Question

Explain why Scandium (Z=21) is a transition element while Zinc (Z=30) is not.

Hint:

Elements of group 12 (Zn, Cd, Hg) are not true transition elements because their d-orbitals are completely filled.

Answer 1

Step 1: Definition of a Transition Element.
A transition element is defined as an element whose atom has an incomplete (n−1)d subshell or which forms at least one ion with an incomplete d-subshell. The presence of partially filled d-orbitals is the key requirement for an element to be classified as a transition element.

Step 2: Electronic Configuration of Scandium (Z = 21).
Scandium has the electronic configuration:
Sc: [Ar] 3d¹ 4s²

Here, the 3d subshell contains one electron, which means it is incompletely filled. Even when Sc forms its common ion Sc³⁺, the d-orbital initially participates in bonding during electron removal. Since the atom of scandium has a partially filled d-orbital, it satisfies the definition of a transition element.

Step 3: Electronic Configuration of Zinc (Z = 30).
Zinc has the electronic configuration:
Zn: [Ar] 3d¹⁰ 4s²

In this case, the 3d subshell is completely filled with 10 electrons. When zinc forms its common ion Zn²⁺, its configuration becomes:
Zn²⁺: [Ar] 3d¹⁰

Thus, both the atom and its stable ion have a completely filled d-subshell. Since there is no partially filled d-orbital in either form, zinc does not meet the criteria for being a transition element.

Step 4: Conclusion.
Scandium is considered a transition element because it has a partially filled d-subshell in its atomic state. Zinc, however, has a completely filled d-subshell in both its atomic and ionic states, and therefore it is not classified as a transition element.