How many elements are there in 4^(th) period? Prove it.

1.	How many elements are there in 4^(th) period? Prove it.
Answer» Solution :In fourth period, 18 ELEMENTS are present. In this period electrons are entering into fourth energy level, ie, n=4. It starts with the filling of 4s-orbitals. However, after the 4s, but before the 4p orbitals, there are five 3D-orbitals also to be filled. Thus, nine orbitals (ONE 4s, five 3d and three 4p) have to be filled. These nine orbitals can ACCOMMODATE `(9xxx2=18)` 18 electrons. Hence, `4^(th)` period contain 18 elements in it.

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