|Name, symbol||flerovium, Fl|
|Flerovium in the periodic table|
|Atomic number (Z)||114|
|Group, block||group 14 (carbon group), p-block|
|Element category||post-transition metal|
|Standard atomic weight (Ar)|||
|Electron configuration||[Rn] 5f14 6d10 7s2 7p2 (predicted)|
|2, 8, 18, 32, 32, 18, 4 (predicted)|
|Melting point||340 K (67 °C, 160 °F) (predicted)|
|Boiling point||420 K (147 °C, 297 °F) (predicted)|
|Density near r.t.||14 g/cm3 (predicted)|
|Heat of vaporization||38 kJ/mol (predicted)|
|Oxidation states||0, 1, 2, 4, 6 (predicted)|
|Ionization energies||1st: 823.9 kJ/mol (predicted)
2nd: 1601.6 kJ/mol (predicted)
3rd: 3367.3 kJ/mol (predicted)
|Atomic radius||empirical: 180 pm (predicted)|
|Covalent radius||171–177 pm (extrapolated)|
|Naming||after Flerov Laboratory of Nuclear Reactions (itself named after Georgy Flyorov)|
|Discovery||Joint Institute for Nuclear Research (JINR) and Lawrence Livermore National Laboratory (LLNL) (1999)|
|Most stable isotopes of flerovium|
Flerovium is a superheavy artificial chemical element with symbol Fl and atomic number 114. It is an extremely radioactive synthetic element. The element is named after the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna, Russia, where the element was discovered in 1998. The name of the laboratory, in turn, honours the Russian physicist Georgy Flyorov (Флёров in Cyrillic, hence the transliteration of "yo" to "e"). The name was adopted by IUPAC on 30 May 2012.
In the periodic table of the elements, it is a transactinide element in the p-block. It is a member of the 7th period and is the heaviest known member of the carbon group; it is also the heaviest element whose chemistry is currently known. Initial chemical studies performed in 2007–2008 indicated that flerovium was unexpectedly volatile for a group 14 element; in preliminary results it even seemed to exhibit properties similar to those of the noble gases. More recent results show that flerovium's reaction with gold is similar to that of copernicium, showing that it is a very volatile element that may even be gaseous at standard temperature and pressure, that it would show metallic properties, consistent with it being the heavier homologue of lead, and that it would be the least reactive metal in group 14.
About 90 atoms of flerovium have been observed: 58 were synthesized directly, and the rest were made from the radioactive decay of heavier elements. All of these flerovium atoms have been shown to have mass numbers from 284 to 289. The most stable known flerovium isotope, flerovium-289, has a half-life of around 2.6 seconds, but it is possible that this isotope may have a nuclear isomer with a longer half-life of 66 seconds; this would be one of the longest half-lives of any isotope of a superheavy element. Flerovium is predicted to be near the centre of the theorized island of stability, and it is expected that heavier flerovium isotopes, especially the possibly doubly magic flerovium-298, may have even longer half-lives.
From the late 1940s to the early 1960s, the early days of the synthesis of heavier and heavier transuranium elements, it was predicted that since such heavy elements did not occur naturally, they would have shorter and shorter half-lives to spontaneous fission, until they stopped existing altogether at around element 108 (now known as hassium). Initial work in the synthesis of the actinides appeared to confirm this. The nuclear shell model, introduced in the late 1960s, stated that the protons and neutrons formed shells within a nucleus, somewhat analogous to electrons forming electron shells within an atom. The noble gases are unreactive due to their having full electron shells; thus it was theorized that elements with full nuclear shells – having so-called "magic" numbers of protons or neutrons – would be stabilized against radioactive decay. A doubly magic isotope, having magic numbers of both protons and neutrons, would be especially stabilized, and it was calculated that the next doubly magic isotope after lead-208 would be flerovium-298 with 114 protons and 184 neutrons, which would form the centre of a so-called "island of stability". This island of stability, supposedly ranging from copernicium (element 112) to ununoctium (118), would come after a long "sea of instability" from elements 101 (mendelevium) to 111 (roentgenium), and the flerovium isotopes in it were speculated in 1966 to have half-lives in excess of a hundred million years. It took thirty years for the first isotopes of flerovium to be synthesized. More recent work suggests that the local islands of stability around hassium and flerovium are due to these nuclei being respectively deformed and oblate, which make them resistant to spontaneous fission, and that the true island of stability for spherical nuclei occurs at around unbibium-306 (with 122 protons and 184 neutrons).
