As explained here, AutoCorrect can be used to type in unformatted chemical formulae which then automatically add the correct subscripts and superscripts, or to type in an abbreviation which automatically expands to the long word or phrase. But it takes a long time to add each entry individually.
The link below is to a file created by Dave Rado:
https://wordmvp.com/Downloads/AutoCorrect.zip
Download the file and save it to your laptop. Opening the document will give you this message:
Pressing Backup will export your AutoCorrect table which can then be saved. Dozens of entries can then be added at the same time. Next time you open the document in the file above, press Restore and your expanded table is imported. Over time you can add hundreds of entries very easily.
The website below explains how to achieve the same result with a macro on the Quick Access toolbar:
Chemistry Word VBA macros 