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铊(拼音:tā,注音:ㄊㄚ,粤拼:taa1;英语:thallium)是化学元素,符号为Tl,原子序为81,是质软的灰色贫金属,在自然界并不以单质存在。铊金属外表和锡相似,但会在空气中失去光泽。两位化学家威廉·克鲁克斯和克洛德-奥古斯特·拉米在1861年独立发现了这一元素。他们都是在硫酸反应残留物中发现了铊,并运用了当时新发明的火焰光谱法对其进行了鉴定,观测到铊会产生明显的绿色谱线。其名称“Thallium”由克鲁克斯提出,来自希腊文中的“θαλλός”(thallos),即“绿芽”之意。翌年,拉米用电解法成功分离出铊金属。
铊在氧化后,一般拥有+3或+1氧化态,形成离子盐。其中+3态与同样属于硼族的硼、铝、镓和铟相似;但是铊的+1态则比其他同族元素显著得多,而且和碱金属的+1态相近。铊(I)离子在自然界中大部分出现在含钾矿石中。生物细胞的离子泵处理铊(I)离子的方式也和钾(I)类似。
在商业开采方面,铊是硫化重金属矿提炼过程的副产品之一。总产量的60至70%应用在电子工业,其余则用于制药工业和玻璃产业。[2]铊还被用在红外线探测器中。放射性同位素铊-201(以水溶氯化铊的形态),在核医学扫描中可用作示踪剂,例如用于心脏负荷测试。
水溶铊盐大部分几乎无味,且都是剧毒物,曾被用作杀鼠剂和杀虫剂以及谋杀工具。这类化合物的使用已经被多国禁止或限制。铊中毒会造成脱发。[3]
性质
编辑铊金属非常软,可延展性很高,在室温下可以用刀切割。它具有金属光泽,但在接触空气之后,会变为蓝灰色,与铅相似。长期置于空气中的铊会形成厚厚的氧化表层。要保存它的光泽,可以将其浸泡在油里。当接触水后,会形成氢氧化铊。硫酸和硝酸能快速溶解铊,分别形成硫酸亚铊和硝酸亚铊,而氢氯酸则会使铊表面形成一层不可溶的氯化铊。[4]其标准电极电势为−0.34,比铁的−0.44稍低。
同位素
编辑铊共有25种同位素,原子量介乎184和210之间。稳定同位素有203Tl和205Tl,而204Tl则是最稳定的放射性同位素,半衰期有3.78年。[5]
202Tl(半衰期12.23天)可以在回旋加速器中合成,[6]而204Tl可以在核反应堆中对铊的稳定同位素进行中子活化制成。[5][5][7]
201Tl(半衰期73小时)会以电子捕获的方式进行衰变,并释放Hg X射线(约70至80 keV)以及总丰度为10%、能量分别为135和167 keV的光子。[5]它既能提供良好的示踪效果,又不会使病人承受过大的辐射剂量,所以是核医学成像的理想示踪剂。它是铊元素核子心脏负荷测试中最常用的同位素。[8]
208Tl(半衰期3.05分钟)是钍衰变链的自然产物之一。它所释放的2615 keV伽马射线是自然背景辐射中的一大主要高能特征。
化学性质
编辑铊的两个主要氧化态为+1和+3。当处于+1态时,铊化合物和钾或银的化合物十分相近,因此在元素刚被发现后不久,一些欧洲化学家(英国除外)曾把它当做碱金属。[9]:126
氧化态为+3的化合物与相对应的铝(III)化合物相似。它们具有较高的氧化性,如Tl3+ + 3 e– → Tl(s)反应的还原电势为+0.72 V。氧化铊(III)是一种黑色固体,在800 °C以上温度会分解,形成氧化铊(I)和氧气。[4]
化合物
编辑铊(III)
编辑铊的化合物类似于铝的化合物,是一种强氧化剂,但元素本身是不稳定的,也就是说铊三价离子和铊原子都是强还原剂。有些熟悉的混合价化合物像是三氧化四铊和二氯化铊就含有铊三价离子和铊原子。另一个铊三价离子的化合物—三氧化二铊,是一种黑色的固体,熔点约为800℃,由铊原子氧化而成。[4]
铊最简单的化合物是三氢化铊(TlH3),不过太不稳定而不能大量存在,这是因为铊在+3氧化态时的不稳定,以及铊6s和6p轨域与氢的1s轨域重叠不良所致。[10]与三氢化铊(TlH3)相比,铊的三卤化物更稳定,然而它的化学性质不同于其余较轻的13族元素,且在13族中是最不稳定的。例如:氟化铊(TlF3),具有β-BiF3(与三氟化钇相同)结构而不是13族较轻元素的三氟化物结构,且不会在水溶液中形成氟化铊复合阴离子(TlF4−)。铊的三氯化物和三溴化物在室温时能自身氧化还原得到单卤化物,三碘化铊(TlI3)含有线性三碘阴离子(I3−)并且是一价铊的化合物。[11]三价铊的倍半硫属卤化物并不存在。[12]
