Isotope

Notation

An isotope 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 and
by the Atomic number Z, placed in bottom and on the left of the symbol.

Carbon-14, ${} _ {\ 6} ^ {14} \ operatorname {C}$ and carbon 12 ${} _ {\ 6} ^ {12} \ operatorname {C}$ are isotopes.

Properties of the isotopes

The chemical properties of the isotopes of the same element are identical because these isotopes have the same number of electrons.

On the other hand, as the core does not comprise the same number of neutrons, the mass of the atoms varies. This difference in atomic mass makes it possible to separate the isotopes from the same element by Spectrometry mass or centrifugation and thus to distinguish them. Moreover, the proportion of neutrons in the core can make the atom unstable: this is why they can be radioactive.

Stability of the isotopes

There exist eighty twelve different chemical elements in nature, of the Hydrogène ₁H with the Plutonium ₉₄Pu; the Technetium ₄₃Tc and the Promethium ₆₁Pm not having neither stable isotope, nor radioactive isotope of sufficiently long period.

The core of an atom consists of protons which are pushed back under the action of the electromagnetic interaction (the charges electric of comparable nature are pushed back) but which attracts each other under the action of the strong interaction; it contains also neutrons which attract each other under the action of the strong interaction but are not (very very little) concerned steps the electromagnetic interaction. In a core, stability is thus ensured by the strong interaction, and the neutrons which, moving away the protons from/to each other by their only presence, decrease the electromagnetic interaction, from where the following properties:

For these hundreds of natural isotopes, the respective numbers of protons and neutrons seem to comply with certain rules:

the number of neutrons is about equal to that of the protons for the light elements; starting from the ₂₄Cr, the number of neutrons becomes higher than the number of protons, the surplus exceeding 50% for the heaviest elements;
certain particularly stable cores contain protons or neutrons (or both) equal to one of the numbers (known as magic) of the series:

2,8,20,50,82,126 ;

Why these values: They correspond to cores having of the complete layers of neutrons or protons. The rules of full layers are rather simple and rather similar to those of the layers of electrons To find these values one considers the successive layers of the center towards the periphery of the core: s1/2 p3/2 1/2 d5/2 3/2 f7/2 5/2 g9/2 7/2 H 9/11/2.

Rules:

The level S contains 1+ 1/2.2 protons and or neutrons is 2 and constitutes the first layer. The level p contains 1+ of them 3/2.2+1+1/2.2 either 6 to add to the layer S or 6+2= 8.

The third layer contains the levels S and D one finds 10 easily to add to 8 is 20.

The fourth layer contains the levels p and F is 20 what should make 20+20= 40 but the potential creates by the center of the core makes that the first element of the level G is close to the 4th layer and it is necessary to add to this g9/2 layer is 10 nucleons thus the 4th layer is for 50 nucleons.

The fifth layer comprises the levels S D G (amputee of G 9/2) and added h11/2 is 76 addition to the 50 preceding fact a total of 126. One finds there in particular the lead of which the most stable isotope contains 82 protons and 126 neutons. It is doubly magic. the following value is 184.

They are spherical cores. Between these values are cores deformed in cigar or saucer, less stable. The core with 126 protons could not be still created; it will undoubtedly be unstable but one approaches this value by combining 2 atoms.

the elements of odd number Z have less stable isotopes than the elements of even number Z.

Use of the isotopes

A very known example of couple of isotopes is consisted the Carbone: carbon is present in great majority under its isotope of atomic weight 12 (“carbon 12”); in addition, one can find in small quantity the isotope of atomic weight 14 (the Carbon-14), which is chemically strictly equivalent to the carbon 12, but which is radioactive. Indeed, the additional neutrons of the core make the atom unstable. It disintegrates by giving Azote and by emitting a radiation beta.

The proportion of the stable isotope compared to the unstable isotope thus varies during time. It is on this variation that the methods of radioactive Datation are based by couple of isotopes (of which most known is the method of Carbon dating). It is certainly the most important application of the concept of isotope. The isotopic tracer are another application of this concept.

A major application is the separation of isotopes 235 and 238 of uranium, this separation is obtained by gas diffusion or centrifugation of UF₆. The mode by centrifugation is with the hands of some Canadian, Russian, European industrialists… Centrifugation is obtained by a cascade of centrifugal machines which raise gradually the rate of U 235 in the mixture U 235-U 238 which can be used for an enrichment of 5% with applications civil of fission or 90% for military applications. The Centrifugeuse S are narrow cylinders turning at high speed. It is known that the centrifugal force is equal to M.w ² .r where M is the unit mass, W the angular velocity of rotation, R the ray of the cylinder. To avoid a mechanical rupture one chooses R small and in order to have an appreciable force one chooses W very high (the force is proportional to the square of rotation). With a few thousands of centrifugal machines one obtains the required result.

Anecdotes

In the animated series Simpson , the team of baseball of Springfield is called the Isotopes whose sponsor is not other than the Nuclear plant of Springfield and the beer Duff.

See too

radioactive Half-life
Isobar Radioactivity
Table of the isotopes
isotopic Chronology

Be-X-old: Ізатоп Simple: Isotope Zh-min-nan: Tông-ūi-sò͘ Zh-yue: 同位素

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