Use the symbols templates or frameworks on the Equation toolbar to edit the equation. Use the equation feature designed for math but works okay for chemistry. You can click Write button and use your mouse or writing pad to write the formula here.
There are multiple equations in the drop-down list then scroll down and select one of them to meet your actual needs.
Word and click the Insert tab then you see the corner of the page two options which you need them for your documents. Hold down Alt key while typing. Insert Equation by Handwriting. They are used to help Word identify.