铊(I)
编辑铊(I)单卤化物是稳定的。与大型的Tl +阳离子一致,氯化物和溴化物具有氯化铯结构,而氟化物和碘化物是氯化钠结构的变形。与类似的银化合物一样,氯化铊、溴化铊和碘化铊有感光性[13]。铊(I)化合物的稳定性与13族其他化合物的差别在于:已知铊有稳定的氧化物、氢氧化物和碳酸盐,也有许多硫属化合物存在。[14]
复盐 Tl4(OH)2CO3显示其具有以羟基为中心的铊三角形,在[Tl3(OH)]2+的固态结构中反复出现。[15]
有机铊化合物
编辑铊的有机化合物倾向变成热不稳定的,与第13族热稳定性降低的趋势一致。铊在水溶液中形成稳定的[Tl(CH3)2]+离子;像是等电子二甲基汞和二甲基铅离子,它具有直线型的结构。三甲基铊和三乙基铊与镓、铟的化合物一样,都是低熔点的易燃液体。
历史
编辑1861年,威廉·克鲁克斯和克洛德-奥古斯特·拉米(Claude-Auguste Lamy)利用火焰光谱法,分别独自发现了铊元素。[16]由于在火焰中发出绿光,所以克鲁克斯提议把它命名为“Thallium”,源自希腊文中的“θαλλός”(thallos),即“绿芽”之意。[17][18]
在罗伯特·威廉·本生和古斯塔夫·基尔霍夫发表有关改进火焰光谱法的论文,[19]以及在1859至1860年发现铯和铷元素之后,科学家开始广泛使用火焰光谱法来鉴定矿物和化学物的成分。克鲁克斯用这种新方法判断硒化合物中是否含有碲,样本由奥古斯特·霍夫曼数年前交给克鲁克斯,是德国哈茨山上的一座硫酸工厂进行铅室法过程后的产物。[20][21]到了1862年,克鲁克斯能够分离出小部分的新元素,并且对它的一些化合物进行化学分析。[22]拉米所用的光谱仪与克鲁克斯的相似。以黄铁矿作为原料的硫酸生产过程会产生含硒物质,拉米对这一物质进行了光谱分析,同样观察到了绿色谱线,因此推断当中含有新元素。他友人弗雷德·库尔曼(Fréd Kuhlmann)的硫酸工厂能够提供大量的副产品,这为拉米的研究带来了化学样本上的帮助。[23]他判断了多种铊化合物的性质,并通过电解法从铊盐产生了铊金属,再经熔铸后制成了一小块铊金属。
拉米在1862年伦敦国际博览会上“为发现新的、充裕的铊来源”而获得一枚奖章。克鲁克斯在抗议之后,也“为发现新元素铊”而获得奖章。两人之间有关发现新元素的荣誉之争议持续到1862至1863年。争议在1863年6月克鲁克斯获选为英国皇家学会院士之后逐渐消退。[24][25]
铊一开始的最大用途是杀鼠剂。在多次意外之后,美国于1972年2月经第11643号行政命令禁止使用铊杀鼠剂。其他国家也接连实施禁令。[26]
存量及生产
编辑铊在地球地壳中并不属于稀有的物质,含量约为0.7 mg/kg,[27]主要存在于黏土、土壤和花岗岩中的钾基矿物内。然而在商业上从这些矿物开采铊却并不容易。铜、铅、锡等重金属硫化矿中含有微量的铊元素,这才是其最大的实际来源。[28][29]
含有铊的矿物包括硒铊银铜矿(TlCu7Se4)、硫砷铊铅矿(TlPbAs5S9,亦称红铊铅矿)以及红铊矿(TlAsS2)等。[30]黄铁矿中也含有微量的铊,铊是黄铁矿加工生产硫酸过程中的一种副产品。[2][31]
铊也可以从铅和锡矿的冶炼过程中取得。海床上所发现的锰结核含有铊,但如此的开采成本高昂,不切实际。开采过程还可能对生态环境造成破坏。[32]另外,以锑、砷、铜、铅或银为主要成分的一些矿物可以含有16%至60%的铊,但这类矿物极为罕见,所以并未成为商业开采的主要来源。[27]位于马其顿南部的阿尔沙尔矿场(马其顿语:Алшар)是历史上唯一一处开采铊的矿场。矿藏是几种稀有铊矿物的来源,如红铊矿,估计总的铊含量仍有500吨。[33]
铊是铜、锡和铅冶炼过程的副产品,[27]可以从烟气或熔渣中萃取出来。[27]这些物质都含有许多铊以外的矿物杂质,所以首先要进行纯化。原料经碱或硫酸浸洗后,可洗出铊元素,经沉淀移除更多的杂质。最后产生的硫酸铊可以经电解把铊金属堆积在铂片或不锈钢片上。[31]美国地质调查局估计,铊的全球总年产量为10吨左右。[27]产量在1995年至2009年间从15吨下降到10吨,降幅为33%。如果铊有更大的实际应用,例如仍在实验阶段的含铊高温超导体,根据目前铊矿藏的存量,产量是能够重新提高的。[27]
