The elemene 'X' has only three shells in which the difference in number of electrons between last shell and second last shell is equal to number of electron in the first shell. This element 'X' burns in oxygen to produce compound y, which turns acidified potassium dichromate paper green. Y turns moist blue litmus paper red due to the formation of componund Z. Identify X, Y and Z.

Explanation

The element X is sulphur which burns in oxygen to produce compound Y which `(SO_2)` turns acidified potassium dichromate paper to green. When moist blue litmus paper is introduced to `SO_2`, it turns red due to formation of compound Z which is `H_2SO_3` .
` S + O_2 to SO_2 , SO_2 + H_2O to H_2SO_3`
`SO_2 + K_2Cr_2O_7 H_2SO_4 to K_2SO_4 + underset("green")(Cr_2(SO_4)) + H_2O`