\usepackage for Markdown LaTeX formulas.

[athyfr]

I need to use a LaTeX package inside a formula, as LaTeX doesn't have the feature I need by default (isotope formatting).

Why I need a package

Doing it the naive way creates an undesirable result (the spacing is weird):

$$ ^{210}_{85}At + \rightarrow ^{206}_{83}Bi + ^{4}_{2}He + energy $$

So I need to use a package to format it correctly. (The mhchem package)

How would I include a LaTeX package when the LaTeX code is inside a Markdown file? (for $LaTeX here$ format)

[jgm]

You will need to put the \usepackage{mchem} command in the document's preamble.
This can be done by using --include-in-header (with a file containing the code you want to add) or through YAML metadata (setting the variable header-includes):

---
header-includes: |
  \usepackage{mhchem}
...