Physical and Chemical Properties
Protactinium is an actinide which is positioned in the periodic table to the left of uranium and to the right of thorium, and many of its physical properties are intermediate between those two actinides. So, protactinium is more dense and rigid than thorium but is lighter than uranium, and its melting point is lower than that of thorium and higher than that of uranium. The thermal expansion, electrical and thermal conductivities of these three elements are comparable and are typical of "poor metals". The estimated shear modulus of protactinium is similar to that of titanium. Protactinium is a metal with silvery-gray luster that is preserved for some time in air. Protactinium easily reacts with oxygen, water vapor and acids, but not with alkali metals.
At room temperature, protactinium crystallizes in body-centered tetragonal structure which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic (fcc) upon cooling from high temperature, at about 1200 °C. The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×10−6/°C.
Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature. It becomes superconductive at temperatures below 1.4 K. Protactinium tetrachloride is paramagnetic at room temperature but turns ferromagnetic upon cooling to 182 K.
Protactinium exists in two major oxidation states, +4 and +5, both in solids and solutions, and the +3 and +2 states were observed in some solid phases. As the electron configuration of the neutral atom is 7s26d15f2, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f0 configuration. Both +4 and +5 states easily form hydroxides in water with the predominant ions being Pa(OH)3+, Pa(OH)2+
2, Pa(OH)+
3 and Pa(OH)4, all colorless. Other known protactinium ions include PaCl2+
2, PaSO2+
4, PaF3+, PaF2+
2, PaF−
6, PaF2−
7 and PaF3−
8.
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