Atom

m_p = 1,67265 ×10^-27 kg.

Around the core “a cloud” of identical particles is: the electron S; dimensions of this electronic cloud (about a Angström, or 10^-10 m) correspond to those of the atom.

The electrons have an electric charge negative equalizes with:

E = -1,602 173 ×10^-19 C,

m_e = 9,109 53 ×10^-31 kg.

The electric charge of an atom is neutral, because the number of electron S (negatively charged) of the electronic cloud is equal to the number of Proton S (positively charged) constituting the core. Thus, the electric charges are cancelled from a macroscopic point of view.

The atoms are likely to take care electrically while gaining or by losing one or more electrons: one speaks then about Ion S. Owing to the fact that an electron has an electric charge negative , if an atom gains one or more electrons, the load of the atom becomes negative (Anion), and if it loses some, the load of the atom becomes positive (Cation).

The physical properties and chemical of the atoms depend primarily on the number of Proton S which compose their core. Also, the atoms are classified according to this number, called Atomic number.

The matter made up of an unspecified whole of of the same atoms atomic number is an element, or chemical element. Atoms having the same atomic number, but different mass numbers (different number of neutrons), are called Isotope S.

The various artificial or natural chemical elements were ordered according to their properties in the periodic Tableau of the elements.

History of the atom

See also: chronological Plank of microscopic physics

The concept of atom is particularly well admitted by the general public, however, paradoxically, the atoms cannot be observed by average optics and only some rare physicists handle isolated atoms. The atom is thus a primarily theoretical model. Although this model is not called today any more into question, it evolved/moved much during time to fulfill the requirements of the new physical theories and to correspond with the various experiments carried out.

Antiquity: a philosophical and intuitive concept

It is probable so much that several Peuple S developed the concept of “grain composing the matter”, this concept seems obvious when one parcels out a lump of earth, or by looking at a sand dune. In the European Culture , this concept appears for the first time in the ancient Greece at fifth century BC, at the Philosophe S Présocratiques, in particular Leucippe, approx. 460 - 370 front J. - C., Démocrite and, later, Épicure.

It is about a design, a priori of the world, which belongs to the research of the principles of reality, seeks which characterizes the first Philosophe S: it is supposed that the matter cannot divide indefinitely, that there is thus a conservation of the elements of the world, which change or combine according to varied processes. The decomposition of the world in four element S (Water, Air, Ground, fire) can thus supplement this thesis. The Atomisme is a concurrent solution, which is born from the opposition to be it and nothing: the atom is a piece to be which eternally preserves, without what, the things would end up disappearing. It was, undoubtedly, a major philosophical turning, at the origin of the Matérialisme and criticism of the Religion. However, even if the Empirisme epicurean tries to establish this assumption on scientific bases, the atom remains an intuition without confirmations.

The chemistry of the XVIIIe century - elements

Since millenia, one noticed that the products change: the Fire, the Metallurgy (transformation of the Ore into Metal), the Corrosion (degradation of metal), the Life, the Cooking of food, the decomposition of the Organic matter… For example, for Empédocle, the transformations of the Matière were explained in the following way: there were four types of elements (water, air, ground, fire) which joined and dissociated, according to the love or of hatred that they went - famous “the hooked atoms”. With the Middle Ages, the Alchimiste S studied these transformations and noticed that they follow quite precise rules. Towards 1760, British chemists start to be interested in gases produced by the reactions, in order to measure volume of it and to weigh them. Thus, Joseph Black, Henry Cavendish and Joseph Priestley discovers various “airs” (i.e. gas): the “air fixes” (the carbonic gas), the “flammable air” (the Dihydrogène), the “phlogistiqué air” (the Diazote), the “déphlogistiqué air” (the Dioxygène)… (The term “Phlogistique” comes from the theory of the German chemist Georg Ernst Stahl, at the beginning of the 18th century, to explain combustion; this theory was swept by Lavoisier.)

