Elements of group-15, 16 and 17 form compounds with hydrogen with general formula MH3, H 2 M and HX (called hydrogen halides). Stability of these hydrides, along with M-H bond strength and basic nature decreases down the group. Boiling point of these hydrides mainly depends on the strength of their inter molecular forces.
Bond angle (HMH) among hydrides of group-16 decreases down the group because:

1. A Electronegativity of the central atom decreases down the group
2. B Size of central atom decreases down the group
3. C Bond pair-bond pair repulsion decreases down the group with the decrease in difference of electronegativity between hydrogen and central atom
4. D Lone-pair bond pair repulsion increases down the group