Astropedia

Dalton and Atoms



Engraving of a painting of John Dalton. Click here for original source URL.

Aurora borealis, the northern light. Click here for original source URL.

John Dalton was a self-educated English scientist who worked in the early 1800s, around the time that Beethoven composed symphonies and Napoleon ruled much of Europe. He had been born into a family of Quaker weavers in 1766. By the age of 12 he was teaching other youngsters in a Quaker school in his village. He went on to learn math, astronomy, foreign languages, and observational methods from John Gough, a blind philosopher who lived nearby. He published a theory about the aurora borealis, or northern lights, at the age of 22. Dalton correctly inferred that the aurora was caused in part by Earth’s magnetism. He later published a less successful theory about the causes of color blindness, which he suffered from. Dalton never attended university.

Science often boils down to seeing patterns in observational data. Dalton’s particular skill was to be able to synthesize the available information. He could move quickly from partially complete data to brilliant insights about the "big picture," much as some people can see a few pieces from an incomplete jigsaw puzzle and deduce the entire picture. These sudden moments of insight are the rarest and most pleasurable feelings a scientist can have. Dalton also followed the example of Galileo in not trusting the opinions of previous authorities on a subject. He wrote "Having been in my progress so often misled by taking for granted the results of others, I have determined to write [only] what I can attest by my own experience." Dalton did his own experiments and he did his own thinking.

Dalton reached his theory of atoms by experimenting with gases. Using homemade laboratory equipment, he studied how they mixed in the air, got absorbed into water, or combined together. He discovered that air is not a uniform medium, but rather a mixture of gases including nitrogen and oxygen. A single gas is a chemical element. Dalton also found that gases combine only in certain proportions. A combination of gases is a chemical compound. For example, hydrogen and oxygen combine explosively to form steam, but only in proportions of one part hydrogen to eight parts oxygen, by weight. Dalton figured out the proportions for other compounds too. The specific proportions intrigued him. "I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the...gases..." he wrote in 1803.


A scan of the first page of John Dalton's "A New System of Chemical Philosophy", published in 1808. Click here for original source URL.
By writing about the "ultimate particles" of each element, Dalton gave new life to the old Greek idea of atoms. An atom is therefore the particle of each element that cannot be subdivided by ordinary chemical means. Dalton inferred that the fixed proportions meant that the compounds were being formed by fixed numbers of atoms of each element, with different elements having atoms of different weight. A molecule is a linked set of two or more atoms that correspond to a unique compound. Dalton’s measurements of water could be explained if oxygen atoms have 16 times the weight of hydrogen atoms, and if molecules of water vapor (steam) each have two atoms of hydrogen and one of oxygen. Thus, Dalton was the first to describe the composition of water as H2O, meaning it has two hydrogen atoms and one of oxygen. No known process could break down hydrogen and oxygen so they were considered fundamental.

The same ideas applied to all materials — solid, liquid, and gaseous. For example, chemists knew that most materials can be broken down into simpler chemicals. Burning wood produces carbon dioxide, water, and carbon in the form of charcoal. Water can be broken down with an electric current into hydrogen and oxygen. However, carbon cannot be broken down any further. It is an element. We can summarize Dalton’s insight be saying that an atom is the microscopic, indivisible counterpart of a chemical element, and a molecule is the fundamental counterpart of a chemical compound.

Dalton published the results of his elegant experiments in 1810. These ideas, which are known as the atomic theory, form the basis for modern chemistry. The atomic theory includes the following key ideas:


All elements are composed of atoms.


Each element has unique chemical properties and each element is composed of atoms of a unique weight.


Compounds are formed when atoms are linked in certain proportions into molecules. Each compound has unique chemical properties.


Elements and compounds combine (and separate) in a processes called chemical reactions.


Elements — and their microscopic constituents, atoms — are fundamental and cannot be broken down into different materials.


One element cannot be changed into a different element by chemical reactions.

Dalton’s work met with some resistance. After all, no one had ever seen or isolated an individual atom! They were apparently far too small to be seen with a microscope. The atomic theory was a fatal blow to alchemy — the ancient idea that elements could be transformed from one to another. Today we think of alchemy as a non-scientific pursuit. Yet over the centuries many scientists have tried to turn a base metal like lead into gold. Both lead and gold are dull, heavy, malleable metals. It does not unreasonable that one could be converted into the other. The great Newton himself spent hundreds of hours in his laboratory doing experiments in alchemy. Now we know that chemical reactions operate can join and separate elements but they cannot transform one element into a different one.


Author: Chris Impey