The enrgy level fo 4s-orbital is less than 3d- orbital because :

4s orbitals has less energy than 3d orbital

4s orbitals has less energy than 3d orbital

4s orbitals has less energy than 3d orbital

The energy of the electron in the 3d orbital is less than that in the 4s orbital in the hydrogen atom

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 ?

Assertion: When the atoms of first transition series ionize, the 4s-orbital electrons are ionized before 3d-orbital electrons. Reason: The energy of 3d-orbital electron is lower than that of 4s-orbital electrons.