A Level - spd Notation of Transition Elements

The transition elements all have valence electrons in a d sub-shell, which is part of the reason that they make such good catalysts and form coloured solutions of their ions. 

Depending upon your exam board, a transition element is either defined as an element which forms ions with an incomplete d sub-shell or a element with an incomplete d sub-shell. Personally, I settle with the former definition, and for that reason both zinc (Zn) and scandium (Sc) are not classed as transition elements.

The electron configuration of scandium (Sc, above) shows that the 3d sub-shell contains only one electron. Scandium only forms Sc⁺ ions by losing the 3d electron as it has the highest energy, thus forming an ion without an incomplete d sub shell - not a transition element!

The story is similar for zinc (Zn, above), which forms Zn²⁺ ions, by losing the two electrons in the 4s, forming an ion with a complete 3d sub-shell - not a transition element! It loses the 4s electrons and not the 3d because it more energetically beneficial for it to have an empty 4s sub-shell and a full 3d sub-shell than it is for it to have a full 4s sub-shell and a partially-full 3d sub-shell. 

Now we have covered the elements in the series that are exempt from the title of 'transition element', let's turn out attention to Ti - Zn. By the time we get to chromium (Cr), there are 4 electrons in the 3d sub-shell and 2 still in the 4s:

However, the 3d sub-shell is really close to being half-full which would impart some stability (release some energy). As the energy gap between 3d and 4s is only very small, the energy required to promote a 4s electron to the 3d sub-shell in order to half-fill it is comparable to the energy released by half-filling it, therefore, this is exactly what happens:

Thus making the electron configuration of chromium 1s²2s²2p⁶3s²3p⁶4s¹3d⁵.

A similar phenomenon happens for copper, where the 3d can be completely filled by promoting a 4s electron:

This makes copper's electron configuration 1s²2s²2p⁶3s²3p⁶4s¹3d¹⁰.

So, in summary:

• Zinc and scandium are sometimes not considered transition metals as their ions do not have an incomplete d sub-shell. 
• Chromium promotes a 4s electron to half-fill the 3d sub-shell. 
• Copper promotes a 4s electron to fill the 3-d sub-shell completely.