Science, Tech, Math › Science Transition Metal Colors in Aqueous Solution Share Flipboard Email Print Heinrich van den Berg/Getty Images Science Chemistry Molecules Basics Chemical Laws Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated January 27, 2019 The transition metals form colored ions, complexes, and compounds in aqueous solution. The characteristic colors are helpful when performing a qualitative analysis to identify the composition of a sample. The colors also reflect interesting chemistry that occurs in transition metals. Transition Metals and Colored Complexes A transition metal is one that forms stable ions that have incompletely filled d orbitals. By this definition, technically not all of the d block elements of the periodic table are transition metals. For example, zinc and scandium aren't transition metals by this definition because Zn2+ has a full d level, while Sc3+ has no d electrons. A typical transition metal has more than one possible oxidation state because it has a partially filled d orbital. When transition metals bond to one more neutral or negatively charged nonmetal species (ligands), they form what are called transition metal complexes. Another way to look at a complex ion is as a chemical species with a metal ion at the center and other ions or molecules surrounding it. The ligand attaches to the central ion by dative covalent or coordinate bond. Examples of common ligands include water, chloride ions, and ammonia. Energy Gap When a complex forms, the shape of the d orbital changes because some are nearer the ligand than others: Some d orbitals move into a higher energy state than before, while others move to a lower energy state. This forms an energy gap. Electrons can absorb a photon of light and move from a lower energy state into a higher state. The wavelength of the photon that is absorbed depends on the size of the energy gap. (This is why splitting of s and p orbitals, while it occurs, does not produce colored complexes. Those gaps would absorb ultraviolet light and not affect the color in the visible spectrum.) Unabsorbed wavelengths of light pass through a complex. Some light is also reflected back from a molecule. The combination of absorption, reflection, and transmission results in the apparent colors of the complexes. Transition Metals May have More Than One Color Different elements may produce different colors from each other. Also, different charges of one transition metal can result in different colors. Another factor is the chemical composition of the ligand. The same charge on a metal ion may produce a different color depending on the ligand it binds. Color of Transition Metal Ions in Aqueous Solution The colors of a transition metal ion depend on its conditions in a chemical solution, but some colors are good to know (especially if you're taking AP Chemistry): Transition Metal Ion Color Co2+ pink Cu2+ blue-green Fe2+ olive green Ni2+ bright green Fe3+ brown to yellow CrO42- orange Cr2O72- yellow Ti3+ purple Cr3+ violet Mn2+ pale pink Zn2+ colorless A related phenomenon is the emission spectra of transition metal salts, used to identify them in the flame test. Continue Reading Do You Know Which Elements Are Transition Metals? Find Chemistry Definitions From A to Z Learn Key Properties of the Transition Metals What Is an Aqueous Solution? Chemistry Definition and Example Learn What Aqueous Means in Chemistry Why Are Transition Metals Called Transition Metals? 3 Common Chemical Reactions that Take Place in Water How Does a Flame Test Work? Transition Metal Definition Chemistry Vocabulary Terms You Should Know Learn How Firework Colors Work Chemistry Definition of Ligand See How To Work an Aqueous Solution Chemical Reaction Problem Here's How Metallic Bonding Works A List of All the Elements That Are Metals What Is a Complex Ion?