应用
编辑已淘汰的用途
编辑硫酸亚铊无臭无味,曾被广泛用作杀鼠剂和杀虫剂。自1972年起,美国已禁止硫酸亚铊的使用,[26]其他国家也接着陆续实施禁令。[2]人们曾使用铊盐来治疗癣等皮肤感染病,以及减轻肺结核病人夜间盗汗的情况。不过这一用途颇为有限,因为铊盐的治疗指数区间较窄,更先进的相应药物也很快将其淘汰了。[34][35][36]
光学
编辑溴化铊和碘化铊晶体硬度较高,而且能够透射波长极长的光线,所以是良好的红外线光学材料,商品名为KRS-5和KRS-6。[37]氧化亚铊可用来制造高折射率玻璃,而与硫或硒和砷结合后,可以制成高密度、低熔点(125至150 °C)玻璃。这种玻璃在室温下特性和普通玻璃相似,耐用、不溶于水,且具有特殊的折射率。[38]
电子
编辑硫化亚铊的电导率会随红外线的照射而变化,所以能应用于光敏电阻。[34]硒化铊被用于辐射热测量计中,以探测红外线。[39]在硒半导体中掺入铊,可以提高其效能,所以一些硒整流器中含有这种含铊半导体。[34]另一项铊的应用是在伽马射线探测器中的碘化钠里作掺杂物。碘化钠晶体内掺入少量铊,可以增强它产生电离闪烁的效果。[40]氧分析仪中的一些电极也含有铊元素。[2]
高温超导
编辑科学家正在进行有关铊高温超导体方面的研究,潜在应用包括磁共振成像及发电和电力传输等。这些研究在1988年首个铊钡钙铜氧超导体被发现之后开始。[41]铜酸铊超导体的临界温度超过120 K。一些掺汞的铜酸铊超导体在常压下的临界温度甚至超过130 K,几乎达到已知临界温度最高的铜酸汞超导体。[42]
医学
编辑在核医学广泛使用锝-99m之前,半衰期为73小时的铊-201曾经是核心动描记所使用的主要放射性同位素。今天,铊-201也被用于针对冠心病危险分层的负荷测试当中。[43]这一同位素的产生器与用来生成锝-99m的类似。[44]产生器中的铅-201(半衰期9.33小时)会经电子捕获衰变成铊-201。铅-201则是在回旋加速器中通过(p,3n)或(d,4n)反应分别对铊进行质子或氘核撞击而产生的。[45][46]
铊负荷测试
编辑铊负荷测试是闪烁扫描法的一种,它通过测量铊的含量来推算组织血液供应量。活心肌细胞拥有正常的钠钾离子交换泵。Tl+离子会与K+泵结合,进入细胞内。[47]运动以及腺苷、双嘧达莫等血管扩张剂都可以造成冠状动脉窃流。扩张了的正常动脉血液量和流速都会增加,梗死或缺血的组织则会呈现较小的变化。[48]这种血液重组现象是缺血性冠心病的征兆。通过比对负荷前后的铊分布情况,可以判断需要进行心肌血管重建术的组织部分。[47]
其他用途
编辑一种汞铊合金在铊含量为8.5%时形成共晶系统,其熔点为−60 °C,比汞的熔点还要低20 °C。这种合金被用于温度计和低温开关当中。[34]在有机合成方面,铊(III)盐(如三硝酸铊和三乙酸铊)可以为芳香烃、酮类、烯烃等的转化反应作试剂。[49]铊是镁海水电池阳极板的合金材料成分之一。[2]可溶铊盐加入镀金液中,可以加快镀金速度和降低镀金层的粒度。[50]
甲酸铊(I)(Tl(CHO2))和丙二酸铊(I)(Tl(C3H3O4))的等量混合水溶液称为克列里奇溶液(Clerici solution,亦称轻重矿分离液)。它是一种无臭液体,颜色会随铊盐浓度的降低而从黄色变为清澈。溶液在20 °C密度为4.25 g/cm3,是已知最重的水溶液之一。人们利用矿物在克列里奇溶液上漂浮的原理,测量各种矿物的密度。然而由于铊的毒性和溶液的腐蚀性,这种方法逐渐被淘汰了。[51][52]
碘化铊可以添加在金属卤化物灯中,优化灯的温度和颜色。[53][54]它可以使灯光靠近绿色,这对水底照明非常有用。[55]
毒性及污染
编辑铊及其化合物毒性极高,在处理时的安全措施需格外严格。迄今已有多件因铊中毒而死亡的案例。[56]铊需避免与皮肤接触,而在熔化铊金属时,也需保证充分的通风。铊(I)化合物的水溶性高,可以轻易透过皮肤吸收。根据美国劳工部,铊的允许暴露限值为,平均8小时内每平方米不超过0.1毫克。[57]经皮肤进入体内的铊可以超过经呼吸吸收的量。[58]铊对于人类是一种怀疑致癌物。[59]由于毒性高、几乎无味、可溶于水,所以历史上因意外或犯罪导致铊中毒死伤的案例并不鲜见。[25]