Antoine Laurent de Lavoisier (French chemist) states in 1773 that: Nothing is lost, nothing is not created, all changes (formulated in a slightly different way at the time) meaning by there that:

• the mass is preserved during the chemical reactions.
  Les scientific had observed that if one weighed the solid matter before and after combustion, one had a variation of mass; this comes from an exchange with the air (oxygen is incorporated and weighs down, the carbonic gas and the steam from go away and reduce). It is enough to realize it to make burn in a closed bell, and to weigh the bell in entirety, solid nap and gas (including): the total mass does not change. ;
• the substances break up into “elements”, it is the organization of these elements which changes during a reaction.

This concept marks the true birth of the Chimie. The chemists thus started to count the elements of which all the substances are made up and to create a systematic nomenclature - oxygen: who generates acids (οξυs means “acid” in Greek) - hydrogen: who generates water… For example, in 1774, Lavoisier, while following work of the British chemists, establishes that the air is composed in “vital air” (dioxygene) and in “foul air and mephitic, mofette” (diazotizes); into 1785, it breaks up water (by making pass from the steam on iron heated to the red) and thus shows that it is not an element, but that water is decomposable in elements (it is in fact a Pyrolyse). The term of “analysis” comes besides from this concept of decomposition, Lusis (λυσιs) means “dissolution” in Greek: the products are broken up (by acid attack, by burning them, by distilling them…) until obtaining recognizable simple substances easily (hydrogen, oxygen, carbon, iron…).

There is thus the first experimental observation of the decomposition of the matter in elementary substances.

Physique of the XVIIIe century - particles

Another step, taken in parallel, comes from the study of the properties of gases and heat (Thermodynamique).

The Fluide S (liquids and gas) are studied in Europe since Antiquity, but it is in the middle of the 17th century that one really starts to determine their properties, with the invention of the Thermomètre (thermoscope of Santorre Santario, 1612), of the Baromètre and the pumped vacuum (Evangelista Torricelli, 1643), the study of the expansion of the gases (Gilles Nobody Roberval, 1647), the Atmospheric pressure (Blaise Pascal and Florin Perrier, 1648), the relations between pressure and volume ( Robert Boyle in 1660, Edmé Mariotte in 1685), concept of Absolute zero (Guillaume Amontons, 1702)…

Rene Descartes (mathematician, physicist and philosopher French) puts forward the idea, in 1644, that the gases are composed of whirling particles. But it is still only one coloured design, without experimental support; in the same way, Descartes thought that it was also a swirl of “subtle matter” which involved the rotation of planets (this was put at fault by Isaac Newton with the gravitation in 1687).

However, this concept of corpuscles inspired by other scientists. The Swiss mathematicians Jakob Hermann (1716) and Leonhard Euler (1729), but especially the Swiss physicist Daniel Bernoulli (1733), carry out calculations by supposing that the gases are made of particles entrechoquant itself, and their results are in agreement with the experiment. It is the “kinetic” design of gases, i.e. the explanation of the temperature and the pressure by particles moving.

Another science develops at the end of the 18th century: the Crystallography. What intrigues the scientists, it is the observation of the geometrical forms of the natural crystals, and their capacity to be cleaved according to smooth plans respecting these symmetries. Taking up the idea of classification of the living beings of Carl von Linné, one starts to seek and classify the minerals (Jean-Baptiste Rome of Isle, French mineralogist, 1772). The abbot Rene-Just Haüy (French crystallographer), in 1781, supposes that the shape of the crystals reflects the symmetry of a “building bloc”, the crystal being an assembly of these bricks. One finds here this concept of elementary component of the matter.

XIXe century - triumph of the atom

At this stage, three concepts arose:

• the chemical bodies are decomposable in elementary substances;
• the gases are composed of corpuscles which fly and are entrechoquent;
• the crystals are composed of cells whose form determines the form external of the crystal.

John Dalton (chemist and British physicist), in 1804, measurement masses of the reagents and the products of reaction, and in deduced that the substances are made up of spherical atoms, identical for an element, but different from one element to another, in particular by the mass of these atoms. He also discovers the concept of partial Pression (in a mixture of gas, the contribution of a gas given to the total pressure). He was the first to put forward the ideas of the atomic Théorie.

