the oxidation number of an element is the number of electrons that are gained or lost by the element to form a chemical bond. the net oxidation number of the ion is its charge. compound (NH₄)₂Cr₂O₇ is made of cation - NH₄⁺ and anion - Cr₂O₇²⁻ oxidation number of ion - -2 (oxidation number of Cr x 2 Cr atoms) +(oxidation number of O x 7 O atoms )= -2 oxidation number of Cr = y oxidation number of O = -2 (y x 2) + (-2 x 7) = -2 2y - 14 = -2 2y = 12 y = + 6 therefore oxidation number of Cr is +6