Flerovium was first synthesized in December 1998 by a team of scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, led by Yuri Oganessian, who bombarded a target of plutonium-244 with accelerated nuclei of calcium-48:
* → 289
+ 3 1
A single atom of flerovium, decaying by alpha emission with a lifetime of 30.4 seconds, was detected. The decay energy measured was 9.71 MeV, giving an expected half-life of 2–23 s. This observation was assigned to the isotope flerovium-289 and was published in January 1999. The experiment was later repeated, but an isotope with these decay properties was never found again and hence the exact identity of this activity is unknown. It is possible that it was due to the metastable isomer 289mFl.
Glenn T. Seaborg, a scientist at the Lawrence Berkeley National Laboratory who had been involved in work to synthesize such superheavy elements, had said in December 1997 that "one of his longest-lasting and most cherished dreams was to see one of these magic elements"; he was told of the synthesis of flerovium by his colleague Albert Ghiorso soon after its publication in 1999. Ghiorso later recalled:
I wanted Glenn to know, so I went to his bedside and told him. I thought I saw a gleam in his eye, but the next day when I went to visit him he didn't remember seeing me. As a scientist, he had died when he had that stroke.— Albert Ghiorso
Seaborg died a month later, on 25 February 1999.
Road to confirmation
In March 1999, the same team replaced the 244Pu target with a 242Pu one in order to produce other flerovium isotopes. This time two atoms of flerovium were produced, decaying via alpha emission with a half-life of 5.5 s. They were assigned as 287Fl. This activity has not been seen again either, and it is unclear what nucleus was produced. It is possible that it was the meta-stable isomer 287mFl.
The now-confirmed discovery of flerovium was made in June 1999 when the Dubna team repeated the first reaction from 1998. This time, two atoms of element 114 were produced; they alpha decayed with a half-life of 2.6 s, different from the 1998 result. This activity was initially assigned to 288Fl in error, due to the confusion regarding the previous observations that were assumed to come from 289Fl. Further work in December 2002 finally allowed a positive reassignment of the June 1999 atoms to 289Fl.
In May 2009, the Joint Working Party (JWP) of IUPAC published a report on the discovery of copernicium in which they acknowledged the discovery of the isotope 283Cn. This implied the discovery of flerovium, from the acknowledgement of the data for the synthesis of 287Fl and 291Lv, which decay to 283Cn. The discovery of the isotopes flerovium-286 and -287 was confirmed in January 2009 at Berkeley. This was followed by confirmation of flerovium-288 and -289 in July 2009 at the Gesellschaft für Schwerionenforschung (GSI) in Germany. In 2011, IUPAC evaluated the Dubna team experiments of 1999–2007. They found the early data inconclusive, but accepted the results of 2004–2007 as flerovium, and the element was officially recognized as having been discovered.
Using Mendeleev's nomenclature for unnamed and undiscovered elements, flerovium is sometimes called eka-lead. In 1979, IUPAC published recommendations according to which the element was to be called ununquadium (with the corresponding symbol of Uuq), a systematic element name as a placeholder, until the discovery of the element is confirmed and a permanent name is decided on. Some scientists in the field called it "element 114", with the symbol of (114) or 114.
According to IUPAC recommendations, the discoverer(s) of a new element has the right to suggest a name. After the discovery of flerovium and livermorium was recognized by IUPAC on 1 June 2011, IUPAC asked the discovery team at the JINR to suggest permanent names for those two elements. The Dubna team chose to name element 114 flerovium (symbol Fl), after the Russian Flerov Laboratory of Nuclear Reactions (FLNR), named after the Soviet physicist Georgy Flyorov (also spelled Flerov); earlier reports claim the element name was directly proposed to honour Flerov. In accordance with the proposal received from the discoverers IUPAC officially named flerovium after the Flerov Laboratory of Nuclear Reactions (an older name for the JINR), not after Flyorov himself. Flyorov is known for writing to Stalin in April 1942 and pointing out the silence in scientific journals in the field of nuclear fission in the United States, Great Britain, and Germany. Flyorov deduced that this research must have become classified information in those countries. Flyorov's work and urgings led to the development of the USSR's own atomic bomb project.
