How does the periodic table tell you the subscripts for elements in chemical formulas?
The periodic table does not tell you the subscripts for elements in chemical formulas. For the main group (or representative) elements, which are all of the elements except the transition and inner transition elements, it does give you the number of valence electrons for the elements in that group. This can help you to determine chemical formulas of some substances, especially ionic compounds, the diatomic molecules, binary covalent compounds, and other smaller compounds like water. For more complex molecules, you will need to use percent composition of the elements, molar mass, mole ratios, and molecular formula weight.
Group number: Valence electrons