[Jules18]

How do you know whether AsI₃ is ionic or covalent?

I was asked on an exam to name it, and I called it Arsenic triioide when the prof said it was Arsenic (III) iodide. 

I asked a sophomore TA one time how to tell when it's covalent, and she said the beginning column of the p subshell elements (the end of the transition metals) is her dividing line.  In other words, if they're both on the left right of that line, it's a covalent compound, and if they're on opposite sides it's ionic.
But apparently this is an exception, so she must have been mistaken.

And according to another exam written by students in the chem club, Sb₂Te₃ is ionic, so the dividing line isn't the metal-nonmetal dividing line. 
It's like you have to know the electronegativity differences or something, which they never give you. 
Does anyone have another rule?


~Jules~