Uranium

Uranium, U, is the name of element 92. Wikipedia has an article which provides a lot of information about the element.  This article will focus on things Wikipedia does not stress:

Nuclear properties

Information sources

Japan Atomic Energy Agency (JAEA) maintains an on-line chart of nuclides which includes decay properties of many predicted nuclides - unlike charts published by Korea Atomic Energy Research Institute (KAERI) or the (U.S.) National Nuclear Data Center (NNDC). This chart gives separate numerical values for partial half-lives against fission, beta emission (both β^- and β⁺), and alpha emission. This reference provides the most focused look at the most significant predicted Pu isotopes. Other references used are cited.

Predicted and observed properties

Isotopes from the neutron dripline down to ²³⁹U, plus ²³⁷U are predicted or observed to decay primarily by beta emission. Half lives are predicted to increase, as A declines, from around 0.001 sec at the dripline to 8 sec at ²⁴⁷U and predicted to reach 3/4 hr in ²⁴¹U. ²⁴⁰U has an observed half-life of 14.1 hr, and ²³⁷U has a half-life of 6.75 days, making those the longest-lived beta-emitting isotopes.

²³⁸U, plus isotopes in the band ²³⁶U to ²³²U decay by alpha emission, Of these, only ²³²U has a half-life less than 10⁵ yr. They are, to say the least, well-studied.

Positive beta decay sets in at ²³¹U, which is almost entirely free of alpha decay (branch ratio = 0.0001). As A declines, alpha decay becomes increasingly important until positive beta decay disappears (below BR = 0.0001) at ²²⁷U. Only alpha decay has been observed in the 12 lightest isotopes, down to ²¹⁵U.

Neutron count in ²¹⁸U is 126, Half-lives for alpha emitting isotopes with N < 126 are in the millisecond range, which is longer than half-lives for isotopes with N slightly above 126. This is to be expected below a neutron shell closure.

²¹¹U has been predicted () to decay by fission, and for half-lives to drop below 10^-09 level for isotopes down to ¹⁹⁸U. Isotopes in the band ¹⁹⁷U to ¹⁹²U are predicted to have half-lives in the 10^-09 - 0.001 sec range.

Occurrence

Formation

a) Outside Earth

U isotopes from the neutron dripline down to ²³³U can form during supernovae and neutron star mergers. Rapid capture of electrons occurs in both, and some material ejected from a disintegrating neutron star has A high enough to beta-decay directly to U. Decay chains to lighter isotopes are blocked by ²³²Th, ²³¹Pa, and ²³⁰Th to ²²⁶Th. Below that, lower Z elements block decay chains.

b) On Earth

²³⁸U does not form on earth. Molar concentration ratio between ²³⁴U and ²³⁵U is 0.0075 and that of lighter isotopes is effectively 0. Neutrons from ²³⁸U spontaneous fission, produce small amounts of ²³⁵U and ²³⁶U (at 0.20 branch ration). Neutron capture by ²³⁶U produces the 6.75 day isotope ²³⁷U, so fresh ²³⁸U is the result of two neutron captures by the same nucleus within a few days of each other. With neutrons as rare as they are even in rich uranium ore, the chances of that happening are tiny.

It should be noted that the 0.8 branch ratio for fission in ²³⁵U is one of two cases of induced nuclear fission contributing to earth's energy sources. The other is ²³⁹Pu (0.64 branch ratio for fission).

Persistence

As is true of the other elements, most U isotopes are short lived. Only 9 have half-lives over a day. Of these, 5 have half-lives exceeding 10⁵ yr, and 3 have half-lives exceeding 2×10⁶ yr. Considering only material evolving from an initial supernova or kilonova, ²³³U (1.59×10⁵ yr) and ²³⁴U (246×10⁵ yr) can be expected to survive long enough to be incorporated into forming planetesimals. In the case of ²³⁴U, those primordial nuclei will eventually fall below the equilibrium concentration between ²³⁴U and ²³⁸U. By the time fully mature planets form, only ²³⁴U will remain. The half-life of ²³⁶U is 2.34×10⁷ yr which is roughly the span of time required for a mature terrestrial (rocky) planet to form. ²³⁶U would have been a part of the early earth, and its decay energy one of its heat sources. Eventually, the primordial nuclei decayed, and were replaced by those produced by the slow formation by neutron capture which continues to this day.

Atomic properties

Wikipedia's article "Uranium" discusses the element's properties in detail. Among those details is the presence of the element on earth. One thing that article did not touch on is the chemical activity of uranium in this context. The element reduces many metals, including iron. Earth's core is depleted in U, which means its mantle is enriched proportionately. Its crustal concentration is higher still.

References

2. "Nuclear Properties for Astrophysical Applications"; P. Moller & J. R. Nix; Los Alamos National Laboratory website; search by "LANL, T2", then "Nuclear Properties for Astrophysical Applications".

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

(12-05-20)