In the pre-16 model of the atom, students are introduced to ... s [less than] p [less than] d As energy level increases, the size of the orbitals increases and they become more diffuse ie. a 1s ...

A further quandry arises when we examine the ionisation energies of the d-block elements. We find that it is the 4s orbital electrons that are ionised first before the 3d orbitals. We have to conclude ...