Chromium (Z = 24) and molybdenum (Z = 42) display experimentally determined ground-state electron configurations that differ from those predicted by a simple application of the subshell filling order. The chemistry of their ions also reflects these differences.
Use the information below to analyze and explain these observations.
• Atomic number: Cr = 24, Mo = 42
• First ionization energies (approximate): Cr = 653 kJ mol⁻¹; V = 650 kJ mol⁻¹
(a) Write the electron configuration that would be predicted for a chromium atom if subshells were filled strictly in order of increasing principal quantum number followed by increasing azimuthal quantum number.
(b) The experimentally observed ground-state configuration of chromium is [Ar]4s13d5. Explain why this arrangement is lower in energy than the configuration you wrote in part (a). Your explanation should address electron–electron repulsion and exchange energy.
(c) Write the ground-state electron configuration for a Cr3+ ion, and draw an orbital-box diagram for its 3d subshell.
(d) Using qualitative Coulombic reasoning and your configurations, predict which element has the larger first ionization energy—vanadium (Z = 23) or chromium (Z = 24)—and justify your prediction.