Nuclear stability and isotopes
The physical basis of the chemical periodicity governing the periodic table is the electron shell closures at each noble gas (atomic numbers 2, 10, 18, 36, 54, 86, and 118): as any further electrons must enter a new shell with higher energy, closed-shell electron configurations are markedly more stable, leading to the relative inertness of the noble gases. Since protons and neutrons are also known to arrange themselves in closed nuclear shells, the same effect happens at nucleon shell closures, which happen at specific nucleon numbers often dubbed "magic numbers". The known magic numbers are 2, 8, 20, 28, 50, and 82 for protons and neutrons, and also 126 for neutrons. Nucleons with magic proton and neutron numbers, such as helium-4, oxygen-16, calcium-48, and lead-208, are termed "doubly magic" and are very stable against decay. This property of increased nuclear stability is very important for superheavy elements: without any stabilization, their half-lives would be expected by exponential extrapolation to be in the range of nanoseconds (10−9 s) when element 110 (darmstadtium) is reached, because of the ever-increasing repulsive electromagnetic forces between the positively charged protons that overcome the limited-range strong nuclear force that holds the nucleus together. The next closed nucleon shells and hence magic numbers are thought to be at the centre of the long-sought island of stability, where the half-lives to alpha decay and spontaneous fission lengthen again.
Initially, by analogy with the neutron magic number 126, the next proton shell was also expected to occur at element 126, too far away from the synthesis capabilities of the mid-20th century to achieve much theoretical attention. In 1966, new values for the potential and spin-orbit interaction in this region of the periodic table contradicted this and predicted that the next proton shell would occur instead at element 114, and that nuclides in this region would be as stable against spontaneous fission as many heavy nuclei such as lead-208. The expected closed neutron shells in this region were at neutron number 184 or 196, thus making 298Fl and 310Fl candidates for being doubly magic. 1972 estimates predicted a half-life of about a year for 298Fl, which was expected to be near a large island of stability with the longest half-life at darmstadtium-294 (1010 years, comparable to that of 232Th). After the synthesis of the first isotopes of elements 112 through 118 at the turn of the 21st century, it was found that the synthesized neutron-deficient isotopes were stabilized against fission. In 2008 it was thus hypothesized that the stabilization against fission of these nuclides was due to their being oblate nuclei, and that a region of oblate nuclei was centred on 288Fl. Additionally, new theoretical models showed that the expected gap in energy between the proton orbitals 2f7/2 (filled at element 114) and 2f5/2 (filled at element 120) was smaller than expected, so that element 114 no longer appeared to be a stable spherical closed nuclear shell. The next doubly magic nucleus is now expected to be around 306Ubb, but the expected low half-life and low production cross section of this nuclide makes its synthesis challenging. Nevertheless, the island of stability is still expected to exist in this region of the periodic table, and nearer its centre (which has not been approached closely enough yet) some nuclides, such as 291Uup and its alpha- and beta-decay daughters,[a] may be found to decay by positron emission or electron capture and thus move into the centre of the island. Due to the expected high fission barriers, any nucleus within this island of stability decays exclusively by alpha decay and perhaps some electron capture and beta decay, both of which would bring the nuclei closer to the beta stability line where the island is expected to be. Electron capture is needed to reach the island, which is problematic because it is not certain that electron capture becomes a major decay mode in this region of the chart of nuclides.
Several experiments have been performed between 2000 and 2004 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus 292Fl by bombarding a plutonium-244 target with accelerated calcium-48 ions. A compound nucleus is a loose combination of nucleons that have not yet arranged themselves into nuclear shells. It has no internal structure and is held together only by the collision forces between the target and projectile nuclei.[b] The results revealed how nuclei such as this fission predominantly by expelling doubly magic or nearly doubly magic fragments such as calcium-40, tin-132, lead-208, or bismuth-209. It was also found that the yield for the fusion-fission pathway was similar between calcium-48 and iron-58 projectiles, indicating a possible future use of iron-58 projectiles in superheavy element formation. It has also been suggested that a neutron-rich flerovium isotope can be formed by the quasifission (partial fusion followed by fission) of a massive nucleus. Recently it has been shown that the multi-nucleon transfer reactions in collisions of actinide nuclei (such as uranium and curium) might be used to synthesize the neutron-rich superheavy nuclei located at the island of stability, although production of neutron-rich nobelium or seaborgium nuclei is more likely.
