What are transition metals?

The d-block elements are found between Group 2 and Group 3 in the periodic table. Across the d-block, electrons are filling d-orbitals and the highest energy sub-shell is a d sub-shell.

A transition element is a d-block element that forms at least one ion with an incomplete d sub-shell. Scandium and zinc are not classed as transition metals as they do not have any ions with partially filled d-orbitals.

scandium forms only the Sc3+ ion, in which d-orbitals are empty.
zinc forms only the Zn2+ ion, in which the d-orbitals are completely full.

The electron configuration of an element is the arrangement of the electrons in an atom of the element. Electrons occupy orbitals in order of energy level.

The sub-shell energy levels in the third and fourth energy levels overlap
The 4s sub-shell fills before the 3d sub-shell.

From scandium to zinc, the 3d orbitals are being filled. The pattern is regular except for chronium and copper, which do not follow the Aufbau principle for placing electrons in orbitals:

chronium - the 3d and 4s orbitals all contain one electron with no orbital being completely filled.
copper - the 3d orbitals are full, but there is only one electron in the 4s orbital

In these two elements it is suggested that electron repulsions between outer electrons are minimised, resulting in an increased stability of the chronium and copper atoms.

In chronium atoms, the 4s and 3d orbitals are all half-filled.
In copper atoms, the 3d orbitals are all filled.

In their reactions, transition element atoms lose electrons to form positive ions. Transition metals lose their 4s electrons before the 3d electrons. The 4s orbitals are filled first, but the 3d and 4s energy levels are very close together and once electrons occupy orbitals, the 4s electrons have a higher energy and are lost first.

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F325 - Equilibria, Energetics and Elements