A brief history of chemistry

Mythology - The Egyptians believed that chemistry was revealed by the god Thoth. This god of language knew the divine words which other gods could not resist. Thot was the god of scribes and magicians. He appeared in the form of the sacred ibis or baboon.

Nefertari listening to Thot reciting magic formulas

Thot with the sribe Nebmertouf

- The Greeks assimilated Thoth with their god Hermes.

Hermès Trismégiste

- Later alchemists of the Middle Ages believed that Hermes had noted all chemical knowledge on an emerald table.

Tabula Smaragdina

- Christian apologist Tertullian says (around 200 AD) very seriously that the fallen angels of the apocalypse taught men the proceeds from this damnable art.

The first attempts to explain the strange properties of matter - For the Greek philosopher Democritus, everything we see and think is formed of very small material things he calls "atoms" (Monism): "In reality, there is nothing but indivisible atoms in an infinite void" - The Greek philosopher Aristotle said that there is a unique raw matter which can only have four qualities: hot, cold, dry and wet. According to him, the fire would be nothing other than this "materia prima" which would be added to the hot and dry. The same would be true for air where the dry and wet are added to the raw matter and for water where it would be the cold and wet. The air would turn into fire, if the humide is replaced by the dry (Dry wood burns better than wet wood) and turn to water, if the hot is replaced by cold (Often moisture condenses from the cooled air).

Earth, water, air and fire - The biggest charlatan of the Middle Ages, the alchemist Paracelsus (he claimed to have made a living in the flesh, the homunculus!), reduced the field to three "principles": sulfur, quicksilver (mercury) and salt. By combining the first two, we get the famous philosopher's stone that would allow nothing less than to transform lead into gold and make a young man an old man! (Like all the fine speakers (even today), Paracelsus had huge success: He was endowed with the first chair of chemistry in the world to the University of Basel in 1527!) - The German chemist Georg Stahl (1700) states "the sublime theory": He says that when a body is burning in the air, it loses something, that something we see burning and that seems to escape out of the burning body: the "phlogiston". He superbly ignores the experiment of Jean Rey, who had already found a half-century before, by burning tin that its weight increases !. (In 1630, Jean Rey, calcining 2 pounds 6 ounces of the purest tin, got 2 pounds 13 ounces residues, "which gave him an incredible astonishment, because he was unable to imagine from where the 7 ounces had come from") - Asked about the theories of matter, the great English physicist Newton said with disdain: "I do not forge assumptions". Between 1700 and 1789 (publication of the "Traité élémentaire de chimie" by Lavoisier) all true chemists stuck to this prudent reserve.

The first chemical experiments -20000: First dyes in cave paintings (Lascaux, Altamira) -5000 Manufacture of antimony and copper (Mesopotamia) -4000: Early Bronze Age, use of cosmetics and perfumes in Egypt, mummies enbaumages by bitumen. -1200: Beginning of the Iron Age, enamel manufacturing in Egypt 700: Distillation methods developed by the Arabs 800: The Arab Geber discovered arsenic salts, sulfur and mercury 1200 Spanish Arabs discovered sulfuric acid 1200 Albertus Magnus discovered nitric acid and its properties 1250 Roger Bacon discovered gunpowder 1650 Van Helmont discovers gas 1660 Robert Boyle distinguished between compound substances and mixtures 1755 Karl Wilhelm Scheele discovers the carbon dioxide and chlorine 1766 Henry Cavendish isolates hydrogen 1774 Joseph Priestley isolates oxygen 1781 Henry Cavendish realises the synthesis of water by the hydrogen and oxygen 1783 Antoine Laurent Lavoisier makes the decomposition of water into hydrogen and oxygen

The fundamental laws of chemistry 1) The law of conservation of mass (Lavoisier 1772)

Lavoisier, the father of chemistry Aged 30 years, Lavoisier repeated the famous experiment of the calcination of tin in a vase filled with air and sealed. He found that the ashes had a greater mass than that of the tin used. However the mass of the air in the vessel was decreased exactly by the value that the mass of the tin increased. As the mass of the vase and its contents had not changed during the operation, an inescapable conclusion was: Nothing, not even phlogiston came out of the burning metal. On the contrary, the oxygen had combined with the tin.

The balance is in equilibrium when the acid and marble are separated. The balance remains in equilibrium after the reaction which produces a release of gas. Conclusion: In a chemical reaction, the total mass of the reactants is equal to the sum of the masses of the products. (Lavoisier's Law) "Rien ne se crée, rien ne se perd, ni dans les oeuvres de l'art, ni dans ceux de la nature" (Lavoisier's formulation) 2) The law of constant proportions

Joseph-Louis Proust At the time, a great challenge for chemists was to make the difference between compounds and mixtures (melting and boiling temperatures!). Once this difference was well established, they could take care of compounds and their composition. In 1807 Proust measured using the balance the masses of the elements present in a compound (e.g. the masses of oxygen and hydrogen obtained by analysis of the compound water or the masses of sulfur and copper which combine to form the compound copper sulfide).

