Name: Promethium Symbol: Pm Atomic Number: 61 Atomic Weight: 145. Density: 7.22 g/cc Melting Point: 1167.85 C Boiling Point: 2726.85 C Electron Configuration: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f5 5s2 5p6 6s2 Chemical Group: Rare Earth Metals Detailed Technical Data The data on this page is more precise and in some cases more up-to-date than the data engraved on the wooden tiles. But as always, never believe one source if the number really matters. Density, melting point and boiling point data is particularly variable and uncertain. There is no stable isotope for this element: The atomic weight given is for the longest-lived isotope.