Question
Fri June 08, 2012 By: Tejaskumar Hiteshkumar Gajjar
 

According to (n+l) rule, in the 6th period of periodic table electrons must be arranged as 6s,4f,5d and 6p. But in this period, after filling the 6s-orbital(i.e. in Barium(Z=56)), the next electron (i.e. 57th electron of Lanthanum) enters the 5d-orbital instead of 4f-orbital. Why is it so ?

Expert Reply
Thu June 14, 2012
The next electron (i.e. 57th electron of Lanthanum) enters the 5d-orbital instead of 4f-orbital [Xe]5d16sbecause it gives stability due to 5d after that there is filling of the 4f orbital i.e. 14 electrons are filled up giving us the lanthanide series of 14 elements.
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