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John Dalton was the son of an English weaver from Cumbria who made a living as a teacher at the Presbyterian college in Manchester. Privately, he carried out his own investigation. In 1807, he would be the first person to use the term “atom” to describe the smallest particle of any element.
What did Dalton do?
He studied gasses and discovered that elements can combine with other elements to form compounds. He didn’t know how many atoms joined to form a compound so he had to make educated guesses. He did that by observing the ratios with which the compounds reacted. With this, he was able to calculate some relative masses of different elements. The precise measurement of course were impossible at the time. To explain the difference between the solid, liquid and gaseous phase, he though that the atoms were surrounded by and atmosphere of heat. The solid phase of a substance simply had less heat than the liquid of gaseous phase which all things considering, does make sense.
Dalton’s atomic theory, 1807
All matter is made up of tiny particles called atoms.
Each atom is a solid particle, surrounded by an atmosphere of heat.
Atoms can’t be made or destroyed
Atoms of the element are identical
Atoms of different elements have different masses, colours etc.
Atoms can join to form compounds.
Dalton’s insight
Dalton did realise that he could make compounds using different particles and he did manage to reach quite a few conclusions about the atomic mass by measuring the masses of the reaction products. He did not realise that two identical particles could also form a compound. This is why his carbon dioxide has only one particle of carbon and one particle of oxygen. This lack of understanding had a detrimental effect on his conclusions on the atomic masses.
Assignments
1. Find the formula of pyruvic acid and draw it like Dalton would do.
2. How did Dalton explain the difference between a molecule of water and a molecule of ice?
3. How did Dalton know the amount of particles in a molecule?
4. Use Dalton’s symbols to write down the reaction scheme for the combustion of alcohol.
Dalton did not realise that two identical particles could also combine to form a molecule. This lead him to make a mistake while calculating with Hydrogen and Oxygen.
5. With which five (modern) elements would he have made the same mistake?
6. Can you give an example of a reaction that he would have never done right? (Hint: it’s a reaction that occurs in the top layer of the atmosphere)
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