|Name, symbol||flerovium, Fl|
|Flerovium in the periodic table|
|Standard atomic weight|||
|Element category||post-transition metal|
|Group, block||group 14 (carbon group), p-block|
|Electron configuration||[Rn] 5f14 6d10 7s2 7p2 (predicted)|
|per shell||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·cm−3 (predicted)|
|Heat of vaporization||38 kJ·mol−1 (predicted)|
|Oxidation states||0, 1, 2, 4, 6 (predicted)|
|Ionization energies||1st: 823.9 kJ·mol−1 (predicted)
2nd: 1601.6 kJ·mol−1 (predicted)
3rd: 3367.3 kJ·mol−1 (predicted)
|Atomic radius||empirical: 180 pm (predicted)|
|Covalent radius||171–177 pm (extrapolated)|
|CAS Registry Number||54085-16-4|
|Naming||after Flerov Laboratory of Nuclear Reactions (itself named after Georgy Flyorov)|
|Discovery||Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory (1999)|
|Most stable isotopes|
Flerovium is a superheavy artificial chemical element with symbol Fl and atomic number 114. It is an extremely radioactive element that has only been created in the laboratory and has not been observed in nature. 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, honors the Russian physicist Georgy Flyorov. The name was adopted by IUPAC on May 30, 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 currently placed as the heaviest known member of the carbon group. 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, and that while it would show metallic properties, consistent with it being the heavier homologue of lead, it would also be the least reactive metal in group 14.
About 80 atoms of flerovium have been observed to date: 50 were synthesized directly, while the rest were made from the radioactive decay of even heavier elements. All of these flerovium atoms have been shown to have mass numbers from 285 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 flerovium 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 in nature, they would have shorter and shorter half-lives to spontaneous fission, until they stopped being able to exist altogether at around element 108 (now known as hassium). Initial work in the synthesis of the actinides appeared to confirm this. However, the nuclear shell model was introduced in the late 1960s, which stated that the protons and neutrons formed shells within a nucleus, just like 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 centering around elements 112 (copernicium) to 118, would come just 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 was not until thirty years later, however, that the first isotopes of flerovium would be synthesized. More recent work, however, suspects that the local islands of stability around hassium and flerovium are due to these nuclei being respectively deformed and oblate, which make them resistant towards 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:
94Pu + 48
20Ca → 292
114Fl* → 289
114Fl + 3 1
A single atom of flerovium, decaying by alpha emission with a lifetime of 30.4 seconds, was detected. This gave a calculated half-life of 2–23 s. This observation was assigned to the isotope flerovium-289 and was subsequently published in January 1999. However, while the experiment was later repeated, 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 is due to a metastable isomer, namely 289mFl.
Glenn T. Seaborg, a scientist at the Lawrence Berkeley National Laboratory who had been involved in work to synthesize such superheavy elements, stated in December 1997 that "one of his longest-lasting and most cherished dreams was to see one of these magic elements"; he received notice of the synthesis of flerovium from 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
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, alpha decaying with a half-life of 5.5 s. They were assigned as 287Fl. Once again, this activity has not been seen again and it is unclear what nucleus was produced. It is possible that it was a meta-stable isomer, namely 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 what was found in 1998. 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 therefore implied the de facto discovery of flerovium, from the acknowledgment 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. Whereas they found the early data inconclusive, the results of 2004–2007 were accepted as identification of 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. The recommendations were mostly ignored among scientists, who called it "element 114", with the symbol of (114) or even simply 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 founder of the Russian Flerov Laboratory of Nuclear Reactions (FLNR), the Soviet physicist Georgy Flyorov (also spelled Flerov). However, 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 conspicuous silence in scientific journals within 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 eventual 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 much 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 simple exponential extrapolation to be in the range of nanoseconds (10−9 s) by element 110 (darmstadtium), because of the ever-increasing repulsive electromagnetic forces between the positively charged protons that overcomes the limited-range strong nuclear force that holds the nucleus together. Therefore the next closed nucleon shells and hence magic numbers are thought to be at the center 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, much too far away from the synthesis capabilities of the mid-20th century to achieve much theoretical attention. However, in 1966, new predictions arrived that expected the next proton shell to occur instead at element 114, because of changed values for the potential and spin-orbit interaction in this region of the periodic table, and that the stabilization would make nuclides in this region as stable against spontaneous fission as many stable 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 in the vicinity of a large island of stability with the longest half-life at darmstadtium-294 (1010 years, comparable to that of thorium-232). This prediction However, 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 actually 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 centered around 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 center (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 center of the island. Due to the expected high fission barriers, any nucleus within this island of stability exclusively decays 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. However, electron capture is needed to actually 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 arranged themselves into nuclear shells yet. 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 therefore 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. However, 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 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, with just enough neutrons to alpha decay to nuclides close enough to the center of the island to possibly undergo electron capture and move inwards to the center, 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 means that, while this may be the best hope currently to synthesize nuclei on the island of stability, 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; therefore, flerovium will behave similarly to its lighter congeners in many respects. However, notable 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 much 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. Consistent with this, its first ionization energy of 8.539 eV should be the highest in group 14. However, 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. Flerovium may hence also be able to show the +4 and +6 oxidation states, although the +4 and +6 states should still be much less stable than the +2 state, following periodic trends, and may only be able to be stabilized in flerovium fluorides.
The closed-shell electron configuration of flerovium results in the metallic bonding in metallic flerovium being much 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. 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 atom (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 projected to be the second member of the 7p series of chemical elements and the heaviest known member of group 14 in the periodic table, below lead. 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; thus 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 indeed would not be formed from the direct reaction of flerovium with oxygen), while flerovane (FlH4), which should have Fl–H bond lengths of 1.787 Å, is predicted to be much 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, while the lead(II) value is only 1.87.
Flerovium(II) should be much 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, while spin-orbit effects would destabilize flerovium dihydride (FlH2) by almost 2.6 eV. 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, while 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 eV, 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 actual lighter congener Pb2+.
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. The adsorption properties of the resultant atoms on a gold surface were compared with those of the noble gas 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 flerovium atoms should decay before adsorption. In the second reaction, 2 atoms of 288Fl and possibly 1 atom of 289Fl were detected. Two of the three atoms portrayed adsorption characteristics associated with a volatile, noble-gas-like element, which has been suggested but is not predicted by more recent calculations. These experiments did however provide 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, which provided data that agreed with previous data that supported flerovium having a noble-gas-like interaction with gold.
The experimental support for a noble-gas-like flerovium was soon to weaken abruptly, however. 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 and 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 somewhat in between normal metals and noble gases in terms of absorption 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 in fact showed that flerovium was in reality more reactive than copernicium, in exact 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 potentially be recognised as being discovered.
- 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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