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The answer mentioned above is correct but there is also a shortcut built in which is math autocorrect. It is much like LaTeX. By default it's inactive but you can activate it and is really helpful if you want to write big equations. For example, if you want to type $\ce{H2}$H then you just have to type H_2.

Many more options are available. For example, if you want a superscript character, the caret (^) sign will be converted as superscript. Many such shortcuts are covered in these videos or you can simply search for How to insert mathematical equation (like LaTeX) in Ms-Office: Tips and tricks on YouTube.

These methods will be especially helpful if you are a fast at typing. Moreover, it will save your time which is lost while switching between keyboard and mouse and searching proper option in Word.