Find the oxidation number of the elements which are underlined in the compounds given below. Among these find out the elements which show variable oxidation numbers.
`MnO_2`, `Mn_2O_7`, `K_2Cr_2O_7`, `KCrO_3`, `MnCl_2`,` MgO`, `MgCl_2`, `Al_2O_3`, `AlCl_3`
(Hint : Oxidation number O= -2, Cl=-1, K=+1)

Explanation

MnO_2.
1*x + 2(-2) =0
x-4 = 0
x = -4

.Mn_2O_7.
2(x) + 7(-2) = 0
2x-14 = 0
2x=14
x= 7

.K_2Cr_2O_7.
2(1) + 2(x) +7(-2) = 0
2x-12 = 0
2x =12
x= 6

.KCrO_3.
1(1) + 1(x)+3(-2) = 0
x-5 = 0
x = 5

.MnCl_2.
1(x) + 2(-1) = 0
x-2 = 0
x = 2

MgO
x + (-2) = 0
x = 2

.MgCl_2.
1(x) + 2(-1) = 0
x-2 = 0
x = 2

.Al_2O_3.
2(x) + 3(-2) = 0
2x - 6 = 0
2x = 6
x = 3

.AlCl_3.
x + 3(-1) = 0
x -3 = 0
x= 3