The nucleus was discovered in 1911, as a result of Ernest Rutherford's
efforts to test Thomson's "plum pudding model" of the atom.[10] The
electron had already been discovered by J. J. Thomson. Knowing that atoms
are electrically neutral, J. J. Thomson postulated that there must be a
positive charge as well. In his plum pudding model, Thomson suggested that
an atom consisted of negative electrons randomly scattered within a sphere
of positive charge. Ernest Rutherford later devised an experiment with his
research partner Hans Geiger and with help of Ernest Marsden, that involved
the deflection of alpha particles (helium nuclei) directed at a thin sheet
of metal foil. He reasoned that if J. J. Thomson's model were correct, the
positively charged alpha particles would easily pass through the foil with
very little deviation in their paths, as the foil should act as
electrically neutral if the negative and positive charges are so intimately
mixed as to make it appear neutral. To his surprise, many of the particles
were deflected at very large angles. Because the mass of an alpha particle
is about 8000 times that of an electron, it became apparent that a very
strong force must be present if it could deflect the massive and fast
moving alpha particles. He realized that the plum pudding model could not
be accurate and that the deflections of the alpha particles could only be
explained if the positive and negative charges were separated from each
other and that the mass of the atom was a concentrated point of positive
charge. This justified the idea of a nuclear atom with a dense center of
positive charge and mass.
The term *nucleus* is from the Latin word nucleus, a diminutive of nux
('nut'), meaning 'the kernel' (i.e., the 'small nut') inside a watery type
of fruit (like a peach). In 1844, Michael Faraday used the term to refer to
the "central point of an atom". The modern atomic meaning was proposed by
Ernest Rutherford in 1912.[11] The adoption of the term "nucleus" to atomic
theory, however, was not immediate. In 1916, for example, Gilbert N. Lewis
stated, in his famous article *The Atom and the Molecule*, that "the atom
is composed of the *kernel* and an outer atom or *shell.*"[12]Similarly,
the term *kern* meaning kernel is used for nucleus in German and Dutch.
The nucleus of an atom consists of neutrons and protons, which in turn are
the manifestation of more elementary particles, called quarks, that are
held in association by the nuclear strong force in certain stable
combinations of hadrons, called baryons. The nuclear strong force extends
far enough from each baryon so as to bind the neutrons and protons together
against the repulsive electrical force between the positively charged
protons. The nuclear strong force has a very short range, and essentially
drops to zero just beyond the edge of the nucleus. The collective action of
the positively charged nucleus is to hold the electrically negative charged
electrons in their orbits about the nucleus. The collection of negatively
charged electrons orbiting the nucleus display an affinity for certain
configurations and numbers of electrons that make their orbits stable.
Which chemical element an atom represents is determined by the number of
protons in the nucleus; the neutral atom will have an equal number of
electrons orbiting that nucleus. Individual chemical elements can create
more stable electron configurations by combining to share their electrons.
It is that sharing of electrons to create stable electronic orbits about
the nuclei that appears to us as the chemistry of our macro world.
Protons define the entire charge of a nucleus, and hence its chemical
identity. Neutrons are electrically neutral, but contribute to the mass of
a nucleus to nearly the same extent as the protons. Neutrons can explain
the phenomenon of isotopes (same atomic number with different atomic mass).
The main role of neutrons is to reduce electrostatic repulsion inside the
nucleus.
Protons and neutrons are fermions, with different values of the strong
isospin quantum number, so two protons and two neutrons can share the same
space wave function since they are not identical quantum entities. They are
sometimes viewed as two different quantum states of the same particle, the
*nucleon*.[13][14] Two fermions, such as two protons, or two neutrons, or a
proton + neutron (the deuteron) can exhibit bosonic behavior when they
become loosely bound in pairs, which have integer spin.
In the rare case of a hypernucleus, a third baryon called a hyperon,
containing one or more strange quarks and/or other unusual quark(s), can
also share the wave function. However, this type of nucleus is extremely
unstable and not found on Earth except in high-energy physics experiments.
