THE GENERAL FEATURES OF TRANSITION METAL CHEMISTRY

This page explains what a transition metal is in terms of its electronic structure, and then goes on to look at the general features of transition metal chemistry. These include variable oxidation state (oxidation number), complex ion formation, coloured ions, and catalytic activity. You will find some of this covered quite briefly on this page with links to other parts of the site where the topics are covered in more detail. The electronic structures of transition metalsWhat is a transition metal? The terms transition metal (or element) and d block element are sometimes used as if they mean the same thing. They don't - there's a subtle difference between the two terms. We'll explore d block elements first: d block elements You will remember that when you are building the Periodic Table and working out where to put the electrons using the Aufbau Principle, something odd happens after argon. At argon, the 3s and 3p levels are full, but rather than fill up the 3d levels next, the 4s level fills instead to give potassium and then calcium. Only after that do the 3d levels fill.
	Note:  If you aren't sure about atomic orbitals and electronic structures, you really need to follow this link before you go on. It takes you to a page explaining atomic orbitals and then on to other pages about electronic structures.If you do follow the link, use the BACK button on your browser (or the History file or Go menu) to return quickly to this page.
The elements in the Periodic Table which correspond to the d levels filling are called d block elements. The first row of these is shown in the shortened form of the Periodic Table below. The electronic structures of the d block elements shown are: Sc [Ar] 3d14s2 Ti [Ar] 3d24s2 V [Ar] 3d34s2 Cr [Ar] 3d54s1 Mn [Ar] 3d54s2 Fe [Ar] 3d64s2 Co [Ar] 3d74s2 Ni [Ar] 3d84s2 Cu [Ar] 3d104s1 Zn [Ar] 3d104s2 You will notice that the pattern of filling isn't entirely tidy! It is broken at both chromium and copper.

Réference: http://www.chemguide.co.uk/inorganic/transition/features.html