The Danish Peace Academy

SCIENCE AND SOCIETY

John Avery
H.C. Ørsted Institute, University of Copenhagen

Chapter 11 ATOMS IN CHEMISTRY

Dalton

As we saw in an earlier chapter, atomism was originated by the Greek philosopher, Leucippus, in the 5th century B.C., and it was developed by his student Democritus. The atomists believed that all matter is composed of extremely small, indivisible particles (atoms).They believed that all the changes which we observe in matter are changes in the groupings of atoms, the atoms themselves being eternal.

The rational philosophy of Democritus was not very popular in his own time, but it was saved from being lost entirely by the Athenian philosopher Epicurus. Later, the Roman poet, Lucretius, published a long, philosophical poem, De Natura Rerum, in which he maintained that all things (even the gods!) are composed of atoms. In 1417, a single surviving manuscript copy of De Natura Rerum was discovered and printed.

The poem became very popular, and in this way, the ideas of Democritus were transmitted to the experimental scientists of the 17th century, almost all of whom were believers in the atomic theory of matter.

Christian Huygens, for example, believed that light radiating from a flame is a wavelike disturbance produced by the violent motion of atoms in the flame. Sir Isaac Newton was also a believer in the atomic theory of matter. He believed (correctly) that chemical compounds are composed of atoms bonded together by forces which are fundamentally electrical in nature. The universally talented Robert Hooke came near to developing a kinetic theory of gases based on atomic ideas; but he lacked the mathematical power needed for such a theory.

At the beginning of the 19th century, an honest, ingenious, colorblind, devout, unmarried English provincial schoolteacher named John Dalton (1766-1814) gave the atomic theory of matter new force by relating it to the observed facts of chemistry. Dalton was born in Cumberland, the son of a Quaker weaver, and he remained in the North of England all his life. At the early age of 12, he became a teacher; and he remained a teacher in various Quaker schools until 1800, when he became the Secretary of the Manchester Literary and Philosophical Society.

One of Dalton’s early scientific interests was in meteorology, and he recorded the capricious weather of the Lake District in a diary which ultimately contained more than 200,000 entries. In speculating about water vapor in the atmosphere, John Dalton began to wonder why the various gases in the atmosphere did not separate into layers, since some of the gases in the mixture were less dense than others.

The only way that Dalton could explain the failure of the atmosphere to stratify was to imagine it as composed mainly of empty space through which atoms of the various gases moved almost independently, seldom striking one another. In this picture, he imagined each of the gases in the atmosphere as filling the whole available volume, almost as though the other gases in the mixture were not there.

Dalton believed the pressure on the walls of a vessel containing a mixture of gases to be due to the force of the atoms striking the walls; and he believed that each of the gases behaved as though the other gases were not there. Therefore he concluded that the total pressure must be the sum of the partial pressures, i.e. the sum of the pressures which would be exerted by each of the gases in the mixture if it occupied the whole volume by itself. This law, which he confirmed by experiment, is known as “Dalton’s Law of Partial Pressures”.

Convinced of the atomic picture by his studies of gases, John Dalton began to think about chemical reactions in terms of atoms. Here he made a bold guess - that all the atoms of a given element are of the same weight. He soon found that this hypothesis would explain one of the most important fact in chemistry, the fixed ratio of weights in which chemical elements combine to form compounds. (The law of definite proportions by weight in chemical reactions is known as “Proust’s Law”, after the French chemist, Joseph Louis Proust (1754-1826), who first proposed and defended it.)

In Dalton’s view, molecules of the simplest compound formed from two elements ought to consist of one atom of the first element, united with one atom of the second element. For example, the simplest compound of carbon and oxygen should consist of one atom of carbon, bonded to one atom of oxygen (carbon monoxide). Dalton believed that besides such simple compounds, others with more complicated structure could also exist, (e.g. carbon dioxide).

By studying the weights of the elements which combined to form what he believed to be the simplest chemical compounds, Dalton was able to construct a table of the relative atomic weights of the elements.

For example, knowing that 12 ounces of carbon combine with 16 ounces of oxygen to form carbon monoxide, Dalton could deduce that the ratio of the weight of a carbon atom to the weight of an oxygen atom must be 12/16. His table of relative atomic weights contained some errors, but the principle which he used in constructing it was not only correct, but also very important.

