Transition elements are groups of metallic elements that have partially filled d orbitals. They occupy the d-block of the periodic table (Group 3-12). Examples of elements that fall under this category are Nickel (Ni), Cobalt (Co), Copper (Cu), Zinc (Zn) etc. Transition metals have outstanding properties that distinguishes them from other elements.
One of these properties is their ability to form colored compounds due to their unfilled d electron shells. They form ions that are usually colored in solid compounds and in solution.