Question

why fourth period contains 18 elements instead of 32​

Answer 1

After the filling of 4s orbital, instead of 4p orbitals, there starts the filling 3d orbitals. ... Thus 10 elements are built up by the filling of 3d orbitals. ... Hence the fourth period consists up of 18 and not 8 elements.

Answer 2

Explanation:

After the filling of 4s orbital, instead of 4p orbitals, there starts the filling 3d orbitals. ... Thus 10 elements are built up by the filling of 3d orbitals. ... Hence the fourth period consists up of 18 and not 8 elements.

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