Gay-Lussac and Avogadro

In 1808, John Dalton published his table of atomic weights in a book entitled A New System of Chemical Philosophy. A year later, in 1809, the celebrated French chemist and balloonist, Joseph Louis Gay-Lussac (1778-1850), made public an important law concerning the chemical reactions of gases: Gay-Lussac’s experiments showed that the volumes of the reactants and the volumes of the products were related to each other by the ratios of simple whole numbers.

This law was strikingly similar to Proust’s law of definite proportions by weight, on which Dalton had based his table of relative atomic weights. Gay-Lussac stated that his results were “very favorable to Dalton’s ingenious ideas”; but there were problems in linking Dalton’s ideas with Gay-Lussac’s experiments.

Observation showed, for example, that one volume of hydrogen gas would unite with exactly the same volume of chlorine gas to form the gas of hydrochloric acid. The problem was that, if the temperature and pressure were kept constant, the resulting total volume of gas was the same after the reaction as before, although according to Dalton’s ideas the number of particles should be cut in half!

This was a mystery which Dalton and Gay-Lussac failed to solve; but it was completely cleared up a year later, in 1810, by Amadeo Avogadro (1776-1856), Count of Quaregna and Professor of Philosophy at the University of Vercelli in Italy. Avogadro introduced a bold hypothesis - that a standard volume of any gas whatever, at room temperature and atmospheric pressure, contains a number of particles which is the same for every gas.

(Avogadro himself did not have any idea how many gas particles there are in a litre of gas; but we now know that at room temperature and atmospheric pressure, 22.4 litres contain 602,600,000,000,000,000,- 000,000 particles. This is the same as the number of atoms in a gram of hydrogen. To get some imaginative idea of the size of “Avogardo’s number”, we can think of the fact that the number of atoms in a drop of water is roughly the same as the number of drops of water in all the oceans in the world!)

Avogadro believed that the particles of a gas need not be single atoms, even if the gas contains only a single element. In thes way, he could explain the mysterious proportions of volume observed by Gaylussac. for example, in the reaction where hydrogen and chlorine combine to form hydrochloric acid, Avogadro assumed that every molecule of hydrogen gas consist of two atoms joined together, and similarly, every molecule of chlorine gas consist of two atoms. Then, in the reaction in which hydrochloric acid is formed, the total number of molecules is not changed by the reaction, which fits with Gay-lussac’s observation that the volume occupied by the gasses is unchanged. Although Avogadro completely solved the problem of reconciling Dalton’s atomic ideas with Gay-lussac’s volume ratios, there was a period of 50 years during which most chemists ignored the atomic theories of Dalton and Avogadro. However, it hardly mattered that the majority of chemists where unconvinced, since the greatest chemist of the period, J¨ons Jakob Berzelius (1779-1849), was an ardent disciple of Dalton’s atomism. His belief more that made up for the other chemists’ disbelief!

After studying medicine at the University of Uppsala in Sweden, Berzelius became a chemist; and over a period of ten years, between 1807 and 1817, he analysed more than two thousand different chemical reactions. He showed that all these reactions follow Proust’s law of definite proportions by weight. He also continued Dalton’s work on relative atomic weights; and in 1828 he published the first reasonably accurate table of these weights.

Unfortunately, although Berzelius was a follower of Dalton, he did not appreciate the value of Avogadro’s ideas; and therefore confusion about the distinction between atoms and molecules remained to plague chemistry until 1860. In that year, the first international scientific congress in history was held at Karlsruhe, Baden, to try to clear up the confusion about atomic weights. By that time, Dalton’s atomic theory was widely accepted, but without Avogadro’s clarifying ideas, it led to much confusion. In fact, the chemists of the period were almost at one another’s throats, arguing about the correct chemical formulas for various compounds.

Among the delegates at the Karlsruhe Congress was the fiery Italian chemist, Stanislao Cannizzaro (1826-1910). He had been a revolutionist in 1848, and he was later to fight in the army of Garibaldi for the unification of Italy. Cannizzaro had read Avogadro’s almostforgotten papers; and he realized that Avogadro’s hypothesis, together with Gay-Lussac’s volume ratios, could be used to determine atomic weights unambiguously. He went to the congress filled with missionary zeal; and as a result of his efforts, most of the other delegates saw the light. One of the delegates, Lothar Meyer, said later: “The scales suddenly fell from my eyes, and they were replaced by a feeling of peaceful certainty.”

