If you'd like to write equations in Word just go to Tools, Customize. Under Categories, click on Format. Under the right column (Commands) find Superscript. Click on it and drag it up to the formatting toolbar at the top of your screen. This is where your font, font size, and alignment is located. Release your mouse and the Superscript icon should insert itself into the formatting toolbar. If you don't see your formatting toolbar, go to View, Toolbars, and make sure that formatting is checked. Do the same thing for Subscript. This is the one that will use most often for chemical equations. In your Word document, type H2O4. Highlight the 2 and click on the Subscript tool. Do the same thing for the number 4. Your equation should now look like this:
If you really want to get creative, you can use the equation editor, but you'll probably have to add it where you can use it. Go back to Tools, Customize. Look under the Insert Category and find Equation Editor on the right side. Click and drag the Equation Editor where you want it. You can now make all sorts of symbols for mathematical and chemical equations.