John Dalton: Tiny pieces of matter

Who discovered the atom? Nowadays it’s completely unremarkable to hear someone talk about atoms, but before John Dalton’s pioneering work in the development of modern atomic theory little was understood about them. It is amazing to think that most of the ideas that Dalton proposed at the start of the nineteenth century are still considered accurate today. Of course, some of his hypotheses have been refined, while others have been more significantly altered, but the founding principles of his theory have survived.

Yale University Press’ Little Histories collection is a family of books that takes a closer look at some of the most significant events, ideas, discoveries and people throughout history. As part of our ongoing coverage of the collection, here’s an excerpt from William Bynum’s A Little History of Science, a book that examines the scientific discoveries that radically altered our understanding of the world.

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The modern ‘atom’ was the brainchild of a thoroughly respectable Quaker, John Dalton (1766 – 1844). A weaver’s son, he went to a good school near where he was born, in the English Lake District. He was especially skilled in mathematics and science, and a famous blind mathematician encouraged his scientific ambitions. Dalton settled in nearby Manchester, a thriving and rapidly growing town during the early Industrial Revolution, when factories began to dominate the making of all kinds of goods. Here he worked as a lecturer and private tutor. He was the first person to give talks on colour-blindness, based on his own affliction. For many years, colour-blindness was called ‘Daltonism’. If you know someone who is colour-blind, it is probably a boy, since girls rarely suffer from it.

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Dalton was a leading light in Manchester’s scientific life, and his work was gradually appreciated throughout Europe and North America. He did some important experimental work in chemistry, but his reputation then and now rested on his idea of the chemical atom. Earlier chemists had shown that when chemicals react with each other, they do so in predictable ways. When hydrogen ‘burns’ in ordinary air (part of which is oxygen) the product is always water, and if you measure things carefully, you can see that the proportions of the two gases that combine to form water are always the same. (Don’t try this at home, because hydrogen is very easily burned, and can explode.) The same kind of regularity also happened n other chemical experiments with gases, liquids and solids. Why?

For Lavoisier, in the previous century, this was because elements were the basic units of matter and simply couldn’t be broken down into smaller parts. Dalton called the smallest unit of matter the ‘atom’. He insisted that the atoms of one element are all the same, but different from the atoms of other elements. He thought of atoms as extremely small, solid bits of matter, surrounded by heat. The heat around the atom served to help him explain how his atoms, and the compounds they make when joined with other atoms could exist in various states. For example, atoms of hydrogen and oxygen could exist as solid ice (when they had the least heat), as a liquid water, or as water vapour (when they had the most heat).

Dalton made models with little cut-outs to stand for his atoms. He marked his cardboard cut-outs with symbols, to save space (and time) when writing the names of compounds and their reactions (just as if he were sending a modern text message). At first his system was far too awkward to be used easily, but it was the right idea, so gradually chemists decided to use initials as the symbols for elements (and therefore Dalton’s atoms). So hydrogen became ‘H’, oxygen ‘O’, and carbon ‘C’. Another letter sometimes had to be added to avoid confusion: for example, when helium was discovered later, it couldn’t be H so became ‘He’.

The beauty of Dalton’s atomic theory was that it allowed chemists to know things about these bits of matter that they could never actually see. If all the atoms in an element are the same, then they must weigh the same, so chemists could measure how much one weighed compared to another. In a compound made of different kinds of atoms, they could measure how much of each atom there was in the compound, by relative weight. (Dalton couldn’t actually measure how much an individual atom weighed, so atomic weights were merely compared with the weights of other atoms.) Dalton led the way here, and he didn’t always get it right. For instance, when oxygen and hydrogen combine to form water, he assumed that one atom of hydrogen and one atom of oxygen were involved. Based on his careful weighing, he gave the atomic weight of hydrogen as 1 (hydrogen was the lightest known element), and the atomic weight of oxygen as 7, so he said that they had a weight ratio of 1 to 7, or 1:7. He always rounded his atomic weights to whole numbers and the comparative weights he was working with suggested he was right. In fact, the weight rations in water are more like 1:8. We also now know that there are two atoms of hydrogen in each molecule of water, so the ratio of atomic weights is actually 1:16 – one of hydrogen to sixteen of oxygen. The current atomic weight of oxygen in 16. Hydrogen has retained the magical weight of 1, which Dalton gave it. Hydrogen is not only the lightest atom, it is also the most common one in the universe.

Dalton’s atomic theory made sense of chemical reactions, by showing how elements or atoms combine in definite proportions. So, hydrogen and oxygen do this when they form water, and carbon and oxygen when they make carbon dioxide, and nitrogen and hydrogen when they make ammonium. Such regularity and consistency, as well as increasingly accurate tools for measurement, made chemistry a cutting-edge science in the early nineteenth century. Dalton’s atomic theory provided its foundation.

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