Atom | neutral configuration | cation configuration | Slater Z* | Second Ionization Potential |
K | [Ar]4s1 | [Ar] = (1s)2(2s2p)8(3s3p)8 | Z*=19 � [7(0.35) + 8(0.85) + 2(1.00)] = 7.75 | 31.625 eV |
Ca | [Ar]4s2 | [Ar]3d1 = (1s)2(2s2p)8(3s3p)8(3d)1 | Z*=20 � [18(1.00)] = 2.0 | 11.871 eV |
Sc | [Ar]4s23d1 | [Ar]3d2 = (1s)2(2s2p)8(3s3p)8(3d)2 | Z*=21 � [1(0.35) + 18(1.00)] = 2.65 | 12.80 eV |
Ti | [Ar]4s23d2 | [Ar]3d3 = (1s)2(2s2p)8(3s3p)8(3d)3 | Z*=22 � [2(0.35) + 18(1.00)] = 3.30 | 13.58 eV |
V | [Ar]4s23d3 | [Ar]3d4 = (1s)2(2s2p)8(3s3p)8(3d)4 | Z*=23 � [3(0.35) + 18(1.00)] = 3.95 | 14.65 eV |
Z* nicely parallels the second ionization potential. It is hard to ionize K+ and this is indicated by the large Z*. All of the other elements ionize more readily and the second ionization potentials steadily increase along with Z*.
The electron configuration for Ca+ is d1, the same as any transition metal element. If the ion were 4s1, then the Slater Z* would be = 20 � [8(0.85) + 10(1.00)] = 3.20, which is out line with the observations. Z* = 3.2 suggests an ionization potential closer to Ti+.