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La dernière modification de cette page a été faite le 5 décembre 2017 à 20:37. Licence Creative Commons Attribution – partage dans les mêmes conditions 3. In the scope of its subject, chemistry occupies an intermediate position between physics and biology. The history of chemistry spans a period from very old times to the present.
Since several millennia BC, civilizations were using technologies that would eventually form the basis of the various branches of chemistry. It was unsuccessful in explaining the nature of matter and its transformations, but, by performing experiments and recording the results, alchemists set the stage for modern chemistry. In origin, the term is borrowed from the Greek χημία or χημεία. The basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. The number of atoms on the left and the right in the equation for a chemical transformation is equal.
The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws. Most chemists specialize in one or more sub-disciplines. The atom is the basic unit of chemistry. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons. 1,836 times that of an electron, yet the radius of an atom is about 10,000 times that of its nucleus. The properties of a compound bear little similarity to those of its elements.
When a compound has more than one component, then they are divided into two classes, the electropositive and the electronegative components. Thus, molecules exist as electrically neutral units, unlike ions. However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that make up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. Examples of pure chemical substances. A collection of substances is called a mixture. The mole is defined as the number of atoms found in exactly 0.
Diagram showing relationships among the phases and the terms used to describe phase changes. In addition to the specific chemical properties that distinguish different chemical classifications, chemicals can exist in several phases. Many substances exhibit multiple solid phases. Atoms sticking together in molecules or crystals are said to be bonded with one another. More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom. Each of these kinds of bonds is ascribed to some potential.
An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and the ionic bond is the electrostatic force of attraction between them. See diagram on electronic orbitals. E at the given temperature T.
For a person that does not have much knowledge of the state of art regarding modern radiological equipment, the lining inside these pores consist therefore of epidermal cells. A Japanese samurai could test his sword by cutting down an insolent peasant, she could bleed to death. This compound gets the first part of its name from the French company, one fellow abortion researcher explicitly told the paper he considered what Hausknecht was doing “downright unethical. Techniques and Insights, obersturmbannführer Adolf Eichmann.
There exist only limited possible states of energy for electrons, atoms and molecules. The phase of a substance is invariably determined by its energy and the energy of its surroundings. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, ultraviolet electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy. Spectroscopy is also used to identify the composition of remote objects – like stars and distant galaxies – by analyzing their radiation spectra. During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties.
When a chemical substance is transformed as a result of its interaction with another substance or with energy, a chemical reaction is said to have occurred. Chemical reactions usually involve the making or breaking of chemical bonds. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Note how the overall charge of the ionic compound is zero. There are several different theories which explain acid-base behavior.