锕系元素

title: 性质.
content: 锕系元素皆为银灰色有光泽的放射性金属,半衰期随着原子序的扩大而依次缩短。锕系元素的性质较软,具有较高的密度及可塑性,在空气中会失去光泽。与镧系元素一样,锕系元素的化学性质比较活泼,能形成错合物及可溶于水的氯化物、硫酸盐、硝酸盐及高氯酸盐等,至于它们的氢氧化物、氟化物、硫酸盐及草酸盐等则不溶于水。
title: 争论.
content: 铹(Lr)是否属于锕系元素有争论。最近的色谱分析显示,铹应属于IIIB族,而非锕系。但为了方便叙述,现今仍习惯将其与锕系合称。参见镧系元素中关于镥的地位的争论。
title: 原子半径和离子半径(锕系收缩).
content: 与同族的钪、钇、镧、锕原子半径逐渐增大的规律恰恰相反,从钍到铹则是逐渐减小。这种锕系元素的原子半径和离子半径随原子序数的增加而逐渐减小的现象称为锕系收缩。  锕系元素中,充填最初几个元素的5f电子时,离子半径收缩地比较明显,但此现象后来趋于平缓,使得这些较重的锕系元素的离子半径十分接近。因此锕系元素在化学性质上的差别随着原子序数的增大而逐渐变小,以致逐个地分离锕系元素(尤其是重锕系元素)越来越困难。
title: 分布.
content: 地壳中含量最丰富的锕系元素为钍和铀,它们两者具有很长的半衰期,放射性较微弱,因此能在地壳中以稳定的量存在。主要的含铀矿物有钒酸钾铀矿和钙铀云母等,而钍主要分布在独居石、方钍石和钍石等矿物中。大多数含钍矿物中皆含有铀,反之亦然,且这些矿物中也都含有大量的镧系元素。而镤和锕主要分布在各种铀矿中,含量甚低。至于超铀元素大多不存在于自然界中,必须透过粒子加速器人工合成,只有部分较轻的元素如镎和钚等以痕量存在于铀矿中。
title: 用途.
content: 尽管部分锕系元素已在日常生活中得到了应用,例如烟雾侦测器中的镅和煤气网罩中的钍等,但锕系元素主要用于核武器,或当作核反应堆的燃料,例如铀和钚等。而原子序较大的重锕系元素由于制备的难度较高,且较不稳定,因此只用于学术研究,而没有实际用途。  在核反应堆最重要的同位素是铀-235。它被用于如核反应堆,且在天然铀里有0.72%的铀-235。它强烈吸收热中子,然后放出能量。铀235核子吸收中子后,会裂变成2个较轻的核子和2至3颗中子,如:  Other promising actinide isotopes for nuclear power are thorium-232 and its product from the thorium fuel cycle, uranium-233.  Emission of neutrons during the fission of uranium is important not only for maintaining the nuclear chain reaction, but also for the synthesis of the heavier actinides. Uranium-239 converts via β-decay into plutonium-239, which, like uranium-235, is capable of spontaneous fission. The world's first nuclear reactors were built not for energy, but for producing plutonium-239 for nuclear weapons.  钍除了用作煤气网罩的材料之外,也常和镁、铝等金属做成合金,因为镁钍合金不但坚固,还具有高熔点及高延展性,因此被广泛用于航空工业和导弹的生产中。钍还有着良好的电子发射性能,可用作电子器件中的高效电子发射极。此外钍的半衰期很长,和铀同位素的相对含量被广泛用于估算包括恒星在内的各种物体的年龄(参见放射性定年法)。  钚元素的主要用途是制造核武器,尤其是可以裂变的钚-239。它的临界质量是铀-235的1/3。  Plutonium-238 is potentially more efficient isotope for nuclear reactors, since it has smaller critical mass than uranium-235, but it continues to release much thermal energy (0.56 W/g) by decay even when the fission chain reaction is stopped by control rods. Its application is limited by the high price (about US$1000/g). This isotope has been used in thermopiles and water distillation systems of some space satellites and stations. So Galileo and Apollo spacecraft (e.g. Apollo 14) had heaters powered by kilogram quantities of plutonium-238 oxide; this heat is also transformed into electricity with thermopiles. The decay of plutonium-238 produces relatively harmless alpha particles and is not accompanied by gamma-irradiation. Therefore, this isotope (~160 mg) is used as the energy source in heart pacemakers where it lasts about 5 times longer than conventional batteries.  锕-227是中子源。 Its high specific energy (14.5 W/g) and the possibility of obtaining significant quantities of thermally stable compounds are attractive for use in long-lasting thermoelectric generators for remote use. Ac is used as an indicator of radioactivity in chemical research, as it emits high-energy electrons (2.18 MeV) that can be easily detected. Ac-Ra mixtures are widely used as an intense gamma-source in industry and medicine.  Development of self-glowing actinide-doped materials with durable crystalline matrices is a new area of actinide utilization as the addition of alpha-emitting radionuclides to some glasses and crystals may confer luminescence.
title: 对生物的影响.
content: 由于锕系元素皆具有放射性,对生物而言具有极高的毒性,过度暴露在它们散发出的辐射中会损害人体健康。因此含有锕系元素的核废料的处置是一个至关重要的课题。

Summary: 锕系元素是第89号元素锕到103号元素铹共15种放射性元素的统称。锕系元素也属于过渡元素,只是锕系元素的外层和次外层的电子构型基本相同,新增加的电子则大都填入了从外侧数第三个电子层(即5f电子层)中,所以锕系元素又可以称为5f系。为了区别于元素周期表中的d区过渡元素,故又将锕系元素及镧系元素合称为内过渡元素。由于锕系元素都是金属,所以又可以和镧系元素统称为f区金属。锕系元素用符号An表示。