Typically, atoms tend to form chemical bonds by utilizing their valence electrons, which are the electrons in the outermost shell or energy level. The valence electrons are the ones involved in bonding because they have the highest energy and are more accessible for interaction with other atoms.
However, in certain cases, atoms can form bonds involving electrons from the second-to-last shell, also known as the penultimate shell or the antepenultimate shell. This phenomenon is observed in some transition metal elements and their compounds.
Transition metals, such as copper (Cu) or iron (Fe), have partially filled d orbitals in their second-to-last shell. These d electrons can participate in bonding, in addition to the valence electrons from the outermost shell. This behavior is responsible for the variable oxidation states and the ability of transition metals to form complex compounds.
When atoms utilize electrons from the second-to-last shell for bonding, it can lead to the formation of coordination complexes, where the central atom is surrounded by ligands that donate electrons to form coordinate bonds. These complexes exhibit unique properties, such as color, paramagnetism, and catalytic activity.
In summary, while valence electrons are primarily involved in bonding, certain elements, particularly transition metals, can utilize electrons from the second-to-last shell for bonding, leading to the formation of coordination complexes with distinct characteristics.