Q.i. Write the order of the thermal stability of the hydrides of group 16 elements.

Group 16 elements react with hydrogen to form angular, sp3-hybridised hydrides of the type H2E (H2O, H2S, H2Se, H2Te, H2Po); H2O is a colourless, odourless liquid, while the rest are colourless, foul-smelling, poisonous gases. These hydrides are all weakly acidic, with acidity increasing and thermal stability decreasing from H2O to H2Te, because the H-E bond dissociation enthalpy falls steadily down the group; all except H2O also show reducing power, increasing H2S < H2Se < H2Te. Group 17 elements react with hydrogen (H2 + X2 -> 2HX) to give the hydrogen halides HF, HCl, HBr, HI; acidic strength increases HF < HCl < HBr < HI and thermal stability decreases in the same order HF > HCl > HBr > HI, both tracking the falling H-X bond dissociation enthalpy down the group. HF stands out as the only hydrogen halide that is a liquid at room temperature (b.p. 293 K), because of the strong hydrogen bonding uniquely available to it -- the same hydrogen-bonding logic that makes H2O, among the group 16 hydrides, the only liquid at room temperature too. Group 18 elements are chemically inert towards hydrogen altogether, forming no hydrides under ordinary conditions.