Thorium

Thorium, Th, is the name of element 90. It is an actinide, a seventh-period f-block element. Thorium is common enough for minerals in which Th is the predominant metal to exist. It is also radioactive, but its principal isotope, ²³²Th, has a half-life exceeding 14 billion years, which means slightly less than half of any ²³²Th which formed in the early universe still exists. This element is of interest both for its chemistry and its physics.

NUCLEAR PROPERTIES

Aside from ²³²Th, three Th isotopes have half-lives exceeding one year. They are, in order of decreasing half-life: ²³⁰Th [between 75080 and 75680 yrs⁽¹⁾], ²²⁹Th [7180 to 7500 yrs], and ²²⁸Th [1.9100 to 1.9132 yr]. All these long-lived isotopes are present on earth today because they are daughters of long-lived ancestors: ²³⁸U, ²³⁸U via its fission channel, and ²³²Th, respectively. The half-life of ²³²Th itself is between 1.411E10 yrs and 1.399E10 yrs, with a mean at 1.405E10 yrs so most of the ²³²Th ever formed is still around.

Three more short-lived isotopes are present on earth today. Two of them, ²²⁷Th [18.68+/-0.09 days] and ²³¹Th [25.51+/-0.01 hrs], descend from ²³⁵U. The third, ²³⁴Th [24.10+/-0.03 days], is the alpha daughter of ²³⁸U.

Thirty-four isotopes of Th have been reported, which is about a quarter of the total number predicted to exist⁽²⁾. Most of the predicted ones are neutron-rich, so they will form in quantity in something like a supernova or neutron star merger. With the exception of the seven listed above, Th isotopes disappear within days of the event which formed them.

²³¹Th and ²³⁴Th decay exclusively through beta emission. The five other remaining isotopes all decay by alpha emission. Physicists have observed spontaneous fission in ²³⁰Th and ²³²Th, but it is so rare that these isotopes aren't significant as a neutron source in the way ²³⁸U is. Interestingly, Th has only one isotope whose half-life exceeds 100000 yrs, while U has five and Pu has two. Thorium's lighter isotopes are heavily affected by instability of nuclei with slightly more than 126 neutrons, so no isotopes comparable to ²³³U exist.

ATOMIC PROPERTIES

Thorium is an actinide. Comparison of the physical and chemical properties of Th with those of U reveals substantial differences, differences great enough so that Th and U are often found in different minerals. These comparatively large differences between two elements so close together is different from what happens in the lanthanides. Partly, this is the result of relativistic effects and the greater similarity between f and d energies than is true in lanthanides. In experimental work, though, radioactivity makes close measurement of actinides difficult, both by damaging crystal structure and by introducing impurities into a sample.

REFERENCES

1. "Isotopes of Thorium", Wikipedia
2. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011.

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