Theoretical estimation of the alpha decay half-lives of the isotopes of the flerovium supports the experimental data. The fission-survived isotope 298Fl, long expected to be doubly magic, is predicted to have alpha decay half-life around 17 days. The direct synthesis of the nucleus 298Fl by a fusion–evaporation pathway is currently impossible since no known combination of target and stable projectile can provide 184 neutrons in the compound nucleus, and radioactive projectiles such as calcium-50 (half-life fourteen seconds) cannot yet be used in the needed quantity and intensity. Currently, one possibility for the synthesis of the expected long-lived nuclei of copernicium (291Cn and 293Cn) and flerovium near the middle of the island include using even heavier targets such as curium-250, berkelium-249, californium-251, and einsteinium-254, that when fused with calcium-48 would produce nuclei such as 291Uup and 291Fl (as decay products of 299Uue, 295Uus, and 295Lv), with just enough neutrons to alpha decay to nuclides close enough to the centre of the island to possibly undergo electron capture and move inwards to the centre, though the cross sections would be small and little is yet known about the decay properties of superheavy nuclides near the beta stability line. This may be the best hope currently to synthesize nuclei on the island of stability, but it is speculative and may or may not work in practice. Another possibility is to use controlled nuclear explosions to achieve the high neutron flux necessary to create macroscopic amounts of such isotopes. This would mimic the r-process in which the actinides were first produced in nature and the gap of instability after polonium bypassed, as it would bypass the gaps of instability at 258–260Fm and at mass number 275 (atomic numbers 104 to 108). Some such isotopes (especially 291Cn and 293Cn) may even have been synthesized in nature, but would have decayed away far too quickly (with half-lives of only thousands of years) and be produced in far too small quantities (about 10−12 the abundance of lead) to be detectable as primordial nuclides today outside cosmic rays.
Atomic and physical
Flerovium is a member of group 14 in the periodic table, below carbon, silicon, germanium, tin, and lead. Every previous group 14 element has four electrons in its valence shell, forming a valence electron configuration of ns2np2. In flerovium's case, the trend will be continued and the valence electron configuration is predicted to be 7s27p2; flerovium will behave similarly to its lighter congeners in many respects. Differences are likely to arise; a largely contributing effect is the spin–orbit (SO) interaction—the mutual interaction between the electrons' motion and spin. It is especially strong for the superheavy elements, because their electrons move faster than in lighter atoms, at velocities comparable to the speed of light. In relation to flerovium atoms, it lowers the 7s and the 7p electron energy levels (stabilizing the corresponding electrons), but two of the 7p electron energy levels are stabilized more than the other four. The stabilization of the 7s electrons is called the inert pair effect, and the effect "tearing" the 7p subshell into the more stabilized and the less stabilized parts is called subshell splitting. Computation chemists see the split as a change of the second (azimuthal) quantum number l from 1 to 1⁄2 and 3⁄2 for the more stabilized and less stabilized parts of the 7p subshell, respectively.[c] For many theoretical purposes, the valence electron configuration may be represented to reflect the 7p subshell split as 7s2
1/2. These effects cause flerovium's chemistry to be somewhat different from that of its lighter neighbours.
Due to the spin-orbit splitting of the 7p subshell being very large in flerovium, and the fact that both flerovium's filled orbitals in the seventh shell are stabilized relativistically, the valence electron configuration of flerovium may be considered to have a completely filled shell, making flerovium a very noble metal. Its first ionization energy of 8.539 eV (823.9 kJ/mol) should be the highest in group 14. The 6d electron levels are also destabilized, and may still be able to participate in chemical reactions in flerovium (but not the later 7p elements), which might allow it to behave in some ways like transition metals and allow higher oxidation states such as +4 and +6. They should be less stable than the +2 state, following periodic trends, and may only be stable in flerovium fluorides.
