Rhenium

Rhenium, atomic symbol Re, is element 75 on the periodic table. Rhenium is part of Group 7, Period 6. It is similar in its properties to the "platinum group" transition metals, but is not considered a member of that group. This element has an atomic mass of 186.207 Da, which reflects the fact that Re is composed of two isotopes.

Rhenium is largely used as an alloying agent or (with Pt) as a catalyst. There is, however, at least one application for pure rhenium. The metal does not react with oxygen, even at extreme temperatures, so it has been used for rocket nozzles, where its high melting point allows the engine to run uncooled. Its density and high cost make the technique impractical for large engines, but using rhenium simplifies the design of small engines.

History

Rhenium was discovered in 1925 by German scientists Walter Noddack, Ida Tacke and Otto Berg. They detected the element in Platinum ore while researching and named it after the Rhine River.

Masataka Ogawa reported discovering element 43 (now technetium) in 1908. We now know that what Ogawa found was not technetium, but there is reason to think he may have discovered rhenium.

Nuclear properties and formation

At least 92 isotopes of rhenium have been predicted, ranging from ²⁴⁷Re to ¹⁵⁶Re. Of these, 35 have been observed (plus 21 isomers). ¹⁸⁸Re and heavier isotopes decay by beta emission and have short half lives, peaking at 24.3 hr in ¹⁸⁹Re. Positive beta decay (positron emission or electron capture sets in at ¹⁸⁴Re and is observed down to ¹⁶²Re. Alpha decay appears at ¹⁷⁰Re and is predicted to be a significant decay mode down to ¹⁵⁹Re (N = 84), below which it doesn't occur. Between ¹⁶¹Re and ¹⁵⁶Re, proton decay is observed or predicted to dominate.

Between neutron-rich and proton-rich radioisotopes lie ¹⁸⁷Re to ¹⁸⁵Re, each different. ¹⁸⁵Re is both effectively and observationally stable. ¹⁸⁷Re is radioactive, decaying by beta emission with a half-life of 4.12E10 yrs. Its long life is not the result of inhibited decay but a consequence of its extremely low decay energy of 2.47 keV. The isotope between, ¹⁸⁶Re, is an odd-Z, odd-N nuclide sandwiched between even-Z, even-N stable nuclides. It decays both to ¹⁸⁶W (branch ratio = 0.931) and to ¹⁸⁶Os (branch ratio = 0.069) and has a half-life of 3.72 days. ¹⁸⁶Re also has an isomer, ^186mRe with a half-life of 2.0E05 yrs. This high-spin isomer decays mainly by isomeric transition to the ground state, but also has a beta emission branch, indicating that beta decay is not completely blocked.

Both ¹⁸⁵Re and ¹⁸⁷Re form in supernovae and neutron star mergers. Their difference in abundance is not large, and may simply reflect fast neutron capture cross sections. (Both environments are very hot.) Both can also form by capturing slower neutrons at long intervals (s process). ¹⁸⁵Re forms via neutron capture by ¹⁸⁴W. ¹⁸⁵W has a half-life (t12) of ¹⁸⁵W is 75.1 days, If neutron capture rate is much slower than 175.1 d, most nuclei which form will decay to ¹⁸⁵Re. If the rate is comparable to or lower than 75.1 d, less ¹⁸⁵W will be produced. ¹⁸⁷Re can be produced at low neutron capture rates either via neutron capture by ¹⁸⁶W or via neutron capture by ¹⁸⁵Re to generate ^186mRe followed by a second capture which produces ¹⁸⁷Re. The branch ratio between ¹⁸⁶Re and ^186mRe for the reactions n + ¹⁸⁵Re --> ^186*Re + gammas [n(¹⁸⁵Re,^186*Re)gammas], where * = " " or "m", is not available, so it is unclear how much this second path contributes to ¹⁸⁷Re production.

Aside from the stable pair, one rhenium isotope can be expected to persist long enough after the event which led to its formation to be injected into a collapsing cloud core, ^186mRe. It will become extinct before formation of a stars-and-planets system proceeds far.

References

9-Period Periodic Table of Elements
1	1 H																																																																	2 He
2	3 Li	4 Be																																																																	5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg																																																																	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca																																																																	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr																																																																	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba																																																					57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra																																																					89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
8	119 Uue	120 Ubn																															121 Ubu	122 Ubb	123 Ubt	124 Ubq	125 Ubp	126 Ubh	127 Ubs	128 Ubo	129 Ube	130 Utn	131 Utu	132 Utb	133 Utt	134 Utq	135 Utp	136 Uth	137 Uts	138 Uto	139 Ute	140 Uqn	141 Uqu	142 Uqb	143 Uqt	144 Uqq	145 Uqp	146 Uqh	147 Uqs	148 Uqo	149 Uqe	150 Upn	151 Upu	152 Upb	153 Upt	154 Upq	155 Upp	156 Uph	157 Ups	158 Upo	159 Upe	160 Uhn	161 Uhu	162 Uhb	163 Uht	164 Uhq	165 Uhp	166 Uhh	167 Uhs	168 Uho	169 Uhe	170 Usn	171 Usu	172 Usb
9	173 Ust	174 Usq
Alkali metal Alkaline earth metal Lanthanide Actinide Superactinide Transition metal Post-transition metal Metalloid Other nonmetal Halogen Noble gas predicted predicted predicted predicted predicted predicted predicted predicted predicted