Iridium

Iridium is a chemical element with symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is generally credited with being the second densest element (after osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser.

Iridium was discovered in 1803 among insoluble impurities in natural platinum. Smithson Tennant, the primary discoverer, named iridium for the Greek goddess Iris, personification of the rainbow, because of the striking and diverse colors of its salts. Iridium is one of the rarest elements in Earth's crust, with annual production and consumption of only three tonnes.

The most important iridium compounds in use are the salts and acids it forms with chlorine, though iridium also forms a number of organometallic compounds used in industrial catalysis and in research. Iridium metal is employed when high corrosion resistance at high temperatures is needed, as in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Iridium radioisotopes are used in some radioisotope thermoelectric generators. It is also called the most corrosion-resistant metal, although there is a delicate point here. Iridium does form two oxides with negative free energy of formation (exothermic), while gold's only known oxide has a positive free energy of formation^(1),(2). Considering the abundance of oxygen in the universe, iridium is more likely to be incorporated into cosmic dust.

The bulk metal can tolerate temperatures as high as 2000 °C in a reducing atmosphere, although certain molten salts and halogens are corrosive to to it. Finely divided iridium dust is much more reactive and can be flammable.

Iridium is found in meteorites with an abundance much higher than its average abundance in Earth's crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of dinosaurs and many other species 66 million years ago. It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

ISOTOPES AND FORMATION

At least 92 isotopes of iridium have been predicted, ranging from ²⁵³Ir to ¹⁶²Ir, of which 39 have been observed, as well as 34 isomers. ¹⁹⁴Ir and heavier isotopes decay by beta emission, with half-lives peaking at 19.3 hr in ¹⁹⁴Ir (see below). Among proton-rich isotopes ¹⁹⁰Ir and lighter half-lives peak at 13.2 days in ¹⁸⁹Ir and fall below 1 day at ¹⁸⁷Ir. Positive beta decay (electron capture or positron emission) is the only decay mode active between ¹⁹⁰Ir and ¹⁸⁴Ir. ¹⁸³Ir is reported to have a weak alpha decay branch [branch ratio (BR) = 0.0005]. Between ¹⁸²Ir and ¹⁷⁸Ir, only positron emission is reported, although it is not clear whether it is the only decay mode possible or whether these isotopes have a very weak [BR < 0.0001] alpha decay branch. Between ¹⁷⁷Ir and ¹⁷⁰Ir, positive beta decay predominates over alpha decay (with a weak exception at ¹⁷¹Ir). In ¹⁶⁹Ir and ¹⁶⁸Ir alpha predominates over positive beta decay. Then things get weird. Alpha decay is dominant over proton decay and positive beta decay in ¹⁶⁷Ir, below which positive beta decay no longer matters. Alpha decay is also dominant over proton decay in ¹⁶⁶Ir, but subordinate to it at ¹⁶⁵Ir. In ¹⁶⁴Ir and ¹⁶²Ir, proton decay is predicted to predominate.

¹⁹³Ir and ¹⁹¹Ir are effectively and observationally stable. ¹⁹²Ir is an odd-Z / odd-N nuclide sandwiched between two even-Z, even-N nuclides. As is often the case with such nuclides, it decays via two paths - mainly [BR = 0.9524] beta decay to ¹⁹²Pt but some [BR = 0.0476] electron capture to ¹⁹²Os. In addition, ¹⁹²Ir has two isomers - of which ^192m1Ir is short-lived but ^192m2Ir has a half-life of 241 yrs and is reported to decay by isomeric transition in a two-step process⁽³⁾. {Wikipedia's "Isotopes of Iridium" table does not include this information.} It is a high-spin isomer which may be completely blocked from either kind of beta decay and whose ability to emit a gamma is almost completely inhibited. It's half-life is 1120 times that of ground-state ¹⁹²Ir. [That pattern continues with ^194m2Ir (213 times ground-state half-life) and ^196mIr (97 times).]

Of the two stable isotopes, ¹⁹³Ir is the more abundant, which says something about where it formed. Both ¹⁹³Ir and ¹⁹¹Ir form via beta-decay chains from neutron-rich nuclides produced in supernovae and neutron star mergers via rapid neutron capture (r process), fission infall (especially in neutron star mergers), or high-A starting material (neutron star mergers only). That mechanism should produce ¹⁹³Ir and ¹⁹¹Ir in roughly equal amounts. In practice, ¹⁹³Ir is the more abundant, accounting for 0.627 of all atoms, while ¹⁹¹Ir provides 0.373 of the total. Both ¹⁹³Os and ¹⁹¹Os decay by beta emission, but their half-life ratio is 0.08. If free neutrons are present, but scarce, beta emission will compete with neutron capture as "decay" mechanisms for nuclides. When this occurs, the longer a nuclide's half-life, the more likely it is to capture a neutron before decaying. Due to ¹⁹¹Os's longer half-life, ¹⁹¹Ir will be about 1/10 as abundant as ¹⁹³Ir when neutrons are rare enough for the capture / decay mechanism to be called an "s process". Actual abundance may reflect a true s process or a period of declining neutron flux at the tail end of an r-process event.

Eleven isotopes of iridium, plus seven isomers, will persist more than 0.01 year after an event which led to their formation. Only one, ^192m2Ir, persists long enough to be injected into a cloud core which is beginning to collapse; but it will become extinct before formation of a system of stars and planets.

REFERENCES

1. "Gibbs Energy of Formation of Iridium Dioxide; H.KleykampL, J.Paneth; Journal of Inorganic and Nuclear Chemistry; Volume 35, Issue 2, February 1973, Pages 477-482.
2. "The Thermochemistry of Gold", 0. Kubaschewski and 0. von Goldbeck; (other ref. information missing); found via search for "heat of formation of gold oxide".
3. "Interactive Chart of the Nuclides" (NuDat 3); National Nuclear Data Center (NNDC); "decay radiation results" / "decay scheme".

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