Saw Mui, a 19 year old female from the Internet asks on May 5, 1998,Why does the Periodic Table of the Elements have two separate tables - one above and one below?
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The transition elements are separated from the main group elements in some versions of the periodic table to more clearly show that they are elements with electrons which partly fill d or f sublevels for these.
The main group elements comprise those which have partly filled s and p sublevels.
The lanthanides (rare earth elements) and the actinides (heavy rare earth elements) are usually placed in separate boxes of the periodic table for the same reasons. The lanthanides include those elements in which the 4f subshell is filled with electrons, and the actinides involve those elements which involve the filling of the 5f subshell with electrons. 46
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