(i) O and Te belong to group 16 in the periodic table. We know that for hydrides of these elements, bond dissociation enthalpy decreases down the group. This means energy required to dissociate or break the bond decreases down the group ( O-S-Se-Te-Po) . This means that stability to heat decreases down the group. ( O hydride has most thermal stability and Po hydride has least thermal stability). Hence, thermal stability decreases from H2O to H2Te.
(ii) The heat energy released when new bonds are made between the ions and water molecules is known as the hydration enthalpy of the ion. This increases when ion size decreases. Smaller the size, more the attractions and hence more is the enthalpy. As fluoride ion has smaller size compared to chloride ion, it has higher hydration enthalpy than that of chloride ion.
(iii) Electronic configuration of nitrogen is 1s22s22p3. Nitrogen does not form pentahalides i.e NX5 due to non-availability of the d orbitals in its valence shell.
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