从人体移除铊元素的方法之一是使用能够吸收铊的普鲁士蓝。[60]病人每天需口服最多20克普鲁士蓝,药物通过消化系统后经粪便排出体外。血液透析和血液灌流方法也可以把铊从血液中移除。在治疗的后期阶段,病人需服用额外的钾,把铊从组织中带出来。[61][62]
根据美国国家环境保护局,铊的人为污染源包括水泥工厂所排放的气体、发电厂所燃烧的煤以及金属下水道。矿物加工时对铊进行淋溶的过程是造成水源中铊含量增高的主要原因。[29][63]
参考资料
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3. Z. Kristallogr. 2009, 224 (12): 563–567. Bibcode:2009ZK....224..563S. doi:10.1524/zkri.2009.1213. - ^ * (1861年3月30日)Crookes, William "On the existence of a new element, probably of the sulphur group," Chemical News, vol. 3, pp. 193–194; reprinted in: XLVI. On the existence of a new element, probably of the sulphur group. Philosophical Magazine. April 1861, 21 (140): 301–305.;
- (1861年5月18日)Crookes, William "Further remarks on the supposed new metalloid," Chemical News, vol. 3, p. 303.
- (1862年6月19日)Crookes, William "Preliminary researches on thallium," Proceedings of the Royal Society of London, vol. 12, pages 150–159.
- (1862年5月16日)Lamy, A. "De l'existencè d'un nouveau métal, le thallium," Comptes Rendus, vol. 54, pages 1255–1262.
- ^ Weeks, Mary Elvira. The discovery of the elements. XIII. Supplementary note on the discovery of thallium. Journal of Chemical Education. 1932, 9 (12): 2078. Bibcode:1932JChEd...9.2078W. doi:10.1021/ed009p2078.
- ^ Liddell, Henry George and Scott, Robert (eds.) "θαλλος 互联网档案馆的存档,存档日期2016-04-15.", in A Greek–English Lexicon, Oxford University Press.
- ^ G. Kirchhoff, R. Bunsen. Chemische Analyse durch Spectralbeobachtungen. Annalen der Physik und Chemie. 1861, 189 (7): 337–381. Bibcode:1861AnP...189..337K. doi:10.1002/andp.18611890702.
- ^ Crookes, William. Preliminary Researches on Thallium. Proceedings of the Royal Society of London,. 1862–1863, 12 (0): 150–159. JSTOR 112218. doi:10.1098/rspl.1862.0030.
