Make the chemical formula example truly syntactically incorrect

[wzel]

Subscript before an element can be used to indicate its atomic number, which makes it syntactically legal.

[AllenDowney]

Thanks for reporting this, and thanks for reading Think Python.

I should note that I’m no longer maintaining the second edition, so I’m generally not making additional corrections or updates there.

The third edition is the current version and is actively maintained:

https://allendowney.github.io/ThinkPython/

If you switch to the third edition and still encounter a problem, feel free to open an issue there.