Question

According to the Aufbau principle , the ${19}^{th}$ electron in an atom goes into the ______ orbital.

• A.
  4s
• B.
  3d
• C.
  4p
• D.
  3p

Answer 1

The correct option is A

4s
According to the Aufbau principle , electrons are filled in the orbitals in the order of increasing energy levels. The energy of the orbitals is governed by n+l value.Orbitals with lower energy is filled first.Energy of the orbital is given by n+l value. If the n+l value is the same then the electron goes into the orbital with lower n value.