Neither John Dalton nor Amadeo Avogadro lived to see the triumph of their theories at Karlsruhe, but towards the end of his life, John Dalton was much honored. He was given an honorary degree by Oxford University, invited to soir´ees by the Duke of Sussex, and presented to King William IV of England.

The presentation to the king involved some difficulty, since Dalton was forbidden by his Quaker religion to wear the sword required for court dress. Therefore it was arranged that he should be presented to the king wearing crimson academic robes from Oxford; but here again there was a difficulty: Bright colors were inconsistent with the simple clothes required by the Quakers. Dalton solved this problem by wearing the crimson robes anyway, and saying that he was colorblind, which was perfectly true!

Mendeléev

Among the distinguished delegates listening to Cannizzaro at the Karlsruhe Congress in 1860, was the brilliant young Russian chemist, Dmitri Ivanovich Mendel´eev (1834-1907). He had been born in Tobolsk, Siberia, the youngest child in a family of 14 (some accounts even say 17!). His grandfather had brought the first printing press to Siberia, and had published Siberia’s first newspaper. His father had been the principal of the high-school in Tobolsk, before blindness forced his retirement. Mendeléev’s mother, a part-Mongol woman of incredible energy, then set up a glass factory to support her large family.

When Mendeléev was in his teens, two disasters struck the family: His father died and the glass factory burned down. His mother then gathered her last remaining strength, and traveled to St. Petersberg, where a friend of her dead husband obtained a university place for her favorite son, Dmitri. Soon afterward, she died. After graduating from the university at the top of his class, Dmitri Mendeléev went to Germany to do postgraduate work under Bunsen, (the inventor of the spectroscope and the “Bunsen burner”). In 1860, he attended the First International Congress of Chemistry at Karlsruhe; and like Lothar Meyer, he was profoundly impressed by Cannizzaro’s views on atomic weights.

Returning to St. Petersberg, (where he became a professor of chemistry in 1866), Mendel/eev began to arrange the elements in order of their atomic weights. He soon noticed that when the elements were arranged in this way, their chemical properties showed a periodic variation. Arranged in order of their atomic weights, the first few elements were hydrogen, (helium was then unknown), lithium, beryllium, boron, carbon, nitrogen, oxygen and fluorine. Mendel´eev noticed that lithium was a very active metal, with a valence (combining power) of 1; beryllium was a metal, with valence 2; boron had valence 3; and carbon had valence 4. Next came the non-metals: nitrogen with valence 3; oxygen with valence 2; and finally came fluorine, a very active non-metal with valence 1.

Continuing along the list of elements, arranged in order of their atomic weights, Mendeléev came next to sodium, a very active metal with valence 1; magnesium, a metal with valence 2; aluminium, with valence 3; silicon, with valence 4; phosphorus, a non-metal, with valence 3; sulphur, a non-metal with valence 2; and finally chlorine, a very active non-metal with valence 1. Mendel´eev realized that there is a periodicity in the chemical properties of the elements: The elements of the first period, arranged in order of increasing atomic weight, had the valences 1,2,3,4,3,2,1. The second period exhibited the same pattern: 1,2,3,4,3,2,1.

When he arranged all of the known elements in a table which exhibited the periodicity of their chemical properties, Mendeléev could see that there were some gaps. These gaps, he reasoned, must correspond to undiscovered elements! By studying the rows and columns of his periodic table, he calculated the chemical properties and the approximate atomic weights which these yet-unknown elements ought to have. Mendeléev’s predictions, made in 1869, were dramatically confirmed a decade later, when three of the elements whose discovery he had prophesied were actually found, and when their atomic weights and chemical properties turned out to be exactly as he had predicted! The discovery of these elements made Mendel´eev world-famous, and it was clear that his periodic table contained some deep truth. However, the underlying meaning of the periodic table was not really understood; and it remained a mystery until it was explained by quantum theory in 1926.

Chapter 12: ELECTRICITY AND MAGNETISM.

Suggestions for further reading

1. Frank Greenaway, John Dalton and the Atom, Heinemann, London (1966).
2. Sir Basil Schonland, The Atomists, 1805-1933, Clarendon Press, Oxford (1968).
3. F.J. Moore, A History of Chemistry, McGraw-Hill (1939).
4. W.G. Palmer, A History of the Concept of Valency to 1930, Cambridge University Press (1965).

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