's number was officially fixed today, making the mass of the carbon-12 atom in amu's a measured quantity again.
