5.3.1 (a,b) What Are Transition Elements?

Properties

(a) the electron configuration of atoms and ions of the d-block elements of Period 4 (Sc–Zn), given the atomic number and charge Learners should use sub-shell notation e.g. for Fe: 1s²2s²2p⁶3s²3p⁶3d⁶4s².

(b) the elements Ti–Cu as transition elements i.e. d-block elements that have an ion with an incomplete d-sub-shell

At GCSE "Transition Element" means any element from the d-block (not that you would have called it the d-block at GCSE).

You also learn that Transition Elements have Variable Oxidation States, form colourful compounds and are good catalysts.

This is true but discounts Scandium and Zinc which (almost) only exist as Sc³⁺ and Zn²⁺ in compounds, (almost) only make compounds which are white as solids and dissolve to form colourless solutions, and are not useful as catalysts.

So, chemists have come up with a rather clunky definition of a Transition Element that discounts these two.

Unfortunately, it requires you to learn a little more about Electronic Configurations.

You probably know that up to Argon (element 18) the rules for filling shells apply without exception.

So Argon is 1s²2s²2p⁶3s²3p⁶

The next two elements are Potassium ₁₉K - 1s²2s²2p⁶3s²3p⁶4s¹ or [Ar] 4s¹

And Calcium ₂₀Ca - 1s²2s²2p⁶3s²3p⁶4s² or [Ar] 4s²

Because as you'll (hopefully) recall, the 4s sub-shell is unhelpfully at a slightly lower energy than the 3d - and electrons fill orbitals with the lowest energy first.

The actual energy levels of 3d and 4s are not necessarily the same for each element but rather than worrying about that it's easier to say that electrons are removed from the orbital with the highest principal quantum number first (usually the highest energy).

In the case of Period 4 d-block elements it means that 4s electrons are always removed before 3d electrons.

Scandium (₂₁Sc) has electronic configuration 1s²2s²2p⁶3s²3p⁶4s²3d¹ or [Ar] 4s²3d¹

As Scandium (almost) only does the Sc³⁺ state its ions have electronic configuration 1s²2s²2p⁶3s²3p⁶4s⁰3d⁰ or [Ar] 4s⁰3d⁰ or just 1s²2s²2p⁶3s²3p⁶

Zinc (₂₂Zn) has electronic configuration 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰ or [Ar] 4s²3d¹⁰

As Zinc (almost) only does the Zn²⁺ state its ions have electronic configuration 1s²2s²2p⁶3s²3p⁶4s⁰3d¹⁰ or [Ar] 4s⁰3d¹⁰

Neither Sc³⁺ nor Zn²⁺ has an incomplete d sub-shell (Scandium ions effectively have no d orbitals and all the d orbitals in Zinc ions are full).

This is what stops them forming colourful compounds (which you don't have to be able to explain but which simple of explanations of which are readily available if if you're interested).

And this is what is used to exclude them from the list of proper Transition Elements (but not from being d-block elements)

So, the definition of a Transition Element is now "A d block element which forms ions with an incomplete d sub-shell" - which may be a bit over-complicated but at least works

Just when you thought things were bad enough we come to the electronic configurations of actual Transition Elements.

Filling the orbitals is done exactly as you would imagine except for Chromium and Copper.

Your textbook will probably say that Chromium promotes a paired electron from 4s to 3d to minimise repulsion - which it would.

But that doesn't explain why the same doesn't happen in Sc, Ti, or V.

Rather than worry about exactly what is going on with the relative energies of partly filled sub-shells it really is easier to simply learn that when the 3d subshell is almost half filled or almost filled one electron is allowed to be "promoted".

Fortunately, the rule for removing electrons is the same so:

Cr is 1s²2s²2p⁶3s²3p⁶4s¹3d⁵ but Cr⁺ would be 1s²2s²2p⁶3s²3p⁶4s⁰3d⁵,

Cr²⁺ would be 1s²2s²2p⁶3s²3p⁶4s⁰3d⁴ and Cr³⁺ would be 1s²2s²2p⁶3s²3p⁶4s⁰3d³.

Cu is 1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰ but Cu⁺ would be 1s²2s²2p⁶3s²3p⁶4s⁰3d¹⁰,

Cu²⁺ would be 1s²2s²2p⁶3s²3p⁶4s⁰3d⁹.

This means that Copper (l) compounds don't have an incomplete d-subshell - so they're often (but not always) white.