How We Came to Know the Cosmos: Light & Matter

Discover How We Came to Know the Cosmos

Chapter 6. 19th Century Particle Theories

6.1 The discovery of elements

While some were working to understand the wave nature of light, others were exploring matter, something that, at first, seemed to act just like Democritus’ atoms (discussed in Chapter 1). The British natural philosopher Henry Cavendish discovered hydrogen in 1766,[1] and many more elements were discovered shortly after this.

In 1778, the Swedish chemist Carl Scheele and the French chemist Antoine Lavoisier independently showed that air is mostly composed of nitrogen and oxygen, and within three years, the British natural philosopher Joseph Priestley created water by igniting hydrogen and oxygen.[2]

The British chemist John Dalton stated that matter is composed of atoms at the start of the 19th century. Although he could not prove that atoms exist, he predicted that each element is made of atoms of identical sizes and masses and that atoms can combine to form molecules.[3]

Pages from John Dalton’s ‘A New System of Chemical Philosophy’, depicting different elements.

Figure 6.1
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John Dalton’s A New System of Chemical Philosophy (1808), showing 20 elements and 17 molecules.

Pages from John Dalton’s ‘A New System of Chemical Philosophy’, describing the different elements.

Figure 6.2
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John Dalton’s A New System of Chemical Philosophy (1808). Here the word ‘atom’ refers to what we would now call a molecule.

6.1.1 Periodic tables

The first periodic tables were produced in the 19th century. The German chemist Julius Lothar Meyer, like many others, noticed that if the elements are arranged in order of mass, then they fall into groups that share similar properties. He also noticed that these properties seem to vary periodically.

In 1864, Meyer published a periodic table containing 28 elements classified into six groups, which he updated in 1870. The elements were ordered vertically in terms of atomic weight, with properties depicted by which row the element was in.[4] In 1869, Russian chemist Dmitri Mendeleev independently published his own periodic table, which was similar to Meyer’s. Mendeleev’s table differed, however, because it left gaps for undiscovered elements and changed some of the atomic weights, giving a slightly different order.[5]

The elements Mendeleev had predicted were soon discovered, and it was shown that he had correctly predicted a number of their properties, including their density, melting point, and atomic weight.[6] These tables were later rotated by 90°, like the periodic table we use today. The periodic nature of the elements was explained by the Danish physicist Niels Bohr and the British chemist Charles Bury in the 20th Century (discussed in Chapter 10).

Mendeleev’s periodic table.

Figure 6.3
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Mendeleev’s (1869) periodic table. Here, the elements are ordered vertically in terms of atomic weight and elements in different rows have different properties.

A depiction of a modern periodic table.

Figure 6.4
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A modern periodic table. Here, the elements are ordered horizontally in terms of atomic weight and elements in different columns have different properties.

6.2 Positive and negative ions

In 1834, Michael Faraday coined the term ‘ion’ to describe the matter within a solution - like Volta’s salt-water (discussed in Chapter 5) - that travels from one electrical conductor to another. Faraday dissolved metal and placed it in a tank at one end, next to a positively charged conductor. He later found a new metal had appeared at the other side, next to the negative conductor.

Experiments showed that some substances only moved from the negative conductor, known as the cathode, towards the positive conductor, the anode, and some only travelled the other way. The former were named anions and the latter cations.[7]

The Swedish physicist Svante August Arrhenius elaborated on Faraday’s idea in 1884. Arrhenius showed that when salt - which is composed of sodium chloride (NaCl) - dissolves in water, it breaks up into negatively charged chlorine ions (Cl-) and positively charged sodium ions (Na+).[8]

A diagram showing negatively charged chlorine ions moving towards the anode, and positively charged sodium ions moving towards the cathode.

Figure 6.5
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When sodium chloride (NaCl) is dissolved in water, the negatively charged chlorine ions (Cl-) move towards the positively charged conductor - the anode, and the positively charged sodium ions (Na+) move towards the negatively charged conductor - the cathode.

6.2.1 Geissler tubes

In 1838, Faraday passed an electric current through a glass tube that had had most of its air sucked out. He noticed that this produced a light that began at the cathode and ended at the anode.[9]

In 1857, the German instrument-maker Heinrich Geißler improved upon Faraday’s idea when he invented a way to pump even more air out of the tube. When he did this, Geißler found that the light filled the whole tube. He pumped different gases into the tube and found that they produced lights of different colours.[10] This idea was commercialised by 1910 when Geissler tubes were used to make neon signs.[11] The British chemist William Crookes developed Geißler’s idea in the 1870s, creating Crookes tubes, which were improved upon in the 20th century with cathode ray tubes, leading to the first televisions in the 1930s.[11]

Coloured drawing of Geissler tubes.

Figure 6.6
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Drawing of Geissler tubes, 1869.

6.2.2 The discovery of the electron

When Geißler’s discovery was made, no one knew what carried electrical currents or what it was about the current that made the gas glow. It wasn’t until 1897 that the British physicist Joseph John “J.J.” Thomson determined the mass to charge ratio of the particles that appear to carry the electric current. Thomson determined that they are over 1000 times smaller than the smallest atom. He also found that they all have the same mass, regardless of which gas was pumped into the tube.[12] Thomson had discovered the first elementary particle, the electron.

