Final answer:
The thermal stability of group 6A compounds with hydrogen decreases as you move down the periodic table, due to a decrease in overlap between the hydrogen 1s orbital and the valence orbital of the larger chalcogen atoms.
Step-by-step explanation:
The thermal stability of group 6A compounds with hydrogen decreases as you move down the group in the periodic table. This is due to a decrease in overlap between the 1s orbital of hydrogen and the valence orbital of the chalcogen (group 6A element) as the size of the chalcogen increases. As a result, the larger the atom to which hydrogen is bonded, the weaker the bond becomes. For example, the bond between H and a large atom like tellurium (Te) is weaker than the bond between H and a smaller atom like oxygen (O), leading to lower thermal stability for compounds such as H2Te compared to H2O. Furthermore, the acid strength of binary hydrides increases with increasing atomic size within the group, as indicated by the decreasing pKa values from H2O to H2Te, with H2Te being the strongest acid among them.