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Before 1860 chemists could not agree on something as basic as how heavy an atom of carbon or oxygen is, and so they wrote different formulas for the same compounds. A course outline written for students in Genoa helped settle the question. Its author, Stanislao Cannizzaro, showed how a hypothesis that had been around for nearly 50 years gives one consistent set of atomic weights. This post covers his life, his "law of atoms", his method worked through for carbon, and what a student can take from the way he taught.
Who was Stanislao Cannizzaro?
The Science History Institute biography gives his dates as 1826 to 1910. He was born in Palermo, Sicily, and his interest in chemistry began at medical school there. He joined the failed Sicilian revolution of 1848 and fled to Paris, where he went back to chemistry. Back in Italy he worked in Alessandria, where he discovered the Cannizzaro reaction, the self-oxidation and self-reduction of aldehydes that appears in NCERT Class 12 Unit 8. He then taught at Genoa, without the benefit of a laboratory. In 1860 he supported Garibaldi's campaign in Sicily, and after Italian unification he moved to Rome in 1871, working as both a scientist and a public figure.
The biography describes European chemists of the mid-19th century as being "in a state of confusion and disagreement over the most fundamental concepts of chemistry".
What was the 1858 Sketch?
In 1858 Cannizzaro wrote a letter to his colleague Sebastiano De Luca outlining his course in chemical philosophy at Genoa. It was printed in Il Nuovo Cimento that year, and an English translation, Sketch of a Course of Chemical Philosophy, appeared as Alembic Club Reprint No. 18 in 1911.
He began from Avogadro's hypothesis: equal volumes of gases contain an equal number of molecules, "not however an equal number of atoms", since molecules of different substances can contain different numbers of atoms. Then he asked his students to compare how much of each element appears in many different molecules. The result, which he called the law of atoms, is stated in the translation like this:
"The different quantities of the same element contained in different molecules are all whole multiples of one and the same quantity, which, always being entire, has the right to be called an atom."
He was a teacher first. To make relative weights feel concrete he told students to imagine that half a hydrogen molecule weighs a millionth of a milligram, then explained that no one can know the real value, comparing the trick to the wooden scaffolding engineers remove once a bridge can stand.
Worked example: finding the atomic weight of carbon
His method needs two facts about several gaseous compounds: the molecular mass, which is twice the vapour density measured against hydrogen gas, and the percentage of the element by mass. Using modern rounded values:
Methane: vapour density 8, so molecular mass 16. Carbon is 75.0% by mass: 0.750 × 16 = 12.
Carbon monoxide: vapour density 14, so 28. Carbon 42.9%: 0.429 × 28 = 12.
Carbon dioxide: vapour density 22, so 44. Carbon 27.3%: 0.273 × 44 = 12.
Ethane: vapour density 15, so 30. Carbon 80.0%: 0.800 × 30 = 24.
Benzene vapour: vapour density 39, so 78. Carbon 92.3%: 0.923 × 78 = 72.
Every amount of carbon is a whole multiple of 12: one, one, one, two and six times. So the atomic weight of carbon is 12, and the formulas follow: CH₄, CO, CO₂, C₂H₆, C₆H₆. Cannizzaro's own table reached the same conclusion, that the quantities of carbon in its volatile compounds "are all integral multiples of 12". He noted that carbon's own vapour density could not be measured, and that it did not matter.
Common mistakes:
Forgetting the factor of two. Molecular mass is twice the vapour density relative to H₂, because hydrogen gas is H₂.
Trusting too few compounds. The smallest amount you have found is only an upper limit. If you had only ethane and benzene, you would wrongly conclude 24.
Mixing mass and number percentages. The percentages here are by mass.
The modern form of this reasoning is the mole concept; see our guide to common mole concept errors.
How did a set of lecture notes change chemistry?
In 1860 the first international chemistry congress met at Karlsruhe in Germany to settle what an atom and a molecule are and how to find atomic weights. According to the Science History Institute, the meeting ended without agreement, and the chemists went home to decide for themselves. But many took home a printed copy of Cannizzaro's Sketch. Within a decade his atomic weights had spread, and they were taken up by Lothar Meyer and Dmitri Mendeleev. The same biography says Mendeleev acknowledged that his periodic table was inspired by Cannizzaro's work, which makes sense: you cannot arrange elements by atomic weight until the weights are right. Our profiles of Avogadro's hypothesis and Mendeleev's periodic table tell the before and after.
What can a student take from how Cannizzaro worked?
Teach it to learn it. His landmark was a set of notes written to make students understand, not a research paper.
Follow the history of an idea. He led his students along the path he had taken himself, through earlier theories, so the conclusion felt earned.
Look for the pattern across many cases. One compound tells you little; a table of them gives the law.
If moles, vapour density and atomic mass feel tangled, the Fermi Concept Library covers every concept in chemistry with its own explanation, so you can untangle one idea at a time.
Try this now: propane has vapour density 22 and is 81.8% carbon by mass. How many carbon atoms does one molecule contain?
