Chromate is a polyatomic ion with a charge of negative two. it contains chromium and oxygen in a one-to-four ratio. what is its formula?

Because the ratio of the chromium to oxygen should be 1 is to 4, the first formula that comes to mind is , with 1 chromium atom and 4 oxygen atoms.

The most common oxidation states of chromium are +6, +3, and +2. Less common are the oxidation states +5, +4, and +1 states, which are present in a few stable compounds. 

The oxide ion has a charge of -2. With the chromate formula being , the chromium oxidation state (x) is calculated to be 

                            x + 4*(-2) = -2
                                        x  = -2+8
                                        x  = +6

The chromium oxidation state is +6, which is a known common oxidation state of chromium. 

Testing other 1:4 ratios of Cr and O by multiplying the ratio with different factors would give, 
 
               
               

and so on. 

Determining the oxidation state (x) of chromium for the two possible chromate ion formulas,

1.  

                          2x + 8*(-2) = -2
                                      2x  = -2+16
                                      2x  = +14
                                        x  = +7

2.  

                        3x + 12*(-2) = -2
                                      3x  = -2+24
                                      3x  = +22
                                        x  = +7.3

As observed, using chromate ion formulas in which the 1:4 ratio is multiplied by factors greater than 1, the calculated oxidation states of chromium are not  considered to be possible. Therefore, the only possible formula for the chromate ion with a chromium to oxygen ratio of 1:4, and an ionic charge of -2 is . 
. 

Yet both compounds and elements are considered pure substances. Pure compounds are created when elements combine permanently, forming one substance. For example, when the elements mercury and oxygen combine, and a chemical reaction takes place, mercury (II) oxide is created.

Hope this helps!