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4
(dated) mendelevium.
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, and 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.)