The closed-shell electron configuration of flerovium results in the metallic bonding in metallic flerovium being weaker than in the preceding and following elements; thus, flerovium is expected to have a low boiling point, and has recently been suggested to be possibly a gaseous metal, similar to the predictions for copernicium, which also has a closed-shell electron configuration. The melting and boiling points of flerovium are predicted to be around 70 °C and 150 °C, significantly lower than the values for the lighter group 14 elements (those of lead are 327 °C and 1749 °C respectively), and continuing the trend of decreasing boiling points down the group. Although earlier studies predicted a boiling point of ~1000 °C or 2840 °C, this is now considered unlikely because of the expected weak metallic bonding in flerovium and that group trends would expect flerovium to have a low sublimation enthalpy. In the solid state, flerovium is expected to be a dense metal due to its high atomic weight, with a density variously predicted to be either 22 g·cm−3 or 14 g·cm−3. The electron of the hydrogen-like flerovium ion (oxidized so that it only has one electron, Fl113+) is expected to move so fast that it has a mass 1.79 times that of a stationary electron, due to relativistic effects. For comparison, the figures for hydrogen-like lead and tin are expected to be 1.25 and 1.073 respectively. Flerovium would form weaker metal–metal bonds than lead and would be adsorbed less on surfaces.
Flerovium is the heaviest known member of group 14 in the periodic table, below lead, and is projected to be the second member of the 7p series of chemical elements. The first five members of this group show the group oxidation state of +4 and the latter members have an increasingly prominent +2 chemistry due to the onset of the inert pair effect. Tin represents the point at which the stability of the +2 and +4 states are similar, and lead(II) is the most stable of all the chemically well-understood group 14 elements in the +2 oxidation state. The 7s orbitals are very highly stabilized in flerovium and thus a very large sp3 orbital hybridization is required to achieve the +4 oxidation state, so flerovium is expected to be even more stable than lead in its strongly predominant +2 oxidation state and its +4 oxidation state should be highly unstable. For example, flerovium dioxide (FlO2) is expected to be highly unstable to decomposition into its constituent elements (and would not be formed from the direct reaction of flerovium with oxygen), and flerovane (FlH4), which should have Fl–H bond lengths of 1.787 Å, is predicted to be more thermodynamically unstable than plumbane, spontaneously decomposing into flerovium(II) hydride (FlH2) and hydrogen gas. The only stable flerovium(IV) compound is expected to be the tetrafluoride, FlF4: even this may be due to sd hybridizations rather than sp3 hybridization, and its decomposition to the difluoride and fluorine gas would be exothermic. The gross destabilization of all the tetrahalides (for example, FlCl4 is destabilized by about 400 kJ/mol) is unfortunate because otherwise these compounds would be very useful in gas-phase chemical studies of flerovium. The corresponding polyfluoride anion FlF2−
6 should be unstable to hydrolysis in aqueous solution, and flerovium(II) polyhalide anions such as FlBr−
3 and FlI−
3 are predicted to form preferentially in flerovium-containing solutions. The sd hybridizations would be possible as the 7s and 6d electrons in flerovium share approximately the same energy, perhaps making even higher oxidation states like +6 possible with extremely electronegative elements, such as in flerovium(VI) fluoride (FlF6). In general, the spin-orbit contraction of the 7p1/2 orbital should lead to smaller bond lengths and larger bond angles: this has been theoretically confirmed in FlH2.
Due to the relativistic stabilization of flerovium's 7s27p2
1/2 valence electron configuration, the 0 oxidation state should also be more stable for flerovium than for lead, as the 7p1/2 electrons begin to also exhibit a mild inert pair effect: this stabilization of the neutral state may bring about some similarities between the behaviour of flerovium and the noble gas radon. Due to the expected relative inertness of flerovium, its diatomic compounds FlH and FlF should have lower energies of dissociation than the corresponding lead compounds PbH and PbF. Flerovium(IV) should be even more electronegative than lead(IV); lead(IV) has electronegativity 2.33 on the Pauling scale; the lead(II) value is only 1.87.