The shape of the atomic nucleus can be spherical, rugby ball-shaped
(prolate deformation), discus-shaped (oblate deformation), triaxial (a
combination of oblate and prolate deformation) or pear-shaped.[16][17]
Nuclei are bound together by the residual strong force (nuclear force). The
residual strong force is a minor residuum of the strong interaction which
binds quarks together to form protons and neutrons. This force is much
weaker *between* neutrons and protons because it is mostly neutralized
within them, in the same way that electromagnetic forces *between* neutral
atoms (such as van der Waals forces that act between two inert gas atoms)
are much weaker than the electromagnetic forces that hold the parts of the
atoms together internally (for example, the forces that hold the electrons
in an inert gas atom bound to its nucleus).
The nuclear force is highly attractive at the distance of typical nucleon
separation, and this overwhelms the repulsion between protons due to the
electromagnetic force, thus allowing nuclei to exist. However, the residual
strong force has a limited range because it decays quickly with distance
(see Yukawa potential); thus only nuclei smaller than a certain size can be
completely stable. The largest known completely stable nucleus (i.e. stable
to alpha, beta, and gamma decay) is lead-208 which contains a total of 208
nucleons (126 neutrons and 82 protons). Nuclei larger than this maximum are
unstable and tend to be increasingly short-lived with larger numbers of
nucleons. However, bismuth-209 is also stable to beta decay and has the
longest half-life to alpha decay of any known isotope, estimated at a
billion times longer than the age of the universe.
The residual strong force is effective over a very short range (usually
only a few femtometres (fm); roughly one or two nucleon diameters) and
causes an attraction between any pair of nucleons. For example, between a
proton and a neutron to form a deuteron [NP], and also between protons and
protons, and neutrons and neutrons.
Halos in effect represent an excited state with nucleons in an outer
quantum shell which has unfilled energy levels "below" it (both in terms of
radius and energy). The halo may be made of either neutrons [NN, NNN] or
protons [PP, PPP]. Nuclei which have a single neutron halo include 11Be and
19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C.
Two-neutron halo nuclei break into three fragments, never two, and are
called *Borromean nuclei* because of this behavior (referring to a system
of three interlocked rings in which breaking any ring frees both of the
others). 8He and 14Be both exhibit a four-neutron halo. Nuclei which have a
proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and
27S. Proton halos are expected to be more rare and unstable than the
neutron examples, because of the repulsive electromagnetic forces of the
halo proton(s).
Although the standard model of physics is widely believed to completely
describe the composition and behavior of the nucleus, generating
predictions from theory is much more difficult than for most other areas of
particle physics. This is due to two reasons:
The nuclear radius (*R*) is considered to be one of the basic quantities
that any model must predict. For stable nuclei (not halo nuclei or other
unstable distorted nuclei) the nuclear radius is roughly proportional to
the cube root of the mass number (*A*) of the nucleus, and particularly in
nuclei containing many nucleons, as they arrange in more spherical
configurations:
In other words, packing protons and neutrons in the nucleus gives
*approximately* the same total size result as packing hard spheres of a
constant size (like marbles) into a tight spherical or almost spherical bag
(some stable nuclei are not quite spherical, but are known to be
prolate).[21]
Early models of the nucleus viewed the nucleus as a rotating liquid drop.
In this model, the trade-off of long-range electromagnetic forces and
relatively short-range nuclear forces, together cause behavior which
resembled surface tension forces in liquid drops of different sizes. This
formula is successful at explaining many important phenomena of nuclei,
such as their changing amounts of binding energy as their size and
composition changes (see semi-empirical mass formula), but it does not
explain the special stability which occurs when nuclei have special "magic
numbers" of protons or neutrons.
The terms in the semi-empirical mass formula, which can be used to
approximate the binding energy of many nuclei, are considered as the sum of
five types of energies (see below). Then the picture of a nucleus as a drop
of incompressible liquid roughly accounts for the observed variation of
binding energy of the nucleus:
*Volume energy*. When an assembly of nucleons of the same size is packed
together into the smallest volume, each interior nucleon has a certain
number of other nucleons in contact with it. So, this nuclear energy is
proportional to the volume.
*Surface energy*. A nucleon at the surface of a nucleus interacts with
fewer other nucleons than one in the interior of the nucleus and hence its
binding energy is less. This surface energy term takes that into account
and is therefore negative and is proportional to the surface area.
--
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http://django-oscar.readthedocs.org/en/latest/
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