- ^ Crookes, William. On Thallium. Philosophical Transactions of the Royal Society of London,. 1863, 153 (0): 173–192. JSTOR 108794. doi:10.1098/rstl.1863.0009.
- ^ DeKosky, Robert K. Spectroscopy and the Elements in the Late Nineteenth Century: The Work of Sir William Crookes. The British Journal for the History of Science. 1973, 6 (4): 400–423. JSTOR 4025503. doi:10.1017/S0007087400012553.
- ^ Lamy, Claude-Auguste. De l'existencè d'un nouveau métal, le thallium. Comptes Rendus. 1862, 54: 1255–1262.
- ^ James, Frank A. J. L. Of 'Medals and Muddles' the Context of the Discovery of Thallium: William Crookes's Early. Notes and Records of the Royal Society of London. 1984, 39 (1): 65–90. JSTOR 531576. doi:10.1098/rsnr.1984.0005.
- ^ 25.0 25.1 Emsley, John. Thallium. The Elements of Murder: A History of Poison. Oxford University Press. 2006: 326–327. ISBN 978-0-19-280600-0.
- ^ 26.0 26.1 Staff of the Nonferrous Metals Division. Thallium. Minerals yearbook metals, minerals, and fuels 1. United States Geological Survey. 1972: 1358.
- ^ 27.0 27.1 27.2 27.3 27.4 27.5 Guberman, David E. Mineral Commodity Summaries 2010: Thallium (PDF). United States Geological Survey. [2010-05-13]. 引用错误:带有name属性“USGS-CS2010”的
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- ^ Shaw, D. The geochemistry of thallium. Geochimica et Cosmochimica Acta. 1952, 2 (2): 118–154. Bibcode:1952GeCoA...2..118S. doi:10.1016/0016-7037(52)90003-3.
- ^ 31.0 31.1 Downs, Anthony John. Chemistry of aluminium, gallium, indium, and thallium. Springer. 1993: 90 and 106. ISBN 978-0-7514-0103-5.
- ^ Rehkamper, M; Nielsen, Sune G. The mass balance of dissolved thallium in the oceans. Marine Chemistry. 2004, 85 (3–4): 125–139. doi:10.1016/j.marchem.2003.09.006.
- ^ Jankovic, S. The Allchar Tl–As–Sb deposit, Yugoslavia and its specific metallogenic features. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 1988, 271 (2): 286. Bibcode:1988NIMPA.271..286J. doi:10.1016/0168-9002(88)90170-2.
- ^ 34.0 34.1 34.2 34.3 Hammond, C. R. The Elements, in Handbook of Chemistry and Physics 81st edition. CRC press. ISBN 0-8493-0485-7.
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- ^ Galvanarzate, S; Santamarı́a, A. Thallium toxicity. Toxicology Letters. 1998, 99 (1): 1–13. PMID 9801025. doi:10.1016/S0378-4274(98)00126-X.
- ^ Rodney, William S.; Malitson, Irving H. Refraction and Dispersion of Thallium Bromide Iodide. Journal of the Optical Society of America. 1956, 46 (11): 338–346. doi:10.1364/JOSA.46.000956.
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- ^ Nayer, P. S, Hamilton, O. Thallium selenide infrared detector. Appl. Opt. 1977, 16 (11): 2942. Bibcode:1977ApOpt..16.2942N. doi:10.1364/AO.16.002942.
- ^ Hofstadter, Robert. The Detection of Gamma-Rays with Thallium-Activated Sodium Iodide Crystals. Physical Review. 1949, 75 (5): 796–810. Bibcode:1949PhRv...75..796H. doi:10.1103/PhysRev.75.796.
