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<!--<div style="margin: 0 0 1em 1em; float:right; text-align: center">[[Datoteka:Atom.png|align right|Helium atom (not to scale)]]</div>-->
[[Datoteka:Helium_atom_QM.png|right|thumb|300px|Vizuelni prikaz [[helijum]]ovog atoma prema kvantnomehaničkom modelu. U jezgru, [[proton]]i su ružičasti a [[neutron]]i ljubičasti. Realno jezgro helijuma je sferno simetrično. Gustina sivog oblaka oko jezgra proporcionalna je verovatnoći nalaženja elektrona. 1 [[Fermi (jedinica)|Fermi]] = 10<sup>-15</sup> m. 1 [[Angstrem (jedinica)|Angstrem]] = 10<sup>-10</sup> m]]
* [[1913]]. g. [[Artur van der Bruk|A. van den Bruk]] primetio je da podaci o rasejavanju alfa čestica mogu najlakše da se objasne ako se pretpostavi da je naelektrisanje jezgra umnožak elemetarnog naelektrisanja elektrona i rednog broja elementa Ze.
* [[1914]]. g. [[Henri Mozli|H. Mozli]], mereći frekvencije karakterističnog [[Rentgensko zračenje|X zračenja]], potvrdio je Van den Brukovu hipotezu.
<!--The discovery of the [[electron]] was the first indication that the atom had internal structure. At the turn of the 20th century the accepted model of the atom was [[J. J. Thomson]]'s [[Plum pudding model|"plum pudding" model]] in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it. By the turn of the century physicists had also discovered three types of [[radiation]] coming from atoms, which they named [[alpha decay|alpha]], [[beta decay|beta]], and [[gamma decay|gamma]] radiation. Experiments in [[1911]] by [[Lise Meitner]] and [[Otto Hahn]], and by [[James Chadwick]] in 1914 discovered that the beta decay [[spectrum]] was continuous rather than discrete. That is, electrons were ejected from the atom with a range of energies, rather than the discrete amounts of energies that were observed in gamma and [[alpha decay]]s. This was a problem for nuclear physics at the time, because it indicated that [[conservation of energy|energy was not conserved]] in these decays. The problem would later lead to the discovery of the neutrino (see below).-->
<!--Around the same time that this was happening ([[1911]]) [[Ernest Rutherford]] performed a remarkable [[Geiger-Marsden experiment|experiment]] in which [[Hans Geiger]] and [[Ernest Marsden]] under his supervision fired alpha particles (helium nuclei) at a thin film of [[gold]] foil. The plum pudding model predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. He was shocked to discover that a few particles were scattered through large angles, even completely backwards in some cases. The discovery led to the Rutherford model of the atom, in which the atom has a very small, very dense nucleus consisting of heavy positively charged particles with embedded electrons in order to balance out the charge. As an example, in this model nitrogen-14 consisted of a nucleus with 14 protons and 7 electrons, and the nucleus was surrounded by 7 more orbiting electrons.-->
<!--The Rutherford model worked quite well until studies of [[spin (physics)|nuclear spin]] were carried out by [[Franco Rasetti]] at the [[California Institute of Technology]] in [[1929]]. By [[1925]] it was known that protons and electrons had a spin of 1/2, and in the Rutherford model of nitrogen-14 the 14 protons and six of the electrons should have paired up to cancel each others spin, and the final electron should have left the nucleus with a spin of 1/2. Rasetti discovered, however, that nitrogen-14 has a spin of one.-->
<!--In [[1930]] [[Wolfgang Pauli]] was unable to attend a meeting in [[Tubingen]], and instead sent a famous letter with the classic introduction "Dear Radioactive Ladies and Gentlemen". In his letter Pauli suggested that perhaps there was a third particle in the nucleus which he named the "neutron". He suggested that it was very light (lighter than an electron), had no charge, and that it did not readily interact with matter (which is why it hadn't yet been detected). This desperate way out solved both the problem of energy conservation and the spin of nitrogen-14, the first because Pauli's "neutron" was carrying away the extra energy and the second because an extra "neutron" paired off with the electron in the nitrogen-14 nucleus giving it spin one. Pauli's "neutron" was renamed the [[neutrino]] (Italian for little neutral one) by [[Enrico Fermi]] in [[1931]], and after about thirty years it was finally demonstrated that a neutrino really is emitted during beta decay.-->
<!--In [[1932]] Chadwick realized that radiation that had been observed by [[Walther Bothe]], [[Herbert Becker]], [[Irène Joliot-Curie|Irène]] and [[Frédéric Joliot-Curie]] was actually due to a massive particle that he called the neutron. In the same year [[Dmitrij Iwanenko]] suggested that neutrons were in fact spin 1/2 particles and that the nucleus contained neutrons and that there were no electrons in it, and [[Francis Perrin]] suggested that neutrinos were not nuclear particles but were created during beta decay. To cap the year off, Fermi submitted a theory of the neutrino to [[Nature magazine|Nature]] (which the editors rejected for being "too remote from reality"). Fermi continued working on his theory and published a paper in [[1934]] which placed the neutrino on solid theoretical footing. In the same year [[Hideki Yukawa]] proposed the first significant theory of the strong force to explain how the nucleus holds together.-->
<!--With Fermi and Yukawa's papers the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nucleus, or beta decay, in which they eject an electron (or [[positron]]). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high energy photons (gamma decay).-->
<!--
The study of the strong and weak nuclear forces led physicists to collide nuclei and electrons at ever higher energies. This research became the science of [[particle physics]], the crown jewel of which is the [[Standard Model|standard model of particle physics]] which unifies the strong, weak, and electromagnetic forces.
