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So, to be brutally clear,
are you saying that my code is a competent solution in this context?
If so, thank you for your explanations.
Otherwise, I need an alternative that would meet with approval.

Hi Stefan,

I have so many questions, it is hard to limit myself to the few that are most challenging.
In this case, I replaced xdef with gdef (which I had tried earlier) and this code failed.
I went back to xdef and it worked again. If you have a better alternative to xdef I will try it.
But, for ...

The answer was to use xdef. :roll:

% xdef defines the new control sequence in the global dictionary.
\foreach \Z/\atom/\name/\charge/\ions in \Element{
\expandafter\xdef\csname
ion\trimspaces\atom\endcsname{${\trimspaces\atom}^{\trimspaces\charge}$}
\immediate\write16{def{ion\trimspaces\atom ...

This loop works beautifully for extracting data from the \Element text block.
However, the function definitions made within the loop disappear when the loop finishes.
Is there a way to make the newly defined control sequences permanent globally?

\def\Element{
1 / H / Hydrogen / + / 1+,
11 / Na ...