Why Zn is not transition element

The term Transition element and d-block element are used in such a way that they mean the same thing. But in fact, there are some subtle differences between these two phrases. Zn is a d-block element but why Zn is not transition element. First, we know who the d block elements are.

If we look at the periodic table, we can see that there is no exception in the electron configuration until argon (Ar). But after argon, the electron fills 2s 2p 3s 3p and then goes to the 4s orbital instead of 3d to fill the low energy orbitals first according to the aufbau principle and then the electrons enter 3d orbital after filling the 4s. That means the last electron is going to the d orbital. The elements that have the last electron seen in their electron configuration go to d orbital are called d-block elements.

The d-block elements of period 4 are given below:

Sc (scandium)	[Argon] 3d1 4s2
Ti (titanium)	[Argon] 3d2 4s2
V (vanadium)	[Argon] 3d3 4s2
Cr (chromium)	[Argon] 3d5 4s1
Mn (manganese)	[Argon] 3d5 4s2
Fe (Iron)	[Argon] 3d6 4s2
Co (cobalt)	[Argon] 3d7 4s2
Ni (nickel)	[Argon] 3d8 4s2
Cu (copper)	[Argon] 3d10 4s1
Zn	[Argon] 3d10 4s2

Here the last electron of all goes to the d orbital. So, these are all d block elements. But not all d-block elements are called transition elements.

Among the d block elements, the elements that have d1-9 electrons in the d orbital of the stable cation electron configuration are called transition elements. In other words, a transition metal is an element that produces one or more stable ions and their d orbitals are incomplete. Based on this definition, scandium and zinc are not counted as transition metals although they are members of the d block elements.

What is stable cation

When an element releases electrons to become stable and turns into cation, that state of the element is called stable cation. We know that metals can usually release up to one, two or three electrons. In the case of transition elements, the d orbital of stable cation will remain incomplete. For example:

In the case of cobalt (Co)

Co	[Argon] 3d7 4s2
Co2+ (stable cation)	[Argon] 3d7

Cobalt is stabilized by releasing two electrons from 4s orbital.

In the case of Fe (Iron)

Fe	[Argon] 3d6 4s2
Fe2+ (stable cation)	[Argon] 3d6
Fe3+ (stable cation)	[Argon] 3d5

The stable cation of Fe can be Fe2+ and Fe3+, of which Fe3+ is more stable. Similarly, stable cation of Cu can be Cu+ and Cu2+, of which Cu2+ is more stable. Their electron configuration is as follows: 

Cu	[Argon] 3d10 4s1
Cu+ (stable cation)	[Argon] 3d10
Cu2+ (stable cation)	[Argon] 3d9

But looking at the electron configuration of the stable cations of Sc and Zn, it can be seen that Sc3+ has 0 electrons in d orbital and Zn2+ has 10 electrons in d orbital.

Sc³⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁰

Zn²⁺ = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰

Here it is clearly visible that Sc3+ does not have a d orbital but Zn2+ has a complete d orbital. So, they do not follow the definition of transition element.

Considering the other properties of transition elements, it is seen that they are capable of forming complex ions but Sc and Zn do not form complex ions. Furthermore, the transition elements form colored compounds but Sc and Zn do not form colored compounds.

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