Which one of the following hydride is most volatile

Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Therefore

a) X-H bond enthalpy decreases

b) Thermal stability decreases….H₂O and H₂S are exothermic products,remaining three hydrides are endothermic products.

c) Reducing power increases

d) acidic strength (K_a) increases

e) p^K_a decreases

Down the group as vanderwaal's forces increases

Melting and boiling points increases

Expected BP and MP….H₂O<H₂S<H₂Se<H₂Te

Due to H-bonding in H₂O, it has higher BP and MP than expected.

BP trend…...H₂S<H₂Se<H₂Te<H₂O

MP trend….H₂S<H₂Se<H₂Te<H₂O

Central atom in these hydrides contain two lone pairs, according to Bent rules

As the Electronegativity of central atom decreases, bond angle decreases

VSEPR theory is required to explain the shape of water molecule, VBT can explain the shape of other hydrides;

Bond angle in water is about 105⁰ whereas it is 90⁰ in other hydrides;

Oxygen in water molecule is sp³ hybridized whereas pure p-orbitals of other chalcogens are involved in bonding in their hydrides;