Question

Why is zinc not a transition element?

Answer 1

Hint:The terms transition metal or element and d block element are sometimes used as if they mean the same thing but they are not the same and there's a subtle difference between the two terms. When we put the electrons using the Aufbau Principle, something different happens after argon. Transition elements are characterized by partially filled (n-1) d subshells.

Complete answer:
When we put the electrons using the Aufbau Principle, something different happens after Argon. At argon, the 3s and 3p levels are full, but rather than filling up the 3d levels next, the 4s level fills instead to give potassium (K) and then calcium (Ca). Only after that the 3d levels are filled. The elements in the Periodic Table which correspond to the d levels filling are known as d block elements.
A transition metal is one that forms one or more stable ions which have incompletely filled d orbitals. On the basis of this definition, scandium and zinc do not count as transition metals - even though they are members of the d block.
Zinc has the electronic structure [Ar] \[3{d^{10}}4{s^2}\]. When it forms ions, it always loses the two 4s electrons to give a \[2 + \]ion with the electronic structure [Ar] \[3{d^{10}}\]. The zinc ion has completely filled d orbitals and also it does not meet the definition either.
Therefore, zinc is not a transition element.

Note:
We should note that Scandium (Sc) has the electronic structure [Ar] \[3{d^1}4{s^2}\]. When it forms ions, it always loses the 3 outer electrons and ends up with an argon structure. The \[S{c^{3 + }}\] ion has no d electrons and so does not meet the definition. So, it is also not counted as the transition element.