Complex e-e repulsion plays important role in determining lowest energy state.SCANDIUM EXAMPLESc [Ar] 3d14s2Sc+[Ar] 3d14s1Sc2+[Ar] 3d1Sc [Ar] 3d14s2[Ar] 3d24s1∆E = 2.03 eVSc+[Ar] 3d14s2[Ar] 3d14s1∆E = 6.62 eVSc+[Ar] 3d14s2[Ar] 4s2∆E = 7.98 eVExcite an electron from 4s to 3dPhotoionize by removing either a 3d or a 4s electronIt seems to take less energy to remove a 4s electron than a 3d. Why does it not just populate the 3d states before the 4s? Can really just look at individual orbitals. The overall e-e repulsion is important in determining ground state configurations. 4s is more diffuse than 3d, but it also penetrates more to the nuclear core.In first row transition elements, 4s fills before 3d because the more diffuse nature of 4s reduces e-e repulsion. Exceptions occur when filled or half-filled subshells leads to increased stability.
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DICATIONS IN FIRST TRANSITION ROW•M2+ions all have no 4s electrons; all 3dx. •Removing electrons increases effective nuclear charge for all electrons; decreases diffuseness of 4s. •3d now become more stable than 4s and are filled for all multiply-charged ionic in that first row.
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