Question

Which is the order of the thermal stability of hydrides of the oxygen family?
A.${H_2}Po < {H_2}Te < {H_2}Se < {H_2}S < {H_2}O$
B.${H_2}Po < {H_2}O < {H_2}Te < {H_2}Se < {H_2}S$
${H_2}S < {H_2}O < {H_2}Te < {H_2}Se < {H_2}Po$
${H_2}O < {H_2}S < {H_2}Te < {H_2}Se < {H_2}Po$

Answer 1

The size of the central atom of the molecule increases down the group which makes the bond dissociation energy to decrease. Therefore, thermal stability also decreases down the group.

Complete step by step answer:
The elements of group 16 are Oxygen, Sulphur, Selenium, Tellurium and Polonium. These atoms have six valence electrons. The hydrides of group 16 molecules have bent shape.
The ability of elements to attract electrons towards themselves rather than hydrogen atoms is called electronegativity. The electronegativity of group 16 hydrides is greater. This decreases when moving from top to bottom in the periodic table.
The properties of which are exhibited by the material when the heat is passed from it then, it is said to be thermal properties of material.
When moving from top to bottom the atomic size of the elements increases, which causes the bond dissociation enthalpy of the group 16 elements to decrease because less energy will be required for an atom to dissociate the bonds when the size of atom increases and distance from nucleus also increases. We know that thermal stability increases with increase in the bond dissociation enthalpy and decreases with decrease in the bond dissociation enthalpy.
Therefore, according to the question the atomic size for polonium (Po) will be greatest of group 16 elements and oxygen will have less atomic size. So, the order of thermal stability is –
${H_2}Po < {H_2}Te < {H_2}Se < {H_2}S < {H_2}O$
So, the correct option is (A).

Note: The atomic numbers or the number of protons and number of electrons increase from top to bottom down the group from oxygen. The elements of Group 16 form a covalent bond with the two hydrogen atoms. So, all the hydrides of Group 16 are covalent compounds with general molecular formula, ${H_2}E$, where $E$ is the element of group 16.