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- ^ Thallium-201 production 互联网档案馆的存档,存档日期2006-09-13. from Harvard Medical School's Joint Program in Nuclear Medicine
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外部链接
编辑- Thallium at The Periodic Table of Videos(诺丁汉大学)
- Toxicity, thallium
- NLM hazardous substances databank – Thallium, elemental
- ATSDR – ToxFAQs
- CDC - NIOSH Pocket Guide to Chemical Hazards
IA 1 |
IIA 2 |
IIIB 3 |
IVB 4 |
VB 5 |
VIB 6 |
VIIB 7 |
VIIIB 8 |
VIIIB 9 |
VIIIB 10 |
IB 11 |
IIB 12 |
IIIA 13 |
IVA 14 |
VA 15 |
VIA 16 |
VIIA 17 |
VIIIA 18 | ||||||||||||||||||||
1 | H | He | |||||||||||||||||||||||||||||||||||
2 | Li | Be | B | C | N | O | F | Ne | |||||||||||||||||||||||||||||
3 | Na | Mg | Al | Si | P | S | Cl | Ar | |||||||||||||||||||||||||||||
4 | K | Ca | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | Ga | Ge | As | Se | Br | Kr | |||||||||||||||||||
5 | Rb | Sr | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd | In | Sn | Sb | Te | I | Xe | |||||||||||||||||||
6 | Cs | Ba | La | Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | Tl | Pb | Bi | Po | At | Rn | |||||
7 | Fr | Ra | Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | Nh | Fl | Mc | Lv | Ts | Og | |||||
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Characteristics[edit source]
编辑Physical[edit source]
编辑Energy required to promote an f electron to the d subshell for the f-block lanthanides and actinides. Above around 210 kJ/mol, this energy is too high to be provided for by the greater crystal energy of the trivalent state and thus einsteinium, fermium, mendelevium form divalent metals like the lanthanides europium and ytterbium. (Nobelium is also expected to form a divalent metal, but this has not yet been confirmed.)[10] In the periodic table, mendelevium is located to the right of the actinide fermium, to the left of the actinide nobelium, and below the lanthanide thulium. Mendelevium metal has not yet been prepared in bulk quantities, and bulk preparation is currently impossible.[11] Nevertheless, a number of predictions and some preliminary experimental results have been done regarding its properties.[11]
The lanthanides and actinides, in the metallic state, can exist as either divalent (such as europium and ytterbium) or trivalent (most other lanthanides) metals. The former have fnd1s2 configurations, whereas the latter have fn+1s2 configurations. In 1975, Johansson and Rosengren examined the measured and predicted values for the cohesive energies (enthalpies of crystallization) of the metallic lanthanides and actinides, both as divalent and trivalent metals.[12][13] The conclusion was that the increased binding energy of the [Rn]5f126d17s2 configuration over the [Rn]5f137s2 configuration for mendelevium was not enough to compensate for the energy needed to promote one 5f electron to 6d, as is true also for the very late actinides: thus einsteinium, fermium, mendelevium, and nobelium were expected to be divalent metals.[12] The increasing predominance of the divalent state well before the actinide series concludes is attributed to the relativistic stabilization of the 5f electrons, which increases with increasing atomic number.[14] Thermochromatographic studies with trace quantities of mendelevium by Zvara and Hübener from 1976 to 1982 confirmed this prediction.[11] In 1990, Haire and Gibson estimated mendelevium metal to have an enthalpy of sublimation between 134 and 142 kJ/mol.[11] Divalent mendelevium metal should have a metallic radius of around 194±10 pm.[11] Like the other divalent late actinides (except the once again trivalent lawrencium), metallic mendelevium should assume a face-centered cubic crystal structure.[1] Mendelevium's melting point has been estimated at 827 °C, the same value as that predicted for the neighboring element nobelium.[15] Its density is predicted to be around 10.3±0.7 g/cm3.[1]
Chemical[edit source]
编辑The chemistry of mendelevium is mostly known only in solution, in which it can take on the +3 or +2 oxidation states. The +1 state has also been reported, but has not yet been confirmed.[16]
Before mendelevium's discovery, Seaborg and Katz predicted that it should be predominantly trivalent in aqueous solution and hence should behave similarly to other tripositive lanthanides and actinides. After the synthesis of mendelevium in 1955, these predictions were confirmed, first in the observation at its discovery that it eluted just after fermium in the trivalent actinide elution sequence from a cation-exchange column of resin, and later the 1967 observation that mendelevium could form insoluble hydroxides and fluorides that coprecipitated with trivalent lanthanide salts.[16]Cation-exchange and solvent extraction studies led to the conclusion that mendelevium was a trivalent actinide with an ionic radius somewhat smaller than that of the previous actinide, fermium.[16]Mendelevium can form coordination complexes with 1,2-cyclohexanedinitrilotetraacetic acid (DCTA).[16]
