Select all the true statements. Group of answer choices Many transition metal compounds are colorless and diamagnetic. Lanthanide contraction is responsible for the shrinkage in atomic size between a group in period 5 to period 6. Period 6 contains some of the densest elements known. None of the period 4 transition metals, except Cu, are active enough to reduce H+ from aqueous acid to form hydrogen gas. The most common oxide of uranium is UO3.