Page "Promethium" Paragraph 4

A promethium atom has 61 electrons, arranged in the configuration &# 91 ; Xe &# 93 ; 4f < sup > 5 </ sup > 6s < sup > 2 </ sup >.

In forming compounds, the atom loses its two outermost electrons and one of the 4f-electrons, which belongs to an open subshell.

The element's atomic radius is the third largest among all the lanthanides but is only slightly greater than those of the neighboring elements.

It is the only exception to the general trend of the contraction of the atoms with increase of atomic radius ( caused by the lanthanide contraction ) that is not caused by the filled ( or half-filled ) 4f-subshell.