why in the building of the atom, the filling of 4s-orbital take place

1.	why in the building of the atom, the filling of 4s-orbital take place before 3d-orbital?
Answer» Yes, becoz according to afbau principle, electron first occupy the lowest energy orbital and then filled to higher energy orbital, only after the lower one filled......... For ex- 4s=4+0=4....... For ex- 3d= 3+2=5..... As 4s orbital is lower energy orbital hence, it will filled first............... HOPE IT HELPS?<br>Ssooo the movement is 1s then 2s then 2p 3s then 3p 4s then 3d 4p 5s ....<br>Make a table1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f6s 6p 6d 7s 7p

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