Question

After filling up of 3p orbital, electrons enter into 4s orbital.But not 3d orbital .Why?​

Answer 1

Because according to Aufbau's principle,electrons get filled in the increasing level of energy of the orbitals,after 3p, among 4s(n+l=4) and 3d(n+l=5) energy of 4s is lower due to lower (n+l) value, therefore electrons tend to fill in the 4s orbital first.

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Answer 2

There is a proper sequence of these no. Which is from lower stability to higher stability and stability of 4s is lower than 3d so 4s will be fill up first.