The document discusses transition elements and inner transition elements. Transition elements have an electronic configuration of (n-1)d1-10ns1-2. They are placed in periods 4 to 7 and groups 3 to 12. These constitute the 3d, 4d, 5d and 6d series. Transition elements have a partially filled d orbital in their ground state or oxidation state. Common properties of transition elements include being hard, lustrous metals with high melting points and boiling points. They often form colored compounds due to d-d electron transitions. Many transition elements act as important catalysts in chemical reactions.