Improve the documentation of the data files.
-# periodic_table_of_elements.tsv
+# Periodic Table of the First 18 Elements
-This file is based on a [GitHub project by Goodman Sciences](https://gist.github.com/GoodmanSciences/c2dd862cd38f21b0ad36b8f96b4bf1ee.js).
+These files are based on a [GitHub project by Goodman Sciences](https://gist.github.com/GoodmanSciences/c2dd862cd38f21b0ad36b8f96b4bf1ee.js).
The data was downloaded on 27-Jan-2021. It was reformatted as follows:
+## periodic_table_of_elements.csv
+
* Only the fist 18 data rows were retained.
- * Commas were replaced by tabs.
* Independent discovers were indicated by `|`.
* Discovery teams had spaces replaced by `_`.
+
+## periodic_table_of_elements.tsv
+
+In addition to the changes listed above, the following changes were made:
+
+ * Commas were replaced by tabs in `periodic_table_of_elements.tsv`
* An `id` column was inserted at the front of each row,
* with a copy of the element's symbol.
