Might be a bit late but in GDocs you can use the insert equation tool and select the X subscript b superscript a and you can then the chemical symbol before or after and insert subscript and superscript on both sides. To open MathType to write an equation choose either Insertedit math equation or Insertedit chem formula from the Add-ons menu. In the ribbon at the top click on Insert.
This is an example of an equation represented by subscript.
You can copy and paste formulas created with MathType from Google Docs to Gogle Slides and vice versa. Click on the drop-down menus and select one of the symbols to create an equation. A text box will appear along with a new toolbar with drop-down menus for Greek letters miscellaneous operations relations math operators and arrows. Balance the chemical equation.