Question

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to write a chemical formula, you need to know the involved and their numbers.
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Explanation:

A chemical formula represents a compound. To write it, elements (the basic substances) and their ratio (how many of each element are present) are essential. For example, in water ($H_2O$), the elements are hydrogen ($H$) and oxygen ($O$), and the ratio of hydrogen to oxygen atoms is 2:1.

Answer:

elements; ratio