Question

While filling up of electrons in atomic orbitals the 4s orbital is filled beore 3d oribital but reverse happen during the ionisation of atom. xplain why??

Answer 1

Atomic orbitals are filled in order of increasing energies. Since the energy of 3d orbital is more than 4s orbital, it is filled after 4s orbital. But during ionization, electrons in the outermost orbital are lost. Since, 4s will be the outermost orbital in this case; electrons from this orbital will be ionized first.