Protactinium

In nature, protactinium is found in the Earth's crust, as decay products from ²³⁸U [4]. Protactinium has no stable isotopes, but the isotopes found in nature are ²³¹Pa (T_½ = 3.276 · 10⁴ years), ²³⁴Pa (T_½ = 6.75 hrs.) and ^234mPa (T_½ = 1.17 min.) [4].

In pure solid form, protactinium exist in a body-centered tetragonal crystal form [4]