It was later understood that the electrons were travelling from the cathode towards the anode. When there was less gas in the tube, the electrons could travel faster and sometimes struck the wall of the tube, emitting photons - ‘particles’ of light (discussed in Chapter 8). The gas molecules absorb the photons and emit electrons. They then rapidly reabsorb them, emitting another photon, usually with a lower energy. This is known as fluorescence (discussed in Chapter 10), and its effect appears the most dramatic when the gas molecules absorb ultraviolet photons and emit visible photons.[13]

The role of electrons within atoms was explained after the development of Niels Bohr’s theory of the atom in 1913 (discussed in Chapter 10).

6.3 Radiation

In 1896, the French physicist Antoine Henri Becquerel conducted experiments to see if X-rays, which were discovered less than three months before (discussed in Chapter 5), were produced from naturally fluorescent elements, like uranium. Becquerel left some uranium salts in a draw with a photographic plate and later discovered that the plate was covered in a fog. This meant that the uranium must have emitted some kind of radiation.[14]

A photograph showing dark marks on Becquerel’s photographic plate.

Figure 6.7
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Becquerel’s photographic plate.

Becquerel’s PhD student, the Polish-French physicist and chemist Marie Skłodowska-Curie studied uranium radiation with a device known as a piezoelectric electrometer. This is a device for measuring very small changes in electrical charge that had been invented by her husband, the French physicist Pierre Curie, and his brother Jacques. The electrometer showed that the uranium rays cause the air around a sample to conduct electricity and that the level of conductivity depended on the mass of the sample, irrespective of what temperature it was, or whether it was solid or liquid.[15] Skłodowska-Curie suggested that these rays might be coming from inside the uranium atoms, challenging the idea that atoms are indivisible.

Skłodowska-Curie extracted uranium salts from two types of ore: pitchblende and torbernite. She discovered that both were more radioactive than they should be if the only radioactive substance they contained was uranium and that pitchblende was the most radioactive. Skłodowska-Curie began searching for other radioactive elements within the ore.

Skłodowska-Curie discovered that thorium is also radioactive in April 1898,[16] unaware that the German chemist Gerhard Schmidt had made the same discovery two years before. That year, Pierre stopped his own research and began working on Skłodowska-Curie’s project.

Having been beaten to the discovery of the radioactivity of thorium, the Curies hurried to isolate the other elements in the uranium salts, grinding the ore themselves with pestle and mortars. By July of that year, they had discovered the element polonium, named for Skłodowska-Curie’s homeland of Poland, which did not exist as an independent country at the time. By December 1898, they had discovered radium, named for the amount of radiation it produced, where radium is the Latin word for ‘ray’. The Curies coined the term ‘radioactivity’ during this time.[17]

The Curies found that radioactive substances disappear over time. The time it takes for a radioactive substance to reduce to half its mass is known as its half-life.[18]

Nt = N0(1/2) t/t1/2   (6.1)

Here, N0 is the initial quantity of the radioactive substance, Nt is what remains after time t, and t1/2 is the substance’s half-life.

By 1902, the Curies had ground down and separated over a tonne of pitchblende ore to isolate 0.1 grams of radium chloride. Curie was unable to isolate pure radium until 1910 and was never able to isolate polonium due to its short half-life.

A photograph showing uranium glass glowing green under ultraviolet light.

Figure 6.8
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Uranium glass under ultraviolet light.

6.3.1 Alpha, beta, and gamma-rays

In 1899, the New Zealand physicist Ernest Rutherford discovered that uranium emits two types of radiation, which he termed alpha rays and beta rays.[19] French chemist Paul Villard discovered a third type of radiation in 1900,[20] which Rutherford named gamma-rays.[21] This was the radiation that had fogged Becquerel’s photographic plates.

Becquerel identified beta rays as electrons by measuring their mass to charge ratio in 1900.[22] The emission of beta radiation was eventually explained by electroweak theory, which was developed in the 1960s to describe the weak nuclear force (discussed in Chapter 24).

Alpha rays remained a mystery, although both Rutherford and the Curies predicted that they were some kind of charged particle. In 1907, Rutherford showed that alpha rays are actually helium atoms that are devoid of electrons, which makes them positively charged ions.[23] The emission of alpha radiation was later explained by Heisenberg’s uncertainty principle (discussed in Chapter 16), after the development of nuclear physics (discussed in Chapter 14) and quantum mechanics (discussed in Chapter 15) in the 1920s.

The British physicist William Henry Bragg showed that gamma-rays are part of the electromagnetic spectrum in 1910.[24] In 1914, Rutherford and the British physicist Edward Andrade showed that gamma-rays are slightly more energetic than X-rays.[25] The emission of gamma radiation was finally explained using nuclear physics after Albert Einstein published his theory of special relativity in 1905 (discussed in Book I).

The fact that atoms can emit radiation means that they’re not indivisible. The structure of atoms was explained by the Bohr-Sommerfeld model, which was developed by Niels Bohr and the German physicist Arnold Sommerfeld after 1913 (discussed in Chapter 10 and Chapter 11).

6.4 References

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