# Why does $\mathrm{NO_3}$ have charge $1-$? [closed]

I'm given the following problem:

Write the molecular equation for the reaction $$\mathrm{Iron\;(III)\; Nitrate\; and\; Sodium \;Phosphate}$$

I begin by attempting to determine the empirical formula of $$\mathrm{Iron\;(III)\; Nitrate}$$. In class, we were taught to memorize the following table:

Group     Charge
#1        +1
#2        +2
#3        +3
#5        -3
#6        -2
#7        -1


Additionally, we were asked to memorize the "formula" for Nitrate ($$\mathrm{NO_3}$$).

The $$\mathrm{III}$$ after Iron indicates that it has a $$3+$$ charge. From the table above, $$\mathrm{N}$$ has charge $$3-$$, and $$\mathrm{O}$$ has charge $$2-$$. There are $$3$$ $$\mathrm{O's}$$, giving a net charge of $$-2 \cdot 3=-6$$.

Unable to determine the empirical formula of $$\mathrm{Iron\;(III)\; Nitrate}$$, I looked it up to find $$\mathrm{Fe(NO_3)_3}$$.

If $$\mathrm{NO_3}$$ has charge $$6-$$, then $$3$$ $$\mathrm{NO_3}$$'s has charge $$18-$$. Meanwhile, $$\mathrm{Fe}$$ has only charge $$3+$$.

As it turns out, $$NO_3$$ actually has charge $$1-$$. How is this determined? I'm told its "just something that needs to be memorized" in order to progress in the course.

As it turns out, the simple "adding and subtracting" of charge values (from the table given in the question) we were taught doesn't always (ever?) work. Drawing the Lewis structure for $$\mathrm{NO_3}$$, I obtain: