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Tutorial : Transition Metals
There are about 68 transition metals found over four periods (horizontal lines) of the periodic table.
The transition metals are shown in blue
The fourth period of the table starts with the atom potassium (K). Potassium has one electron in the fourth shell.
The next atom is calcium (Ca) that has two electrons in the fourth shell.
In both potassium and calcium the third shell contains 8 electrons but it is not complete. It can hold up to 18 electrons.
With the next ten atoms in the period, from Scandium (Sc) to Zinc (Zn), the additional electron in each atom is added to the third shell. The third shell is full, with 18 electrons, in the zinc atom.
Before the third shell is full, the electronic structure of the transition metals scandium to copper is less stable. An electron (or sometimes electrons) in third shell sometimes gains the energy to "jump" into the fourth shell (at a higher energy level). Taking iron as an example:
This means that there are two types of iron ions - one type, the iron II ion, is formed when 2 electrons are lost and the other type, the iron III ion is formed when 3 electrons are lost from the outer shell.
Fortunately, the way that the compounds are named tells you the type of ion present in the compound e.g. iron II chloride. The Roman numeral, II, refers to the valency of the original iron atom (the number of electrons that were in the outer shell of the iron atom).
Transition elements have variable valency but the name of the compound formed will always indicate the valency state - the number of electrons in the outer shell - of the transition element.
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