Flerovium(II) should be more stable than lead(II), and polyhalide ions and compounds of types FlX+, FlX2, FlX−
3, and FlX2−
4 (X = Cl, Br, I) are expected to form readily. Fluorine would be able to also form the unstable flerovium(IV) analogues. All the flerovium dihalides are expected to be stable; with the difluoride being water-soluble, spin-orbit effects would destabilize flerovium dihydride (FlH2) by almost 2.6 eV (250 kJ/mol). In solution, flerovium would also form the oxoanion flerovite (FlO2−
2) in aqueous solution, analogous to plumbite. Flerovium(II) sulfate (FlSO4) and sulfide (FlS) should be very insoluble in water, and flerovium(II) acetate (FlC2H3O2) and nitrate (Fl(NO3)2) should be quite water-soluble. The standard electrode potential for the reduction of Fl2+ ions to metallic flerovium is estimated to be around +0.9 V, confirming the increased stability of flerovium in the neutral state. In general, due to the relativistic stabilization of the 7p1/2 spinor, Fl2+ is expected to have properties intermediate between those of Hg2+ or Cd2+ and its lighter congener Pb2+.
Flerovium is currently the heaviest element to have had its chemistry experimentally investigated. Two experiments were performed in April–May 2007 in a joint FLNR-PSI collaboration aiming to study the chemistry of copernicium. The first experiment involved the reaction 242Pu(48Ca,3n)287Fl and the second the reaction 244Pu(48Ca,4n)288Fl: these reactions produce short-lived flerovium isotopes whose copernicium daughters would then be studied. The adsorption properties of the resultant atoms on a gold surface were compared with those of radon, as it was then expected that copernicium's full-shell electron configuration would lead to noble-gas like behaviour. Noble gases interact with metal surfaces very weakly, which is uncharacteristic of metals.
The first experiment allowed detection of three atoms of 283Cn but also seemingly detected 1 atom of 287Fl. This result was a surprise given the transport time of the product atoms is ~2 s, so the flerovium atoms produced should have decayed to copernicium before adsorption. In the second reaction, 2 atoms of 288Fl and possibly 1 atom of 289Fl were detected. Two of the three atoms displayed adsorption characteristics associated with a volatile, noble-gas-like element, which has been suggested but is not predicted by more recent calculations. These experiments provided independent confirmation for the discovery of copernicium, flerovium, and livermorium via comparison with published decay data. Further experiments in 2008 to confirm this important result detected a single atom of 289Fl, and supported previous data showing flerovium having a noble-gas-like interaction with gold.
The experimental support for a noble-gas-like flerovium was soon to weaken abruptly. In 2009 and 2010, the FLNR-PSI collaboration synthesized further atoms of flerovium to follow up their 2007 and 2008 studies. In particular, the first three flerovium atoms synthesized in the 2010 study suggested again a noble-gas-like character, but the complete set taken together resulted in a more ambiguous interpretation, unusual for a metal in the carbon group but not fully like a noble gas in character. In their paper, the scientists refrained from calling flerovium's chemical properties "close to those of noble gases", as had previously been done in the 2008 study. Flerovium's volatility was again measured through interactions with a gold surface, and provided indications that the volatility of flerovium was comparable to that of mercury, astatine, and the simultaneously investigated copernicium, which had been shown in the study to be a very volatile noble metal, conforming to its being the heaviest group 12 element known. Nevertheless, it was pointed out that this volatile behaviour was not expected for a usual group 14 metal.
In even later experiments from 2012 at the GSI, the chemical properties of flerovium were revealed to be more metallic than noble-gas-like. Jens Volker Kratz and Christoph Düllmann specifically named copernicium and flerovium as belonging to a new category of "volatile metals"; Kratz even speculated that they might be gaseous at standard temperature and pressure. These "volatile metals", as a category, were expected to fall between normal metals and noble gases in terms of adsorption properties. Contrary to the 2009 and 2010 results, it was shown in the 2012 experiments that the interactions of flerovium and copernicium respectively with gold were about equal. Further studies showed that flerovium was more reactive than copernicium, in contradiction to previous experiments and predictions.
- Specifically, 291Uup, 291Fl, 291Uut, 287Uut, 287Cn, 287Rg, 283Rg, and 283Ds, which are expected to decay to the relatively longer-lived nuclei 279Mt, 283Mt, 287Ds, and 291Cn.
- It is estimated that it requires around 10−14 s for the nucleons to arrange themselves into nuclear shells, at which point the compound nucleus becomes an nuclide, and this number is used by IUPAC as the minimum half-life a claimed isotope must have to be recognized as a nuclide.
- The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. See azimuthal quantum number for more information.
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- CERN Courier – First postcard from the island of nuclear stability
- CERN Courier – Second postcard from the island of stability
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