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4 state that does not oxidize water. Cerium has no biological role and is not very toxic. Other solid phases occurring only at high pressures are shown on the phase diagram. The energy of the 4f electron is nearly the same as that of the outer 5d and 6s electrons that are delocalized in the metallic state, and only a small amount of energy is required to change the relative occupancy of these electronic levels. This gives rise to dual valence states.
It appears that the valence changes from about 3 to 4 when it is cooled or compressed. At lower temperatures the behavior of cerium is complicated by the slow rates of transformation. Transformation temperatures are subject to substantial hysteresis and values quoted here are approximate. C, γ-cerium starts to change to β-cerium, but the transformation involves a volume increase and, as more β forms, the internal stresses build up and suppress further transformation. C will start formation of α-cerium but this is only from remaining γ-cerium. At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point.
Ce has been experimentally searched for. Ce with a half-life of 284. Ce with a half-life of 137. Ce with a half-life of 33. Ce with a half-life of 32. All other radioactive cerium isotopes have half-lives under four days, and most of them have half-lives under ten minutes. This is so because they are bypassed by the reaction flow of the s-process, and the r-process nuclides are blocked from decaying to them by more neutron-rich stable nuclides.
Ce can only be produced in the r-process. Cerium is a highly electropositive metal and reacts with water. 3 oxidation states to catalyse the reaction. The metal itself was too electropositive to be isolated by then-current smelting technology, a characteristic of rare-earth metals in general. 1830s that ceria was obtained pure. Wilhelm Hisinger was a wealthy mine-owner and amateur scientist, and sponsor of Berzelius.