They’re not alkali metals, but one of these elements trapped in an optical lattice is the main component of the world’s most accurate clock. GBCAs (“G-B-C-A’s”) are agents that use one of these elements to shorten the T1 (“T-one”) relaxation time of water protons in tissue. Ligands like fod (“FODD”) or dpm (“D-P-M”) complexed to these elements can act as chiral shift reagents in NMR. The poor shielding ability of inner electrons causes (10[1])these elements to have a greater-than-expected (10[1])decrease in radii. The Swedish village of Ytterby (10[1])(“IT-er-bee”) names three of these elements, (10[1])whose 4f valence electrons are so bad at shielding that their atoms undergo a namesake “contraction” in radius. For 10 points, name these elements such as gadolinium and cerium which sit on the periodic table above the (10[1])actinides. (10[1])■END■