The periodic table contains a series of fifteen metals beginning with lanthanum and ending with lutetium, which all have rather similar chemical properties. The reason for the mutual resemblance of all these metals is that the series corresponds with the gradual filling of the set of 4f orbitals from lanthanum (   4s2p6d10, 5s2 p6 d1 6s2) to lutetium (   4s2 p6 d10 f14 5s2 p6 d1 6s2) and that the 4f electrons do not greatly affect the chemical properties. The element yttrium (   4s2 p6 d1 5s2), which is closely related electronically to lanthanum and is placed immediately above it in the periodic table, shows a great resemblance to the 4f series of elements. In contrast, scandium (  3s2 p6 d1 4s2 ), the lightest metal of the triad Sc, Y and La, does not resemble the lanthanides very closely. On account of their chemical resemblance and their occurrence together in rare minerals (other more common sources are now known), the element ytterium and the series lanthanum to lutetium inclusive are collectively known as the rare earths. Strictly, rare earth means an oxide of one of these metals.
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