British natural philosopher Henry Cavendish discovered hydrogen in 1766, and many more elements were discovered shortly after this. In 1778, Swedish chemist Carl Scheele and French chemist Antoine Lavoisier independently showed that air is mostly composed of nitrogen and oxygen, and within three years, British natural philosopher Joseph Priestley created water by igniting hydrogen and oxygen.
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.
John Dalton's A New System of Chemical Philosophy (1808). Image credit: John Dalton/Public domain.
1.1 Periodic tables ↑
The first periodic tables were produced in the mid-1800s. German chemist Julius Lothar Meyer, like many others, noticed that if the elements were 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 is in.
In 1869, Russian chemist Dmitri Mendeleev independently published his own periodic table, which was almost identical 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.
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 and melting point, as well as their atomic weight.
These tables were later rotated by 90°, like the periodic table we use today. The periodic nature of the elements was explained by Danish physicist Niels Bohr and British chemist Charles Bury in the 20th Century, using Danish physicist Niels Bohr's theory of the atom, which was developed in 1913.
Mendeleev's (1869) periodic table. Image credit: Dmitri Mendeleev/Public domain.
2. Positive and negative ions ↑
British natural philosopher Michael Faraday coined the term 'ion' in 1834, in order to describe the matter within a solution - like Volta's salt-water - that travels from one electrical conductor to another. Faraday dissolved a metal and placed it in a tank at one end, next to at 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.
Swedish physicist Svante August Arrhenius elaborated on Faraday's idea in 1884, when he suggested that when salt - which is composed of sodium chloride (NaCl) - is dissolved into water, it breaks up into negatively charged chlorine ions (Cl-) and positively charged sodium ions (Na+).
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 negative electrode (the cathode) and ended at the positive electrode (the anode).
In 1857, 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. This idea was commercialised by 1910, when Geissler tubes were used to make neon signs[11a].
Drawing of Geissler tubes, 1869. Image credit: M. Rapine/Public domain.
British chemist William Crookes developed Geißler's idea in the 1870s, creating Crookes tubes, which were improved upon in the 1900s with cathode ray tubes, leading to the first televisions in the 1930s[11b].
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 was not until 1897 that British physicist Joseph John "J. J." Thomson determined the mass to charge ratio of the particles that appear to carry the electrical 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. 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.
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, and its effect appears the most dramatic when the gas molecules absorb ultraviolet photons and emit visible photons.
The role of electrons within atoms was explained after the development of Niels Bohr's theory of the atom in 1913. This explained negatively and positively charged ions as well as fluorescence.
3. Radiation ↑
In 1896, French physicist Antoine Henri Becquerel conducted experiments to see if X-rays, which were discovered less than three months before, were produced from elements that were naturally fluorescent, 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 that was not invoked by an external source of energy.
Becquerel's photographic plate. Image credit: Antoine Henri Becquerel/Public domain.
Polish-French physicist and chemist Marie Sklodowska-Curie studied uranium radiation with an electrometer. This is a device for measuring very small changes in electrical charge, which had been invented by her husband, French physicist Pierre Curie, and his brother Jacques.
Sklodowska-Curie showed that uranium radiation causes the air around it to conduct electricity. The amount of conductivity could be used to measure the amount of radiation. This was found to be proportional to the mass of the uranium, irrespective of what temperature it was, or whether it was solid or liquid.
Sklodowska-Curie went on to study all other known elements and found only one that emitted spontaneous radiation, this was thorium. She then turned her attention to natural ores, which she found to be more radioactive than they should be, assuming the only radioactive substance they contained was uranium.
The Curies ground down samples of radioactive ores until they had isolated the radioactive elements. In 1898, they discovered the elements polonium and radium, the former named for Poland, and the second for its extremely high levels of radioactivity, a term they coined to explain this spontaneous emission.
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.
3.1 Alpha, beta, and gamma rays ↑
Becquerel identified beta rays as electrons by measuring their mass to charge ratio in 1900. The emission of beta radiation was eventually explained by electroweak theory, which was developed in the 1960s to describe the weak nuclear force.
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.
British physicist William Henry Bragg showed that gamma rays are part of the electromagnetic spectrum in 1910. In 1914, Rutherford and British physicist Edward Andrade showed that gamma rays are slightly more energetic than X-rays.
The fact that atoms can emit radiation means that they are not indivisible. The structure of atoms was explained by the Bohr-Sommerfeld model. This was developed by Danish physicist Niels Bohr and German physicist Arnold Sommerfeld after 1913.