In 1807, Louis Joseph Gay-Lussac (French physicist and chemist), establishes the law connecting the temperature and the pressure of a gas. In 1808, it establishes that the gases react in given proportions; the reports/ratios of volumes of the reagents and the products of reaction are small integers. The fact that they are integers, strongly induced to think that the matter is not “continuous” (dominant thought at that time), but made discontinuous elements.

Amedeo Avogadro (Italian physicist), in 1811, states, without proof, that for a fixed temperature and a pressure, a given volume of gas always the same number of molecules contains, and this whatever the gas. It also makes the assumption that the gases are polyatomic, and defines molecules and atoms clearly. Andre-Marie Amp (1814), Jean-Baptiste Dumas (1827) and William Prout (1834) arrives at the same conclusion.

In 1821, John Herapath (British mathematician) publishes a kinetic theory of gases to explain the propagation of the sounds, the phase shifts (Vaporisation, Liquéfaction) and the diffusion of gases. Robert Brown (British botanist), in 1827, observes the movement of grains of pollen in water; the grains go in straight line, and change direction only at the time of a shock with another grain or against a wall. It is of this behavior, the “Brownian Movement”, that the physicists will be inspired to describe the movement of the gas molecules.

Gabriel Delafosse, in 1840, supposes that one can dissociate the elementary component of the crystal and his organization; thus, the building bloc of Haüy could be a network with the nodes of which “molecules would be”; it would be the form of the network which would give the form to the crystal and not necessarily the shape of the molecules. Louis Pasteur (French chemist and biologist), in 1847, establishes the bond between the shape of the molecules and the shape of the crystals (in fact, the molecule gives its form to the network, and the network its form with the crystal). Auguste Bravais (French physicist), in 1849, determines the 32 possible crystal lattices. In 1858, Rudolf Clausius (German physicist) defines the mean free path of a molecule in a gas (average distance traversed between two shocks). On the basis of there, in 1859, James Clerk Maxwell (Scottish physicist) introduced the concept of dispersion statistical speeds of the molecules into the kinetics of gases. This allowed Ludwig Boltzmann (Austrian physicist), in 1858, to estimate the size of the molecules and to define the statistical distribution speeds in a gas.

Dimitri Ivanovitch Mendeleïev (Russian chemist), in 1869, class the atoms by increasing mass, and notices that there is a periodicity in their chemical properties. It thus establishes a table classifying the elements; the holes in this table made it possible to discover new elements.

Assessment

The concept of atom and molecule thus allowed the success of the statistical Thermodynamique, the Chimie and the Cristallographie. To this concept, will correspond of the models which will be refined during the development of physics and particularly specified by the discoveries of the quantum physics during the 20th century, and in particular:

• the discovery of the electron (Joseph John Thomson, 1887);
• experiments of deviation of the particles alpha by the matter (Ernest Rutherford off Nelson, 1911);
• experiments of diffraction of x-rays on the crystals (max von Laue, 1912).

History of the models of the atom

Since Greek antiquity, one supposed that the matter could split of small pieces until obtaining indivisible grains, that it was like “dust in the light”. It is with the experiment of Rutherford that one reaches finally this grain: the particles alpha, while crossing the matter, see their disturbed trajectory, which will finally make it possible to know how this “dust is organized”…

