Hahahah ok, this may be a bit beyond the scope of the class you're taking, but -
Every compound is not purely ionic or covalent, and each has what's called an 'ionic character'. This is figured out using the electronegativity difference of the atoms in the molecule.
In this case, Chromium has an electronegativity of 1.66, and Oxygen is 3.44.
3.44 - 1.66 = 1.78, which is just BARELY considered ionic, chromate's bonds are just 54% ionic.
As for the structure of chromate, that one is a bit tricky. I was able to sketch one out, but it was butt-ugly and I don't think it's right at all. So I guess you'll have to wait for someone else to answer that part hahahah...but with chromiums 3d sublevel with 5 electrons, maybe they're somehow used in the bonds? Because one measly 4s electron for 4 oxygen atoms with 2 holes in each 2p sublevel to fill doesn't seem to cut it.
I wouldn't worry about the structure though - you'll probably never have a need to know it anyway.