Final answer:
The polyatomic ion in NaNO3 is the nitrate ion, represented as NO3-. In sodium nitrate, there is a 1:1 ratio of sodium ions to nitrate ions, so no parentheses or subscripts are needed in this case.
Step-by-step explanation:
The formula for the polyatomic ion in NaNO3 is NO3-, which is known as the nitrate ion. When writing the formula for an ionic compound such as sodium nitrate, NaNO3, we write the sodium ion, Na+, followed by the nitrate ion, NO3-. There is only one nitrate ion needed to balance the charge with one sodium ion, so no parentheses are necessary and no subscripts are added because the subscript would be one.