Which type of materials are typically characterized by their ability to form colored compounds?

Transition metals are characterized by their ability to form colored compounds primarily due to the presence of partially filled d-orbitals. When transition metals form compounds, the electrons in these d-orbitals can absorb specific wavelengths of light, resulting in the promotion of electrons to higher energy levels. This absorption of light leads to the transmission or reflection of specific colors, which is why many transition metal compounds appear vibrant and colorful.

The colors can vary depending on several factors including the metal involved, its oxidation state, the ligands attached to it, and the overall geometry of the compound. For example, copper(II) sulfate appears blue, while potassium dichromate is orange.

This behavior is contrasted with alkali metals, which typically form colorless compounds due to their simple s-orbital configurations and do not typically exhibit the same level of d-orbital complexity. Noble gases, known for their complete valence shells, do not usually form compounds at all, and when they do, they tend to be colorless as well. Halogens can form colored compounds, but they are less commonly known for this property compared to transition metals, which exhibit a broader range of colors across numerous compounds.