In reducing conditions, mendelevium(III) can be easily reduced to mendelevium(II), which is stable in aqueous solution.[16] The standard reduction potential of the E°(Md3+→Md2+) couple was variously estimated in 1967 as −0.10 V or −0.20 V:[16] later 2013 experiments established the value as −0.16±0.05 V.[17] In comparison, E°(Md3+→Md0) should be around −1.74 V, and E°(Md2+→Md0) should be around −2.5 V.[16] Mendelevium(II)'s elution behavior has been compared with that of strontium(II) and europium(II).[16]
In 1973, mendelevium(I) was reported to have been produced by Russian scientists, who obtained it by reducing higher oxidation states of mendelevium with samarium(II). It was found to be stable in neutral water–ethanol solution and be homologous to caesium(I). However, later experiments found no evidence for mendelevium(I) and found that mendelevium behaved like divalent elements when reduced, not like the monovalent alkali metals.[16] Nevertheless, the Russian team conducted further studies on the thermodynamics of cocrystallizing mendelevium with alkali metal chlorides, and concluded that mendelevium(I) had formed and could form mixed crystals with divalent elements, thus cocrystallizing with them. The status of the +1 oxidation state is still tentative.[16]
Although E°(Md4+→Md3+) was predicted in 1975 to be +5.4 V, suggesting that mendelevium(III) could be oxidized to mendelevium(IV), 1967 experiments with the strong oxidizing agent sodium bismuthatewere unable to oxidize mendelevium(III) to mendelevium(IV).[16]
Atomic[edit source]
编辑A mendelevium atom has 101 electrons, of which at least three (and perhaps four) can act as valence electrons. They are expected to be arranged in the configuration [Rn]5f137s2 (ground state term symbol2F7/2), although experimental verification of this electron configuration had not yet been made as of 2006.[18] In forming compounds, three valence electrons may be lost, leaving behind a [Rn]5f12 core: this conforms to the trend set by the other actinides with their [Rn] 5fn electron configurations in the tripositive state. The first ionization potential of mendelevium was measured to be at most (6.58 ± 0.07) eV in 1974, based on the assumption that the 7s electrons would ionize before the 5f ones;[19] this value has since not yet been refined further due to mendelevium's scarcity and high radioactivity.[20] The ionic radius of hexacoordinate Md3+ had been preliminarily estimated in 1978 to be around 91.2 pm;[16] 1988 calculations based on the logarithmic trend between distribution coefficients and ionic radius produced a value of 89.6 pm, as well as an enthalpy of hydration of −3654±12 kJ/mol.[16] Md2+ should have an ionic radius of 115 pm and hydration enthalpy −1413 kJ/mol; Md+ should have ionic radius 117 pm.[16]
Isotopes[edit source]
编辑Main article: Isotopes of mendelevium
Sixteen isotopes of mendelevium are known, with mass numbers from 245 to 260; all are radioactive.[21] Additionally, five nuclear isomers are known: 245mMd, 247mMd, 249mMd, 254mMd, and 258mMd.[2][22] Of these, the longest-lived isotope is 258Md with a half-life of 51.5 days, and the longest-lived isomer is 258mMd with a half-life of 58.0 minutes.[2][22] Nevertheless, the slightly shorter-lived 256Md (half-life 1.17 hours) is more often used in chemical experimentation because it can be produced in larger quantities from alpha particle irradiation of einsteinium.[21] After 258Md, the next most stable mendelevium isotopes are 260Md with a half-life of 31.8 days, 257Md with a half-life of 5.52 hours, 259Md with a half-life of 1.60 hours, and 256Md with a half-life of 1.17 hours. All of the remaining mendelevium isotopes have half-lives that are less than an hour, and the majority of these have half-lives that are less than 5 minutes.[2][21][22]
The half-lives of mendelevium isotopes mostly increase smoothly from 245Md onwards, reaching a maximum at 258Md.[2][21][22] Experiments and predictions suggest that the half-lives will then decrease, apart from 260Md with a half-life of 31.8 days,[2][21][22] as spontaneous fission becomes the dominant decay mode[2] due to the mutual repulsion of the protons posing a limit to the island of relative stability of long-lived nuclei in the actinide series.[23]
Mendelevium-256, the chemically most important isotope of mendelevium, decays through electron capture 90.7% of the time and alpha decay 9.9% of the time.[21] It is most easily detected through the spontaneous fission of its electron-capture daughter fermium-256, but in the presence of other nuclides that undergo spontaneous fission, alpha decays at the characteristic energies for mendelevium-256 (7.205 and 7.139 MeV) can provide more useful identification.[24]
y far.[31]