It's the second-lightest element
The second-
lightest exclusively radioactive element, promethium, has an atomic number of 61
... [DOC] [TLE] Period 2 element - Wikipedia [PAR] The period 2 elements are the chemical elements in the second row (or period) of 
the periodic ... With a density of 0.564 gcm3, lithium is the lightest metal and the 
least dense solid element.