When adding mathematical or chemical equations in Word, use subscripts to make characters much smaller and partially below the rest of the text line. Word converts existing text to subscript or sets subscript as the font type via controls in the Home tab or the Font dialog box.
Open the Home tab and highlight the text that you want converted to subscript.
Click the button labeled X2 in the Fonts group to convert the highlighted text to subscript.
This button actually turns the subscript option on so that any future text is subscripted. To make new characters appear as subscript, place the cursor on the desired location, and then click the X2 button. Click the button again to turn this option off.
Fonts Dialog Box
Highlight and right-click the text to be subscripted.
Select Font in the context menu.
Check the box next to Subscript in the Font tab and click OK.