The transition elements, also known as transition metals, are a group of elements located in the middle of the periodic table. They have unique electronic configurations and exhibit different bonding behaviors compared to the main group elements.
The transition metals have partially filled d orbitals in their electronic configurations, which can accommodate more than 8 electrons. This is because d orbitals have a higher energy level than the s and p orbitals and are more diffuse, allowing for greater electron capacity.
When transition elements form ions, they typically lose their outermost s and p electrons first, as these are in the lower energy levels. However, the d electrons remain relatively stable and tend to be retained in the ion. As a result, transition metal ions often have a variable number of electrons in their d orbitals, exceeding the traditional octet rule.
For example, in the case of Fe³⁺ (iron ion with a +3 charge), the electron configuration of neutral iron (Fe) is [Ar] 3d⁶ 4s². When it loses three electrons to form Fe³⁺, the resulting configuration becomes [Ar] 3d³. As you mentioned, this gives a total of 13 electrons in the valence shell, which is beyond the octet rule.
The unique electronic configurations and bonding behaviors of transition elements contribute to their ability to form complex compounds and exhibit diverse chemical reactivity. The partially filled d orbitals allow for the formation of coordination complexes and the ability to act as catalysts, among other important roles in chemistry.