Question
Thu September 12, 2013 By: Harini A
 

why is it 3d(10 elements) in the fourth period and not 4s(2 elements),4d(10 elements),4p(6 elements)? pls explain...why is '3'd coming there??

Expert Reply
Thu September 12, 2013
The number of elements we decide by considering the subshells according to aufbau principle and the 4s starts fiiling before 3d ,and 5s starts before 4d, so we donot consider 4d ..and so the fourth period  have 18 elements.
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