National Institute of Standards and Technology (NIST)
NIST Atomic Spectra Database -- Ionization Energies Data
http://www.nist.gov/pml/data/ion_energy.cfm


1     1s                   2     S
2     1s2                  1     S
3     1s22s                2     S
4     1s22s2               1     S
5     1s22s22p             2     P
6     1s22s22p2            3     P
7     1s22s22p3            4     S
8     1s22s22p4            3     P
9     1s22s22p5            2     P
10    1s22s22p6            1     S
11    [Ne]3s               2     S
12    [Ne]3s2              1     S
13    [Ne]3s23p            2     P
14    [Ne]3s23p2           3     P
15    [Ne]3s23p3           4     S
16    [Ne]3s23p4           3     P
17    [Ne]3s23p5           2     P
18    [Ne]3s23p6           1     S
19    [Ar]4s               2     S
20    [Ar]4s2              1     S
21    [Ar]3d4s2            2     D
22    [Ar]3d24s2           3     F
23    [Ar]3d34s2           4     F
24    [Ar]3d54s            7     S
25    [Ar]3d54s2           6     S
26    [Ar]3d64s2           5     D
27    [Ar]3d74s2           4     F
28    [Ar]3d84s2           3     F
29    [Ar]3d104s           2     S
30    [Ar]3d104s2          1     S
31    [Ar]3d104s24p        2     P
32    [Ar]3d104s24p2       3     P
33    [Ar]3d104s24p3       4     S
34    [Ar]3d104s24p4       3     P
35    [Ar]3d104s24p5       2     P
36    [Ar]3d104s24p6       1     S
37    [Kr]5s               2     S
38    [Kr]5s2              1     S
39    [Kr]4d5s2            2     D
40    [Kr]4d25s2           3     F
41    [Kr]4d45s            6     D
42    [Kr]4d55s            7     S
43    [Kr]4d55s2           6     S
44    [Kr]4d75s            5     F
45    [Kr]4d85s            4     F
46    [Kr]4d10             1     S
47    [Kr]4d105s           2     S
48    [Kr]4d105s2          1     S
49    [Cd]5p               2     P
50    [Cd]5p2              3     P
51    [Cd]5p3              4     S
52    [Cd]5p4              3     P
53    [Cd]5p5              2     P
54    [Cd]5p6              1     S
55    [Xe]6s               2     S
56    [Xe]6s2              1     S
57    [Xe]5d6s2            2     D
58    [Xe]4f5d6s2          1     G
59    [Xe]4f36s2           4     I
60    [Xe]4f46s2           5     I
61    [Xe]4f56s2           6     H
62    [Xe]4f66s2           7     F
63    [Xe]4f76s2           8     S
64    [Xe]4f75d6s2         9     D
65    [Xe]4f96s2           6     H
66    [Xe]4f106s2          5     I
67    [Xe]4f116s2          4     I
68    [Xe]4f126s2          3     H
69    [Xe]4f136s2          2     F
70    [Xe]4f146s2          1     S
71    [Xe]4f145d6s2        2     D
72    [Xe]4f145d26s2       3     F
73    [Xe]4f145d36s2       4     F
74    [Xe]4f145d46s2       5     D
75    [Xe]4f145d56s2       6     S
76    [Xe]4f145d66s2       5     D
77    [Xe]4f145d76s2       4     F
78    [Xe]4f145d96s        3     D
79    [Xe]4f145d106s       2     S
80    [Xe]4f145d106s2      1     S
81    [Hg]6p               2     P
82    [Hg]6p2              3     P
83    [Hg]6p3              4     S
84    [Hg]6p4              3     P
85    [Hg]6p5              2     P
86    [Hg]6p6              1     S
87    [Rn]7s               2     S
88    [Rn]7s2              1     S
89    [Rn]6d7s2            2     D
90    [Rn]6d27s2           3     F
91    [Rn]5f26d7s2         4     K
92    [Rn]5f36d7s2         5     L
93    [Rn]5f46d7s2         6     L
94    [Rn]5f67s2           7     F
95    [Rn]5f77s2           8     S
96    [Rn]5f76d7s2         9     D
97    [Rn]5f97s2           6     H
98    [Rn]5f107s2          5     I
99    [Rn]5f117s2          4     I
100   [Rn]5f127s2          3     H
101   [Rn]5f137s2          2     F
102   [Rn]5f147s2          1     S
103   [Rn]5f147s27p        2     P
104   [Rn]5f146d27s2       3     F
105   [Rn]5f146d37s2       4     F
