contains in its center a core of a single
particle with positive electric charge, the proton, around which revolves
at high speed, a particle 1840 times smaller, with negative electric
charge equal to the proton: the electron. This is the simplest case.
The oxygen atom is already much more complex: its core is composed of
8 positively charged protons and 8 neutrons, electrically neutral. Around
this core, 2 electrons revolve in an orbit close, and 6 electrons in
an orbit later. As these 8 electrons are negatively charged oxygen atom
is electrically neutral. The connection between the two atoms of hydrogen
and oxygen atom is a kind of sharing of some electrons, and this state
is called the "covalent", we know relatively stable.
Of course, some atoms can be much more
complex, such as uranium, for example, which contains 92 protons, grouped
in the nucleus and 92 electrons distributed in 7 different orbits.
The orbits, in addition to their number,
may have certain other characteristics, and their classification is currently
taking 4 sets of elements:
1 The rank of the orbit from the nucleus:
a) place called "K-shell", b) rank called "L shell, etc..
2 The ellipticity of the orbit.
3 The direction it takes in relation to
a magnetic field determined.
4 The direction of rotation of electrons
themselves (or spin).
These 4 elements are the "quantum
numbers", and the number of
combinations is large enough to represent the various atoms.
Finally the stability of atoms and molecules
is ensured by cohesive forces of various kinds, usually intense in the
case of nuclei (intimate association of protons and neutrons), less strong
with regard to the maintenance of electrons on their orbits , lower still,
as regards the binding of atoms in molecules and the molecules together.
It is DE BROGLIE (1923) that we owe the
notion that particles constituting atoms (eg electrons) are, like photons,
associated with waves, the nature of which has been much discussed: physical
reality ? - Simple mathematical concept? - An expression of probability
of position? Currently, the "probabilistic" seem to prevail.
In any case, this theory of law: material particle / wave is the basis
of the "mechanical wave" that has been successful in physical
chemistry.
However, these miniature worlds that are, without exception, all inert ingredients are subjected frequently to radiation,
(home, artificial or natural - that
is to say, in the latter case, very often from the sun). They are
therefore somehow "bombed" by a rain of photons more or
less energy. In the special case of exposure to atomic radiation,
in addition, the photons of high energy particles (protons, electrons,
neutrons especially). Anyway, impacts occur with moving targets that
are the atomic or molecular electrons, constantly moving on their
orbits and with normal nuclei. The resulting effects of the change.
Next photon energy, we can see:
1 a simple acceleration of the thermal
motion of molecules, constant stirring in all bodies to a temperature
a little far from absolute zero. This effect is often recorded, since
a single red photon absorbed by a molecule can communicate agitation
equal to 50 times that it normally has a temperature of 15/20 ° C;
2 displacement of an electron in an
orbit farther than it normally occupies. In general, a photon of
visible light has nominally sufficient energy to achieve this change
in the electronic state of atoms or molecules, thus taking a state
called "excited". But we need the photon energy corresponds
to that required to move the electron in its new orbit. Energy is
released when the electron returns in one or more times its original
orbit;
3 tear of an electron to the attraction
of the atom's nucleus, or nuclei of the molecule. To do this, it
is necessary, and following the case of photons (or particles) with
an energy higher 3 ... 20 eV. Only then, some ultraviolet rays can
sometimes cause this significant change is called "ionization." This
effect may, however, also be obtained through other channels. The
ionized molecule having lost one of its electrons (negative), is
electrically unbalanced;
4 breakdown of the nucleus, and therefore
the complete dislocation of the atom. This effect is achieved, in
general, particles with high energy produced, for example, by natural
or artificial radioactivity.
Many other effects can be observed, but the case 2 (displacement of an electron from one orbit to another, by a moderate photon energy) is frequently encountered in photochemical processes (Fig. 2). Indeed, when the electron moved back to his original orbit, it releases the energy it has stored. If it is a complex molecule, it can be split into smaller parts. It may be simplistic (to take one of her neighbors one or more oxygen atoms). Sometimes, some body exposed to light become electrically conductive and the result is different
effects, such as a catalytic photochemical. There are also phenomena of light emission of long duration (called phosphorescence) - or short (fluorescence), etc..
))
Fig 2 - Effect photochemical
(simple case). - Around a nucleus n, e an electron revolves
in its orbit (K). Under the action of an external photon phi,
the electron is placed in an orbit farthest (L). Returning
to the orbit (K), it releases energy, origin of the photochemical
effect. The photon, meanwhile, has disappeared.
More simply, in many cases, we
found no effect of preceding genera, the only result is a significant
heating of the body exposed to light.
In practice photochemical had long been known that certain dyes "passed" to light. This transformation is industrially important and has been well studied. Wool, for example, in some yellow rays (UV), and blanch in other (blue). In chemistry, it was found that hydrogen and chlorine are combined explosively in a transparent ball, when he was exposed to sunlight. In diffuse light, the reaction was much slower. In early experiments, they were regarded as mere laboratory curiosities, but the rapid development of photography and its many applications has led in this direction, studies increasingly advanced. "Laws photochemical" have been developed and very interesting discoveries have been made: that substances such photosensitizers. A body, whose chemical properties change very little, if at all, when it is exposed to some radiation, can see the behavior changed completely, when attached to the system a new substance, called photosensitizing, which captures the 'said radiation energy and transfers it to the body previously unaffected. This new concept has received numerous applications, including photography, for the development of photosensitive substances present.