Tiny Pieces of Matter

A Little History Of Science: Tiny Pieces of Matter

Atoms used to have a pretty bad name. Remember the ancient Greeks with their notion of atoms as part of a universe that was random and without purpose? So how is it that for us today, being made up of atoms seems so natural?

The modern ‘atom’ was the brainchild of a thoroughly respect- able 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.

Dalton felt right at home at the Manchester Literary and Philosophical Society. Its active members became a kind of extended family for this shy man who never married. Manchester’s ‘Lit. & Phil.’ was one of many similar societies established from the late eighteenth century in towns and cities throughout Europe and North America. Benjamin Franklin, the electrician, was one of the founders of the American Philosophical Society in Philadelphia. ‘Natural philosophy’ was, of course, what we now call ‘science’. The ‘Literary’ in the Manchester society’s name reminds us that science was not yet separated from other areas of intellectual activity; members would gather to hear talks on all sorts of subjects, from Shakespeare’s plays to archaeology to chemistry. The age of specialisation, when chemists mostly talked to other chemists, or physicists just to other physicists, lay in the future. How exciting to range so broadly!

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.) This same kind of regularity also happened in other chemical experiments with gases, liquids and solids. Why?

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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), or as 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.

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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 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 ratios 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 is 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.

Humphry Davy (1778–1829) was at the centre of this chemistry. Whereas Dalton was quiet, Davy was flamboyant and socially ambitious. Like Dalton, he came from a working-class background, and went to a good local school in Cornwall. He was lucky, too. He was apprenticed to a nearby doctor who was to train Davy to become a family doctor. Instead, Davy used the books that his master owned to educate himself in chemistry (and foreign languages). He moved to Bristol, becoming an assistant in a special medical institution that used different gases to treat patients.

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While there, Davy experimented with nitrous oxide – called ‘laughing gas’ because when you breathed it, it made you want to laugh. Davy’s book on the gas, published in 1800, caused a sensation, for nitrous oxide had become a ‘recreational drug’ and nitrous oxide parties were all the rage. Davy also noted that, after breathing the gas, you didn’t feel pain, and suggested that it might be useful in medicine. It took forty years before doctors took up his suggestion, and the gas is still sometimes used as an anaesthetic in modern dentistry and medicine.

Only the great city of London could satisfy Davy’s ambitions. He got his chance to become lecturer in chemistry at the Royal Institution, an organisation that brought science to the middle- class public. Davy the showman thrived there. His talks on chemistry attracted large crowds – people often went to lectures for fun as well as to learn. Davy became a professor at the Institution, and his research flourished. Along with other chemists, he discovered the chemical use of Volta’s electrical ‘pile’, the first battery. He dissolved compounds in liquids to make solutions and then used the pile to pass an electric current through them, analysing what happened. What he saw is that in many solutions, the elements and compounds were attracted to either the negative or the positive ends (poles) of the pile. Davy identified several new elements this way: sodium and potassium, for instance, which both accumulated around the negative pole. Sodium is part of the compound sodium chloride, the substance that makes the ocean salty, and which we put on our food. Once new elements were discovered, Davy could experiment with them, and work out their relative atomic weights.

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Volta’s pile, with its positive and negative poles, also changed the way chemists thought about atoms and chemical compounds. Positively charged things went towards the negative pole, and negatively charged ones to the positive pole. This helped explain why elements had natural tendencies to combine with each other.

The Swedish chemist Jöns Jacob Berzelius (1779–1848) made this fact central to his famous theory of chemical combination. Berzelius survived a difficult childhood. Both his parents died when he was young and he was brought up by various relatives. But he grew up to become one of the most influential chemists in Europe. He discovered the joys of chemical research when he was training to be a doctor, and was able to work as a chemist in the Swedish capital, Stockholm, where he lived. He also travelled a lot, particularly to Paris and London – exciting places for a chemist.

Like Davy, Berzelius used the Voltaic pile to look at compounds in solution. He discovered several new elements this way, and he published lists of them with ever more accurate atomic weights. He worked out the weights by carefully analysing the relative weights of substances combining to make new compounds, or by breaking down compounds and then carefully measuring the products. His chemical table of 1818 listed the atomic weights of forty-five elements, with hydrogen still as 1. It also gave the known compositions of over 2,000 compounds. It was Berzelius who popularised Dalton’s convention of identifying elements by the first one or two letters of their name: C for carbon, Ca for calcium, and so on. This made the language of chemical reactions much easier to read.

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When compounds have more than one atom of an element in them, he indicated it with a number following the letter. Berzelius placed the number above the letter, but scientists now put it below: O2 means there are two atoms of oxygen. Apart from that, Berzelius wrote chemical formulas much as we do today.

Berzelius was much better with inorganic compounds than with organic ones. ‘Organic’ compounds are ones containing carbon and are associated with living things: sugars and proteins are two examples. Organic compounds are often more complex chemically than inorganic ones, and they tend to react in rather different ways than the acids, salts and minerals that Berzelius was mostly examining. Berzelius thought that the reactions that go on in our bodies (or those of other living things such as trees and cows) could not be explained in quite the same way as those that happen in a laboratory. Organic chemistry was being developed during his lifetime in France and Germany, and although he distanced himself from these chemists, he actually contributed to their research. First, he provided the word ‘protein’ to describe one of the most important kinds of organic compounds.

Second, he realised that many chemical reactions will not take place unless there is a third substance present. He called this third thing a ‘catalyst’. It helped the reaction – often by speeding it up – but it did not actually change during the reaction, unlike the other chemicals that combined or broke down. Catalysts are found throughout nature, and trying to under- stand how they work has been the goal of many chemists since Berzelius’s time.

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Elsewhere in Europe, ‘atoms’ were helping chemists understand their work. There were still a lot of puzzles, however. In 1811, in Italy, the physicist Amedeo Avogadro (1776–1856) made a bold statement. It was so bold that it was neglected by chemists for almost forty years. He declared that the number of particles of any gas in a fixed volume and at the same temperature is always identical. ‘Avogadro’s hypothesis’, as it came to be called, had important consequences. It meant that the molecular weights of gases could be calculated directly, using a formula he devised. His idea, or hypothesis, also helped modify Dalton’s atomic theory, because it explained a curious feature of one of the most studied gases, water vapour. Chemists had long puzzled why the volume of hydrogen and oxygen in a particular amount of water vapour was incorrect if one assumed one atom of hydrogen and one of oxygen combined to make a molecule of water. It turned out that there were two atoms of hydrogen for every atom of oxygen in water vapour.

Chemists discovered that many gases, including both hydrogen and oxygen, exist in nature not as single atoms but as molecules: two or more atoms joined together: H2 and O2, as we would say.

Avogadro’s ideas didn’t seem to make sense, if you believed Dalton’s atomic theory, and Berzelius’s idea of the atoms of elements having definite negative or positive characteristics. How could two negative oxygen atoms bind together? These problems meant that Avogadro’s work was neglected for a long time. Much later on, though, it made sense of many chemical puzzles and is now fundamental to our understanding of the chemist’s atom. Science is often like that: all the pieces only fit together after a long time and then things start to make sense.