• 1675 : Jean Picard observes a green luminescence by agitating a tube of barometer; one will discover a few centuries later that is due to the static electricity and the mercury vapors;
• 1854 : Geissler and Plücker discovers the Cathode rays, of the luminescent green rays when one establishes a strong electric tension in a bulb which one pumped the air (gas low pressure); they thus invent the Gas-discharge lamp, which now clarifies our supermarkets of a white light, our streets and our parkings of an orange light (lamps with sodium);
• 1897 : J.J. Thomson establishes that these cathode rays consist of charged particles negatively torn off with the matter, and thus discovers the electron; it is the first decomposition of the atom;
• 1900 : max Planck watch the quantification of the energy exchanges in the matter (research on the black Body);
• 1911 : Experiment of Rutherford: it bombards a sheet of Or by particles alpha (of the helium cores, positively charged, obtained by radioactivity); it from of deduced that:
  • la majority of the particles goes in straight lines, therefore the matter is “full with holes”;
  • mais some are deviated and even turn back, therefore they meet positively charged matter small islands very concentrated (them + between-they are pushed back).
  It from of deduced the planetary atomic model : the atom consists of a very small positive core and electrons revolving around; this model poses a large problem: while turning, the electrons should lose energy by radiation, and thus be crushed on the core… (e.g.: Capture K)
• 1913 : Niels Bohr joins together the concepts of Planck and Rutherford, and proposes a quantum atomic model : the orbits of the electrons have defined rays, it exists only some “authorized” orbits; thus, the quantified energy exchanges correspond to jumps between the defined orbits, and when the electron is on the lowest orbit, it cannot go down in lower part and be crushed (but this model does not explain why);
• 1914 : the Experiment of Franck and Hertz validates the model of Bohr: they bombard mercury vapor with electrons; the kinetic energy lost by the electrons crossing the vapors is always the same one;
• 1924 : Louis de Broglie postulates the duality wave-corpuscle;
• 1926 : Schrödinger models the electron like a wave, the electron in the atom is not thus more one ball but a “cloud” which surrounds the core; this model, contrary to the others, is stable because the electron does not lose a énergie.

Obsolete models

The models presented in this section are too far away from reality to be able to be used. They are presented here only on a purely historical basis.

The model of J.J. Thomson or model of the far to the prunes (plum pudding)

With the discovery of the electron in 1897, one knew that the matter was made up of two parts: negative, electrons, and positive, the core. In the model imagined then by Joseph John Thomson, the electrons, located particles, bathed in a positive “soup”, with the image of the Pruneau X in the Breton Far (or in the plum- Pudding for the British or like grapes in a cake). This model was invalidated in 1911 by the experiment of one of its former students, Ernest Rutherford.

You can also consult.

The planetary model of Rutherford

The experiment of Rutherford highlights that the positive loads “are not spread out” between the electrons, but are concentrated in small points. It bombarded a fine gold sheet by a beam of particles alpha (particles of positive electric charges). It observed that the particles were deviated slightly, which did not correspond to the result envisaged by the model of Thomson, for which, they should not have crossed it.

Rutherford thus imagines a planetary model: the atom consists of a positive core around of which turn of the negative electrons. Between the core - very small compared to the atom (approximately 100.000 times) - and its electrons, a very large Vide exists.

This model was very quickly put at fault by the Maxwell's equations on the one hand, which predict that any accelerated load rayon of energy, and by the experiments showing the quantification of the energy levels on the other hand.

Approached models usually employed

The model of the hard spheres

The simplest model to represent an atom is an indeformable ball. This model is very much used in Cristallographie. A molecule can be seen like several joined balls, a crystal like piled up balls. One uses sometimes a “burst” representation: the atoms are represented like small spaced balls, connected by features, making it possible to emphasize the privileged directions, the angles and to visualize the number of the connections.

This model corresponds well to certain properties of the matter, like, for example, the difficulty in compressing the liquids and the solids, or the fact that the crystals have quite smooth faces. On the other hand, it does not make it possible to explain other properties, as the shape of the molecules: if the atoms do not have privileged direction, how to explain why the chemical bonds reveal well defined angles?

The model of Bohr

See also: Model of Bohr

A model was developed by Niels Bohr in 1913 starting from the properties highlighted by Planck and Rutherford. In the model of the hard spheres, the atom is a whole, indecomposable object. However, one knows since the middle of the XIXe century that one can of “tear off” particles carrying a negative electric charge, the electrons. In the model of Bohr, the atom is composed of a core charged positively, and electrons turning around, the rays of the orbits of the electrons being able to take only quite precise values.

