Question: how many atoms are there in one molecule of aluminum dichromate?
Answer: How many atoms are represented by one formula unit of aluminum dichromate Al2 Cr2O7 3? If you balance the charging of the magnesium ion and the phosphate polyatomic ion , you get a formula of Mg 3 ( PO 4 ) 2 . Now count 'em up : there exist 3 magnesiums , 2 phosphrous and eight oxygens .