Manganese(II) phosphate has the chemical formula Mn3(PO4)2 and therefore consists of three manganese atoms bonded to two phosphate ions. The phosphate ions each have a charge of negative three.
For transition metals, such as manganese, the Roman numeral immediately following the element in a compound's name refers to the charge of the metal. This is necessary because many transition metals can form ions of several charges. For instance, the (II) immediately following manganese denotes that the manganese in this compound has a charge of positive two. Since phosphate always has a charge of negative three, two phosphate ions and three manganese ions are needed to balance the compound's charge.