While filling up electron in the atomic orbitals, 4s orbital is filled before 3d orbital but reverse happens during the ionisation of the atom. Explain why ?

While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

While filling up of electrons in the atomic orbitals, the 4s-orbital is filled before the 3d-orbital but reverse happens during the ionisation of the atom. Explain why ?

While filling up of electrons in the atomic orbitals, the 4s-orbital is filled before the 3d-orbital. Explain why.

Filling of Electrons in Molecular Orbitals

Why is 4s orbital filled before the filling of 3d orbital?