The subshell energy ordering within the third and fourth principal energy levels often surprises beginning students. In the region bounded by the 3p, 3d, 4s, and 4d subshells, measurable spectra reveal departures from the simple expectation that all subshells of a lower principal quantum number should be filled before any subshells of a higher principal quantum number.
Use your knowledge of atomic structure to analyze this ordering trend.
(a) List, in the order in which they are filled in the ground state of atoms, all subshells beginning with 3p and ending with 4d.
(b) Explain—using the ideas of radial node distribution, electron penetration, and effective nuclear charge—why the 4s subshell is filled before the 3d subshell.
(c) Describe how the filling order you listed in part (a) would change, if at all, for elements with atomic number greater than 56, and account for your reasoning.