Click ! Conclusion: In a compound, the mass ratio of the elements is constant (Proust's law) This fundamental law is in fact due to cooperation of the chemists Proust, Bertollet, Vauquelin, Klaproth and Richter. It was clearly stated by Dalton. 3) The atomic hypothesis of Dalton

John Dalton 1766 - 1844 Dalton was a teacher in a Manchester school. Today he is held for the undisputed creator of the atomic theory. In his famous 1808 book "New chemical system of philosophy", he poses the essential assumptions of atomic theory: 1. An elementary substance is formed of one kind of atoms 2. All atoms of an elementary substance are identical 3. Atoms remain unchanged in chemical reactions 4. In the formation or destruction of compounds, atoms are separated or recombined in a different ways.

Quote Dalton also invented the chemical symbols of the atoms:

Atom symbols by Dalton He imagined their association with these symbols.

Assemblages of atoms by Dalton Unfortunately Dalton had not a clear idea of ​​the molecule, although some of its drawings looked like molecules. Using the atomic hypothesis of Dalton, the laws of Lavoisier and Proust are explained naturally: Explaining the law of mass conservation : According to Dalton, a chemical reaction is a simple rearrangement of atoms, for example:

Chemical reaction by Dalton As all atoms are kept intact keeping their mass, the weight of the reactants (in this case the mass of 6 "red" atoms + mass of six "green" atoms + mass of three "purple" atoms) is equal to the mass of products (in this case the mass of 6 "red" atoms + the mass of six "green" atoms + the mass of three "purple" atoms ) Explanation of the law of constant proportions: According to Dalton, a compound is an association of different atoms. Here, for example, according to Dalton, two samples of the same compound:

The compound Let be "a" the mass of a "green" atom and "b" the mass of an "purple" atom. Here is how we arrive at same ratio of the masses of the atoms "green" and "purple" in each sample: 1st sample: $\frac{m_{green}}{m_{purple}}=\frac{12a}{6b}=\frac{2a}{b}$ 2nd sample: $\frac{m_{green}}{m_{purple}}=\frac{6a}{3b}=\frac{2a}{b}$

The internal structure of the atoms 1) The discovery of the electron

J.J. Thomson In 1897, the English physicist J.J. Thomson examined the electrical discharge that occurs from a metal electrode to an other in a glass enclosure in which there is a high vacuum. He studied the properties of cathode rays:

J.J. Thomson Conclusion: Atoms contain negatively charged particles called electrons. The electrons have a very low mass compared to that of the whole atom. 2) The model of Thompson ("Plum pudding model"):

Electrons are inserted as negative "grapes" into a positive compact mass. 3) The discovery of α (alpha) particles

Pechblende In 1896, the French physicist Henri Becquerel observed that the uranium ore "Pechblende" spontaneously emits a radiation that he called radioactive radiation.

Marie Sklodowska Curie Two years later, Marie Curie isolated the radioactive chemical element radium (Ra) from Pechblende: The properties of radioactivity could then be determined:

Ra radiation revealed on a fluorescent plate a). The radioactive radiation is not simple: it consists of three parts, namely the α , β and &gamma rays b). γ (gamma) rays are not deflected by passing through the charged plates: They are similar to light (electromagnetic radiation), but not visible and much stronger. c) β (beta) rays are negatively charged particles that were successfully identified as being electrons. d) α (alpha) rays are positively charged particles. These particles are ten thousand times heavier than the electrons. e) α particles are atoms of the helium element devoid of their electrons. f) The last point shows that with the discovery of radioactivity, science enters a new era. Since the element (one kind of atoms) radium produced the element(one kind of atoms) helium, one had to admit that atoms can decompose to form other atoms: One of Dalton's assumptions had to be revised ! g) & alpha; particles are projected with tremendous speed outside the radium atoms. The idea quickly came to use these missiles to bombard other atoms to see what might happen. 3. The discovery of the nucleus

Ernest Rutherford In 1911, Ernest Rutherford used α particles to bombard the atoms contained in a very thin sheet of the chemical element gold (Au):

The Rutherford experiment (Geiger and Marsden): 1. α ray source; 2. Thin gold sheet 3. Fluorescent screen That experience revealed at once the true nature of the atom: a) As most of the particles pass straight through the gold atoms, one must admit that atoms are almost entirely empty. b) The whole mass of the atom must be concentrated in a very small central region called the nucleus, because it is only this region that deflects the heavy α particles.

Nucleus and α c) Since the nucleus deflects some α particles (one particle out of is 100000 is deflected !) which pass in its vicinity, it must be positively charged. d) Each atom is formed from a small central nucleus and a peripheral region containing only the electrons: the electron cloud. If the particles pass through this region, they can not be deflected, because the electrons are much too light. (A cannonball is not deflected by a fly!)

Nucleus and atom e) As one particle out of 10.000 is deflected by a gold sheet 10000 atoms thick, we must admit that if we had a sheet of 1 atom thick, one particle out of 100000000 would be deflected. The section $s$ of the nucleus is thus 100000000 times smaller than the section $S$ of the entire atom: $100000000=\frac{S}{s}=\frac{\pi \cdot R^2}{\pi \cdot R^2}=(\frac{R}{r})^2$ so: $\frac{R}{r}=10000$ If the atom was an elephant, the nucleus would take the size of a microbe!