The core is very compact, of a diameter of approximately 10^-15 with 10^-14 m, i.e. the core is a hundred and thousand to a million times smaller than the atom; it carries a positive electric charge. It is also the heaviest part of the atom, since the core accounts for at least 99,95% of the mass of the atom. The electrons are specific, i.e. their ray is allowed quasi null (all at least smaller than than one can estimate). They carry a negative charge. For reasons of legibility, the diagram below is thus not on the scale, with regard to dimensions of the core and the electrons, nor also for the rays of the various orbits (it will be noted here that the number of electrons on the orbits is not predicted by the model).

This vision makes it possible to describe the spectroscopic phenomena fundamental, i.e. the fact that the atoms absorb or emit only certain wavelengths (or color) of light or x-rays. Indeed, the electrons being able to turn only on defined orbits, the jump of an orbit with another is done while absorbing or by emitting a given quantity of energy ( quantum ).

The model of Bohr, breaking up the atom into two parts, a core and a cloud of electrons, is more precise than the model of the hard spheres, for which the surface of the sphere corresponds to the orbit of the external electrons.

However, it presents the great disadvantage of the planetary models: electrons orbits about it around the core are accelerated loads, they should radiate energy,… and should thus come to be crushed on the core. The model does not explain either the shape of the molecules.

The current model: model of Schrödinger

See also: Theory of Schrödinger of the hydrogen atom

Birth of the wave mechanics of Louis de Broglie 1924, generalized by Erwin Schrödinger in 1926 pleasing to propose a new model, whose relativistic aspects were described by Paul Dirac in 1928; it makes it possible to explain the stability of the atom and the description of the spectroscopic terms.

In this model, the electrons are not any more of the balls located in orbit, but of the clouds of probability of presence. This point of view, revolutionist, can shock in first approach. However the representation which one could have of an electron - a small ball? - was dictated by the forms observed in the macroscopic world, transposed without evidence in the microscopic world . It is necessary well to be penetrated owing to the fact that what one knows of the electron rests only on indirect demonstrations: electric current, cathode tube (television)…

Since the Years 1930, one thus models the electron by a “function of wave” whose square of the “standard represents the density of probability of presence”. To represent the properties of the electron accurately, one has only the complicated mathematical functions. This abstraction still rejects many physicists. We will try to give a image this concept of function of wave, necessarily imperfect image.

Imaginons that out of the atom, the electron is a small ball. When the electron is captured by the atom, it “dissolves” and becomes a diffuse cloud, it “evaporates”. When it is torn off atom, it becomes again a small ball, it “recondense”. There exist other examples of object which change form, for example, out of water, salt is in the form of crystals; put in water, it dissolves, and if one evaporates water, one finds crystals. Salt changes form (crystal compact or dissolved in water), but one has salt all the time.

In a way a little more exact: an electron, out of an atom, is represented by a Paquet of waves, which can be considered, in some limiting, like a small ball. The quantum Mécanique shows that such a package of waves is spread out during time; on the contrary, an electron of an atom preserves the structure of the function of wave associated with the orbit which it occupies (as long as it is not ejected an atom). Quantum mechanics thus postulates, not the conservation of the shape (nonknown) of the electron, but the integral of the probability of presence.

In the model of Schrödinger, the clouds corresponding to the various electrons interpenetrate; it is not question of giving an individual representation of the electrons each one about its orbit, as that was in the case of the model of Bohr. That is all the more true as the electrons are identical particles indistinguishable . The Effets of exchange bring to consider that each electron of the atom is at the same time on each orbital occupied (correspondent with a given electronic configuration). The ionization of the atom (the wrenching of an electron of the atom) can then be represented by the diagram simplified below.

To avoid useless complications, one will consider the simplest atom in order to show some diagrams revealing the fundamental points of the model:

• the electronic cloud associated at the fundamental state, revealing (like other states) the possibility for the electron of being within the core, which has consequences in Nuclear physics: orbital electron capture.

• the electronic cloud associated with a linear combination of two orbital associated with the first excited level. This example shows the possibility of obtaining electronic clouds pointing towards the outside of the atom… we are thus prepared with the molecular connections.

