Explain why compounds of Sc3+ are colorless, but compounds of Ti3+ are colored.

The electronic configuration of an atom describes the distribution of electrons in its atomic orbitals. For transition metals like Scandium (Sc) and Titanium (Ti), their configurations are significant as they determine the behavior and properties of the atoms when they form ions.

Take Scandium and Titanium, for instance, which in their neutral states have the configurations [Ar]3d1 4s2 and [Ar]3d2 4s2, respectively. Upon ionization, particularly when forming +3 ions, these elements lose their outermost 4s electrons first, and then the required number of 3d electrons to achieve a positive charge of +3. This process is crucial to understanding how these ions interact with light, as the presence or absence of 3d electrons affects the way they absorb and emit light.

Ionization is the process by which an atom or molecule acquires a positive or negative charge by gaining or losing electrons. For transition metals, the ionization process often includes the loss of 4s electrons before 3d electrons despite the 3d orbitals being filled first. This is due to the 4s electrons having a higher energy level compared to the 3d electrons once the atom is in its ground state.

The specific case of Sc^3+ and Ti^3+ illustrates this point. Scandium loses its three outermost electrons -- two from the 4s subshell and one from the 3d subshell. Titanium, on the other hand, loses two 4s electrons and just two of its 3d electrons. This difference in electron configuration post-ionization plays a pivotal role in the color manifestation of their compounds.

The color in compounds, especially those of transition metals, often stems from d-d electron transitions. These are transitions of an electron from one d-orbital to another within the same atom. In the context of Sc^3+ having a [Ar] configuration with no d electrons, there are no available d orbitals for such transitions; thus, no visible light is absorbed, rendering the compounds colorless.

Conversely, Ti^3+ retains a single d electron in its configuration ([Ar]3d1). This lone electron can be excited to a higher energy d orbital when the ion absorbs certain wavelengths of light. The wavelengths not absorbed are reflected or transmitted, and the compound appears colored as a result of this selective light absorption. This fascinating interplay between electron configuration and light interaction gives rise to the vivid and diverse colors of many transition metal compounds.