-->
== Modeli atomskog jezgra ==
== Izotopi ==
[[Izotop]]ski sastav jezgra određen je brojem neutrona u njemu. (Promenom broja protona, menja se hemijska priroda atoma.) Različiti izotopi istog hemijskog elementa imaju veoma slične (ali ne i identične, videti [[izotopski efekat]]) hemijske osobine jer hemijsu prirodu elementa skoro u potpunosti određuje broj elektrona u elektronskom omotaču atoma. To znači da se različiti izotopi jednog te istog hemijskog elementa vrlo teško mogu razdvojiti hemijskim putem ali mogu različitim fizičkohemijskim procesima i metodama poput centrifugiranja, masene spektrometrije, frakcione destilacije, elektrolize itd. Na primer, obogaćeni uranijum (povećanje koncentracije uranijuma-235 u odnosu na uranijum-238) na industrijskoj skali, dobija se centrifugiranjem uranijumheksafluorida UF6. Za odreživanje starosti materijala organskog porekla (na osnovu odnosa kocentracija izotopa ugljenika-14 i ugljenika-12) koristi se masena spektrometrija. Za dobijanje kiseonika-18 koji se koristi za pravljenje radioaktivnih izotopa za medicinsku dijagnostiku ([[PET]]) koristi se [[frakciona destilacija]] azotdioksida...
<!--The number of protons and neutrons together determine the nuclide (type of nucleus). Protons and neutrons have nearly equal masses, and their combined number, the [[mass number]], is approximately equal to the [[atomic mass]] of an atom. The combined mass of the electrons is very small in comparison to the mass of the nucleus, since protons and neutrons weigh roughly 2000 times more than electrons.-->
=== Raspad jezgra ===
Iz tog ugla gledano i alfa radioaktivni raspad može se smatrati specijalnim oblikom nukearne fisije jer je alfa čestica isto što i atomsko jezgro helijuma-4. Međutim, pod fisijom se obično podrazumeva cepanje teškog jezgra na dva manja slične veličine.
<!--For certain of the heaviest nuclei which produce neutrons on fission, and which also easily absorb neutrons to initiate fission, a self-igniting type of neutron-initiated fission can be obtained, in a so-called [[chain reaction]]. [Chain reactions were known in [[chemistry]] before [[physics]], and in fact many familiar processes like fires and chemical explosions are chemical chain reactions]. The fission or "nuclear" chain-reaction, using fission-produced neutrons, is the source of energy for [[nuclear power]] plants and fission type nuclear bombs such as the two that the [[United States]] used against [[Hiroshima]] and [[Nagasaki]] at the end of [[World War II]]. Heavy nuclei such as [[uranium]] and [[thorium]] may undergo [[spontaneous nuclear fission]], but they are much more likely to undergo decay by alpha decay.-->
<!--For a neutron-initiated chain-reaction to occur, there must be a [[critical mass]] of the element present in a certain space under certain conditions (these conditions slow and conserve neutrons for the reactions). There is one known example of a [[natural nuclear fission reactor]], which was active in two regions of [[Oklo]], Gabon, Africa, over 1.5 billion years ago. Measurements of natural neutrino emission have demonstrated at around half of the heat emanating from the earth's core, results from radioactive decay. However, it is not known if any of this results from fission chain-reactions.-->
== Proizvodnja teških elemenata ==
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As the Universe cooled after the [[big bang]] it eventually became possible for particles as we know them to exist. The most common particles created in the big bang which are still easily observable to us today were protons ([[hydrogen]]) and electrons (in equal numbers). Some heavier elements were created as the protons collided with each other, but most of the heavy elements we see today were created inside of stars during a series of fusion stages, such as the [[proton-proton chain]], the [[CNO cycle]] and the [[triple-alpha process]].
Progressively heavier elements are created during the [[stellar evolution|evolution]] of a star.
Since the binding energy per nucleon peaks around iron, energy is only released in fusion processes occurring below this point. Since the creation of heavier nuclei by fusion costs energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a slow neutron capture process (the so-called ''s'' process) or by the rapid, or ''r'' process. The ''s'' process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The ''r'' process is thought to occur in supernova explosions due to the fact that the conditions of high temperature, high neutron flux and ejected matter are present. These stellar conditions make the successive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells ([[Magic_number_(physics)|magic numbers]]). The ''r'' process duration is typically in the range of a few seconds.-->
== Nuklearna fizika ==
Nuklearna fizika se bavi izučavanjem osobina i procesa u atomskom jezgru, i danas se uglavnom bavi atomskim jezgrom u ekstremnim uslovima, kao što su ekstremno veliki spin, ekstremno visoka pobuđenja stanja, ekstremni oblik poput ragbi lopte ili ekstremni odnos broja neutrona i protona. Takva jezgra se eksperimentalno mogu stvoriti veštački izazvanom fuzijom u ubrzivačima (akceleratorima) čestica.
== LiteraturaIzvori ==
{{Izvori}}
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== Vidi još ==
* [[Spisak nuklearnih čestica]]
* [[Radioativni raspad]]
* [[Fuzija|Nuklearna fuzija]]
* [[Nuklearna fisija]]
* [[Nuklearna medicina]]
* [[Nuklearna fizika]]
* [[Atomski broj]]
* [[Atomska masa]]
* [[Izotop]]
* [[Lista atomskih fizičara]]
== Vanjske veze ==
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