Question

Assertion:
Acidic character of group 16 hydrides increases from \[{H_2}O\] to \[{H_2}Te\].
Reason:
Thermal stability of hydrides decreases down the group.
A. Both assertion and reason are correct and Reason is the correct explanation for assertion.
B. Both assertion and reason are correct but Reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

Hint: As we move down the group, size of the central atom increases and bond length increases and bond energy decreases. Weaker bonds are easy to break and are thermally unstable.

Step by step solution:
As we move down the group atomic number increases.
The number of protons in the nucleus of the atom is equal to the atomic number (Z) and the number of electrons in a neutral atom is equal to the number of protons.
Hence the number of electrons also increases as we move down the group and consequently, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius.
Therefore, down a group the atomic radius increases as the energy levels increase.
On moving down the group thermal stability decreases due to increase in size of central atom (M) by which the bond dissociation energy between central atom and hydrogen (M-H) decreases and thus thermal stability decreases.
So the thermal stability of hydrides of group 16 elements decreases down the group due to increase in size.
\[{H_2}O > {H_2}S > {H_2}Se > {H_2}Te > {H_2}Po\]
But the acidic character of hydrides depends upon dependency to donate \[{H^ + }\]hence depends upon bond strength between central atom and hydrogen which decreases down the group. So it is easy to donate \[{H^ + }\] ion when we move down the group.
Hence acidic character increases down the group.

But thermal stability is not related to acidity.
Hence option (B) is the correct answer.
Note: The chalcogens are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. It consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Polonium (Po) is a radioactive element.