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Table 1 Elemental analysis and comparison

From: PubChem atom environments

CASRS (1970) PubChem compound (2013)
Atomic symbol Occurrence (%) Incidence (%) Atomic symbol Occurrence (%) Incidence (%)
C 74.006 99.644 C 74.129 99.922
O 13.519 82.578 N 10.238 91.629
N 7.258 64.165 O 11.288 91.525
F 1.690 10.020 S 1.564 34.171
S 1.302 19.925 Cl 0.933 19.184
Cl 1.208 14.032 F 1.350 18.153
P 0.282 n.s. Br 0.266 6.468
Br 0.262 n.s. P 0.060 1.159
Si 0.114 n.s. Si 0.056 1.058
I 0.077 n.s. I 0.041 0.968
B 0.063 n.s. B 0.017 0.335
Sn 0.026 n.s. Na 0.012 0.211
Se 0.020 n.s. Y 0.004 0.071
As 0.020 n.s. Sn 0.003 0.064
  1. Incidence is used for ranking. Occurrence is calculated based on counts of non-hydrogen atoms. Incidence refers to the number of structures in the respective repository that contained at least one atom of the particular element. Frequencies determined in PubChem Compound are compared to a 1970 analysis of the Chemical Abstracts Service Registry System (CASRS) by Crowe et al. [46]. Data not supplied by Crowe et al. is indicated as ‘not specified’ (n.s.). Even though the CASRS data was generated using only 28,963 of 596,367 available compounds, the authors found their data to be nearly identical with that obtained by others from the full set.