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The Lanthanide (Lanthanoid) Series
Note: Promethium (Pm) does not occur naturally in the Earth's crust. It is a radio active element that has only been made artificially as a product of uranium fission.
Electronic Structure of the Atoms
It becomes clear (from the information shown in the table) why the properties of these elements are so similar. In most cases, the fifth shell (energy level) and the sixth shell each contain the same number of electrons.
The lanthanides are further divided into the "light" lanthanides (Lanthanum to Samarium) and the "heavy" lanthanides (Europium to Lutetium). The "light" lanthanides are more abundant in the Earth's crust than the "heavy" lanthanides (although even the least abundant lanthanide is more common than silver!)
All lanthanides are considered as Group 3 elements because they form 3+ ions.
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