Chemistry is the science of the stransforming matter.
Chemistry is the scientific discipline involved with elements and compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo during a reaction with other substances.
The modern human experience places a large emphasis upon the material world. From the day of our birth to the day we die, we are frequently preoccupied with the world around us. In fact only a handful of disciplines—certain subsets of religion, philosophy, and abstract math—can be considered completely unrelated to the material world. Everything else is somehow related to chemistry, the scientific discipline which studies the properties, composition, and transformation of matter.
« Chemistry is what separates poverty and famine from abundance. » Robert Brent (first Mayor of Washington, D.C.)
Etymology
The term chemistry comes from the word alchemy . Alchemy appeared before chemistry and this term is directly derived from Arabic al kemi. At least two etymologies exist for the Arabic term, one Egyptian and the other Greek.
- According to the Egyptian hypothesis, kemi comes from the chame « Black » borrowed from the ancient Egyptian kemi (black magic and, by extension, Egypt itself) .
- According to the hypothesis of Greek origin, kemi comes from qumeÐa, khumeia, « mixture of liquids » (qumìc, khumos, « juice, juice »).
Over the centuries, alchemy has developed through important discoveries and then gave birth to chemistry. The term chemistry appeared by a distortion of language. However, chemistry has made great progress to the point of becoming an exact science while there remained alchemists pursuing the quest for the philosopher’s stone, making alchemy an esoteric science.
Chemistry and alchemy had become very different; it was therefore necessary to be able to distinguish them and the two terms remained in the language.
Branches of Chemistry
Chemistry itself has a number of branches:
- Analytical chemistry seeks to determine the composition of substances.
- Biochemistry is the study of chemicals found in living things (such as DNA and proteins).
- Inorganic Chemistry studies substances that do not contain carbon.
- Organic chemistry studies carbon-based substances. Carbon, as described in more detail in this book, has unique properties that allow it to make complex chemicals, including those of living organisms. An entire field of chemistry is devoted to substances with this element.
- Physical chemistry is the study of the physical properties of chemicals, which are characteristics that can be measured
- without changing the composition of the substance.
Basic concepts
An atom (ancient Greek tomoc [atomos], « non cutable ») is the smallest part of a single body that can chemically combine with another. Atoms are the elementary constituents of all solid, liquid or gaseous substances. The physical and chemical properties of these substances are determined by the atoms that constitute them as well as by the three-dimensional arrangement of these atoms.
An atom is made up of an atomic nucleus containing nucleons that keeps around it a number of electrons balancing the positive charge of the nucleus.
A chemical element is the set of atoms that have a given number of protons in their nucleus. This number is its atomic number. For example, all atoms with 6 protons in their nuclei are atoms of the carbon element. These elements are represented in the periodic table, which brings together elements of similar properties.
The chemical bond is the phenomenon that binds the atoms together by exchanging or sharing one or more electrons or by electrostatic forces.
A molecule is an electrically neutral set of atoms associated by covalent bonds.
An ion is a chemical species (an atom or molecule) that has lost or gained one or more electrons. It is called cation when it is positively charged and anion when it is negatively charged.
A chemical compound is a substance resulting from the assembly of several types of atoms from different chemical elements in defined proportions. It is characterized by its chemical formula.
The different representations of the atom
It is known since the eighteenth century there are two kinds of electricity, the positive and negative one qualifies. A corps, in its usual state, possesses as much positive electricity as negative. It is said to be electrically neutral. The electrons are negatively charged. Since all atoms contain electrons, the rest of the atom must be positively charged for the atom to be electrically neutral.
Planetary Model (1901-1915): Perrin, Rutherford, Bohr, Sommerfeld
J. Perrin (1870-1942) proposes in 1901 a planetary representation of the atom: around a positive center, the electrons describe circular orbits. E. Rutherford (1871-1937) shows experimentally, in 1911, that the atom has a very small center positively charged and containing almost all the mass of the atom. The material is therefore mostly empty! In 1913, the Danish N. BOHR (1885-1962) uses the Rutherford model in the particular case of the hydrogen atom and adds other hypotheses which will lead to a correct calculation of the order of magnitude for dimensions of the atom (of the order of a tenth of a nanometer). In 1915, the German A. Sommerfeld (1868-1951) takes up the model of Bohr considering the possibility of elliptical orbits.
Quantum model (from 1925)
The discovery of new properties of matter led physicists to complicate the planetary model of the atom. At present, it is admitted that it is impossible to associate with the electron a trajectory or a well-defined position. All that can be said is that the probability of finding an electron at a given point in space is greater or less. This impossibility of associating atoms with the electron leads to the talk of an electronic procession: we speak of a probabilistic atom, but the simple idea of an atom containing electrons (negative) and a positive nucleus still remains.
Chemistry today
The chemical industry
The chemical industry was born with the industrial revolution in the nineteenth century and today represents an important part of the economic activity of the major industrial countries. This industry deals with two major types of products. Firstly, organic products from petroleum: the syntheses carried out on a large scale are, for example, that of ethylene, propylene and methanol. Second, inorganic products to produce industrial gases, large acids and salts.
The scale of chemical production distinguishes between heavy and fine chemistry. Heavy chemistry produces large quantities of products while fine chemicals consist of the production of relatively small quantities of high value-added compounds for pharmacy, perfumery and cosmetics as well as in many areas of high technology.
Daily chemistry
Chemistry is present in the water we drink every day because it plays a role in preserving natural qualities and disinfecting tap water before consumption. In addition, chemical and biological processes allow the treatment of wastewater in wastewater treatment plants. Chemistry is also present in the diet of everyone: preservatives, nutrients, and food additives (dyes, artificial flavors, sweeteners) are used in the food industry. The packaging of certain foods is also part of the field of chemistry: they preserve food thanks to their properties. They are more and more biodegradable and, using selective sorting after use, they are transformed and upgraded by chemical processes.
Although large-scale use of chemical fertilizers and pesticides is disputed by advocates of organic farming, the use of these product families allows for more abundant crops by enriching poor soils and eliminating pests, parasitic fungi and weeds. In the context of the general awareness of environmental damage, new products that are more efficient and better respect the environment have been developed.
In the field of construction, chemistry also contributes to the manufacture of new materials, insulators, paints, varnishes, mastics, cleaning products and furniture. Chemistry has also contributed with the synthesis of new drugs to the increase in life expectancy recorded since the industrial revolution in developed countries.
Chemistry, health and environment
In the field of « Health-Environment », chemistry is both a source of problems via certain pollutants that it creates or contributes to spreading in the environment, in particular toxic or ecotoxic chemicals whose CMRs are « carcinogenic ». , mutagenic, reprotoxic. Certain products such as drugs, pesticides, catalysts or their residues lost in the environment or present in food can then pose environmental or health problems, in particular with endocrine disruptors).
Chemistry and legislation
At the international level, the Rotterdam Convention, administered by the UN (UNDP, FAO) was adopted by 165 countries in 1998 to better ensure the health of people and the environment against possible damage from the trade in chemicals . Many pieces of legislation deal with chemicals and their residues, which vary from country to country.