Writing Covalent Compound Formulas

You can predict the formula of an ionic compound, based on the loss and gain of electrons to reach a noble gas configuration. (For example, if you react Ca with Cl, you can predict the formula of the resulting salt: CaCl₂.) However, you really can’t make that type of prediction for covalent compounds, because they can combine in many ways, and many different possible covalent compounds may result.

Most of the time, you have to know the formula of the molecule you’re studying. But you may have several different types of formulas, and each gives a slightly different amount of information.