Which atomic nucleus is the most stable
Protons and neutrons form the building blocks of atomic nuclei and thus all chemical elements known to us. There are around ninety naturally occurring elements on earth - from hydrogen to uranium. In the laboratory, however, researchers also artificially create atomic nuclei, thereby constantly expanding the periodic table.
Scientific research into the microcosm began about two hundred years ago with the observation that the chemical elements only combined to form molecules in certain integer ratios. Proof that atoms are the building blocks of matter. At the beginning of the 20th century it was recognized that atoms in turn consist of a positively charged nucleus and negatively charged electrons. And another thirty years passed before protons and neutrons were discovered as the building blocks of the nucleus.
Distribution of protons and neutrons in the atomic nucleus
These so-called nucleons are also not elementary, but are made up of quarks and gluons, bound together by the strong force. In a weakened form, this force also acts in the vicinity of the nucleons and thus connects them to form atomic nuclei. The interaction between the nucleons is very complex, with attractive and repulsive components.
The properties of the atomic nucleus are determined by the strong force in interaction with two further fundamental forces: the electromagnetic and the weak force. These forces determine how many protons and neutrons can be combined in a nucleus and the maximum number of nucleons it can contain. If there are too many protons, the repulsive effect of their positive electrical charge becomes too great and the nuclei disintegrate. Here, the excess proton is converted into a neutron by the weak force, whereby a positron and a neutrino are created and emitted. If the number of neutrons is too high, the excess neutron breaks down into a proton, an electron and an antineutrino. These so-called beta decays always take place when the resulting daughter core is better bound than its parent core. Heavy nuclei with excess protons decay preferentially by emission of a helium nucleus (alpha decay) or by fission.
The map of the nuclides
The forces between the nucleons determine the detailed structure of the atomic nuclei. These include properties such as the number of protons that the respective chemical element determines, the mass of the nuclei, their stability against radioactive decay or their lifespan. These forces also determine how the atomic nuclei were formed shortly after the Big Bang or later in the interior of the stars. The structure of the nuclei is an important key to understanding matter, especially the naturally occurring chemical elements that make up our environment, including all organisms, including ourselves.
If you arrange all atomic nuclei in such a way that the number of neutrons is plotted horizontally and the number of protons is plotted vertically, you get the nuclide map. There are 85 chemical elements on earth with their isotopes - atomic nuclei that belong to the same element but differ in the number of their neutrons. In total, there are about three hundred different stable atomic nuclei. These are so well bound that neither the weak interaction nor the splitting or the alpha decay can spontaneously convert them into even better bound nuclides.
In addition, physicists have synthesized more than 2500 other unstable nuclei in laboratory tests and at particle accelerators. The lifespan of the artificially generated unstable nuclides with an excess of protons or neutrons is short - sometimes only a fraction of a second - but sufficient to investigate their structure. Although unstable nuclides do not occur on earth, they are of crucial importance in stellar processes as an intermediate stage for the generation of many stable atomic nuclei that we find in the solar system and in the universe.
Based on theoretical considerations, it is expected, especially in the area of very neutron-rich nuclides, that there are many more previously unknown atomic nuclei that only decay via the weak interaction. A foray into this Terra Incognita promises new insights both for our understanding of the nuclear forces and for research into the formation processes of the elements in stars. Ultimately, even the evolution of stars is influenced by the properties of the atomic nuclei.
The limits of stability
One of the important experimental and theoretical goals in nuclear physics is to find out the limits of stability. Due to the short range of the strong force, the size of the atomic nuclei is limited. If more and more nucleons are attached to a nucleus, their binding energy decreases more and more until it finally disappears completely. These boundary lines in the isotope diagram, within which nuclei can exist - even if only for a short time - are called protons or neutron break-off edges. For cores near the break-off edge, new and previously unknown properties have been observed and others are predicted. Such cores enable, for example, a critical test of today's best core models by measuring their bare existence, their masses or their decay.
For very large numbers of protons and neutrons, a further limit of stability is reached. The electrical repulsion becomes so great that the nucleus becomes increasingly cigar-shaped and finally splits into two fragments of roughly the same size, which fly apart. At this limit of stability, too, one can find extraordinary phenomena, such as the super-heavy atoms. The size of stable atomic nuclei can be understood quite well in the context of a simple “droplet model”, in which certain properties of an atomic nucleus are described similar to those of a water droplet. Here the radius of the nuclei should increase with the third root of the number of nucleons.
Atomic nucleus of the isotope lithium-11
However, this picture reaches its limits for nuclei with a large excess of neutrons or protons. For light, very neutron-rich nuclei, it has been observed that one or two neutrons are preferentially located in the outermost edge area of the nucleus. Because these neutrons are even outside the range of attraction of the nuclear force, they may be bound to the nucleus by subtle correlations between the neutrons.
In this case, physicists speak of a halo nucleus. Because of this effect, lithium-11 (with three protons and eight neutrons), for example, inflates to the size of the much heavier core of calcium-48 (with 20 protons and 28 neutrons). For heavier neutron-rich nuclei, such as tin-132, the formation of thick neutron membranes is even predicted, with several neutrons at a great distance from the center of the nucleus.
Modern research focuses on questions about the spatial distribution of protons and neutrons in halo nuclei and about the interaction of the outer neutrons with one another and with the nucleons of the inner core. The aim is to develop core structure models in the interplay between experiment and theory, which can then also contribute to the understanding of the formation of elements in stars or in supernovae.
Even comparatively common atomic nuclei can, under certain circumstances, reveal exotic behavior and have molecular-like structures. An important example of this is the atomic nucleus carbon-12, which is an important component of earthly life. Its rate of production in the hot interior of stars is significantly influenced by the fact that it consists of three loosely connected helium nuclei in this environment.
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