Ncert Exercises & Solutions

Which of the following arrangements represents the increasing oxidation number of the central atom?

1. ${CrO}_{2}^{-},$ ${ClO}_{3}^{-},$ ${CrO}_{4}^{2 -},$ ${MnO}_{4}^{-}$

2. ${ClO}_{3}^{-},$ ${CrO}_{4}^{2 -},$ ${MnO}_{4}^{-},$ ${CrO}_{2}^{-}$

3. ${CrO}_{2}^{-},$ ${ClO}_{3}^{-},$ ${MnO}_{4}^{-},$ ${CrO}_{4}^{2 -}$

4. ${CrO}_{4}^{2 -},$ ${MnO}_{4}^{-},$ ${CrO}_{2}^{-},$ ${ClO}_{3}^{-}$

Hint: The oxidation number of oxygen atom is -2.

Writing the oxidation number (O.N.) of Cr, Cl and Mn on each species in the four set ions, then,

(1)

\overset{+ 3}{Cr} O_{2}^{-},

\overset{+ 5}{Cl} O_{3}^{-},

\overset{+ 6}{Cr} O_{4}^{2 -},

\overset{+ 7}{Mn} O_{4}^{-}

(2)

\overset{+ 5}{Cl} O_{3}^{-},

\overset{+ 6}{Cr} O_{4}^{2 -},

\overset{+ 7}{Mn} O_{4}^{-},

\overset{+ 3}{Cr} O_{2}^{-}

(3)

\overset{+ 3}{Cr} O_{2}^{-},

\overset{+ 5}{Cl} O_{3}^{-},

\overset{+ 7}{Mn} O_{4}^{-},

\overset{+ 6}{Cr} O_{4}^{2 -}

(4)

\overset{+ 6}{Cr} O_{4}^{2 -},

\overset{+ 7}{Mn} O_{4}^{-},

\overset{+ 3}{Cr} O_{2}^{-},

\overset{+ 5}{Cl} O_{3}^{-}

Only in the arrangement (1), the oxidation number of central atom increases from left to right, therefore, option (1) is correct.