That is to say ρ ( R , θ, φ) density of probability of presence at the point of Coordinated spherical ( R , θ, φ). For the fundamental state, the density of probability, ρ, is maximum in the center of the atom. Now let us consider the radial density of probability of presence (at the distance R of the core, all the confused directions):

$P\; (R)\; =\; 4\; \backslash \; pi\; r^2\; \backslash \; cdot\; \backslash \; rho\; (R,\; 0,0)$,

ρ (0,0,0) > ρ ( R ₁, 0,0), but P (0) < P ( R ₁).

According to the quantum state of the electron (fundamental, excited…) these clouds can take various forms, which are described in particular by the spherical harmonic . The simplest form is spherical symmetry, shown in particular, above, in the case of the fundamental state, |1s>.

Linear combinations of functions of wave, using distinct spherical harmonics, allow the appearance of an anisotropy which will become essential for the passage of the concept of atom with that of Molécule. The diagram opposite watch a cut of the density of probability of presence of the orbital hybrid |$2sp\_\; \{Z\}$ > of the hydrogen atom, cuts container OZ axis of symmetry of orbital atomic. For this example, the axis OZ becomes a privileged direction, but moreover the density of probability of presence is spread out further for a given orientation.

This model makes it possible to explain:

• the stability of the atom, the loads are accelerated, but they are forced by quantum mechanics (relations of uncertainty);
• the shape of the molecules: preferential orientation of the electronic clouds;
• the organization of the crystals: the electronic cloud behaves like a hard shell;
• spectroscopic effects (the quantification of the energy exchanges): the cloud can take only determined forms, in particular with regard to the distance R ₁ of the maximum of density to the core.

One will note in the case of to finish that relativistic corrections are to be brought, the atoms of high atomic number, for the determination of the internal levels (speeds of the electrons on the orbits of the model of Bohr are then important).

The atomic nucleus

See also: Atomic nucleus

If quantum mechanics made it possible to quickly explain the spectroscopic characteristics of the atoms and the molecules, the heart of the atom, its core, was more difficult to include/understand. The difficulties are here of two orders: one corresponding to the importance of the energy of the particles probes allowing to reach dimensions about the Fermi, the other with the necessary invention of at least an additional interaction allowing the stability of a core made up of protons (which are pushed back electrically) and of neutrons.

This comprehension of the cohesion of the core was to also explain the phenomena of radioactivity alpha, beta and gamma, whose first observations dated from the last decade of the XIXe century.

The decade which preceded the Second world war led to discovered of the two main interactions of the stability of the heart: the strong Interaction and the weak Interaction. The smallness of the range of these two interactions, respectively 10^-15 m and 10^-18 m explains the encountered experimental difficulties. The theoretical difficulties do not miss, either; of physical laws as it is not a question simple as those of the electromagnetism, even complicated by the quantum mechanical , but of the comprehension of all the elementary particles… The invention of the Quark S and the Gluon S thus gives the current vision of the interaction which maintains the nucleons together.

This nuclear physics also leads to the explanation of the nucleosynthesis, explaining the nuclear aspects table of Mendeleïev. One finds oneself there in the expansion of the birth of the universe and the dynamics of stars.

Notation

An atom is usually indicated by its chemical symbol, supplemented by its mass number (equal to the number of Nucléon S of the atom) placed in top and on the left of the symbol.

Example: the Carbone 12 of mass number 12 is noted $\{\}\; ^\; \{12\}\; \backslash \; mathrm\; C\; \backslash ,$.

It is of use to supplement this writing by the Atomic number Z, placed in bottom and on the left of the symbol, to describe a nuclear reaction in which an isotope intervenes.

Carbon 12 is thus noted $\{\}\; ^\; \{12\}\; \_\; \{\backslash \; 6\}\; \backslash \; mathrm\; C\; \backslash ,$.

Thus, carbon-14 $\{\}\; ^\; \{14\}\; \_\; \{\backslash \; 6\}\; \backslash \; mathrm\; C\; \backslash ,$ and carbon 12 $\{\}\; ^\; \{12\}\; \_\; \{\backslash \; 6\}\; \backslash \; mathrm\; C\; \backslash ,$ is two Isotopes.