Energy Levels??

Question

I know that the 1st energy level takes 2 electrons , the 2nd 8 , 18, 32 and so on....but why does the 3rd level only take 8 and not 18

ie Ca : 2,8,8,2 why not 2,8,10 ??

Answer 1

The real rule is that the maximum capacity of the nth level is 2n squared. The second level takes 8, the third level temporarily stops at 8 also (with K and Ca), but its real total (achieved at zinc) is 18. The technical answer is to do with the overlap of energies of the 4s and 3d sublevels.