Why are the Lanthanides and the Actinides given a separate position in the modern periodic table?
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Solution
Lanthanides are 14(Ce-Lu)elements that belong to the IIIB group and have valence electrons in the 4f subshell. Since they are placed after Lanthanum, they are called Lanthanides.
Actinides are 14(Th-Lw)elements that belong to the IIIB group and have valence electrons in the 5f subshell. Since they are placed after Actinium, they are called Actinides.
IIIB group position is in the middle of the periodic table but they occupy the bottom of the periodic table because of these elements fill-up the f subshell and belong to the f-block. The periodic table is separated into four blocks(s, p, d, f) where the extreme left portion is s-block, the right portion is p-block, the middle portion is d-block, and the bottom portion is occupied by f-block elements.
