Our best extrapolations suggest iodine, barium, or one of the lanthanide series of rare-earth elements.

Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series.

The term 'rare earth' is a misnomer: the metals, which include yttrium, and members of the lanthanide series in the periodic table, are comparatively abundant.

Part of the lanthanide series, holmium is a relatively soft and malleable silvery-white metallic element, which is stable in dry air at room temperature.

It is the last element in the lanthanide series, and traditionally counted among the rare earths.

Part of the lanthanide series, holmium is a rare earth element.

He developed the actinide concept which placed the actinide series beneath the lanthanide series on the periodic table.

All elements in the lanthanide series form M ions.

Across the lanthanide series, electrons are added to the 4f shell.

Erbium is a chemical element in the lanthanide series, with the symbol Er and atomic number 68.