Question

The increasing order of electron gain enthalpy of the following substances is:
$O < S < Se < Te$
If the order of bond dissociation enthalpy is correct, enter 1 else 0.

Answer 1

We know that the electron gain enthalpy for an element is the amount of energy released when that element accepts an electron. This process is highly endothermic meaning that energy is given out during the reaction. When there is less electronic repulsion faced by an electron to be occupied in the outermost shell higher will be the amount of energy released.

Complete answer:
On moving down the group, the electron gain enthalpy decreases from Sulphur to Polonium. Oxygen has an unexpectedly lower value of electron gain enthalpy than sulphur because oxygen is smaller in size than Sulphur. The consequence of which is that the incoming electron on oxygen faces more electronic repulsion on oxygen’s outermost surface due to more electron density as a result of which electron affinity decreases.
Lesser the electronic repulsion faced by an electron to get occupied in the outermost shell of an element higher is the amount of energy released by the element.
Therefore we can say that the increasing order of electron gain enthalpy with negative sign for the given elements is in the order;
$O( - 141.4kJ/mol) < Po( - 174kJ/mol) < Te( - 190.0kJ/mol) < Se( - 195.5kJ/mol) < S( - 208.8kJ/mol)$
Hence the correct answer is 0.

Note:
Do not confuse Electron gain enthalpy with electronegativity. Electron gain enthalpy is the enthalpy change when a neutral gaseous atom of an element takes up an extra electron to form an anion. Electronegativity on the other hand is the tendency of the atom of an element in a chemical compound to attract the shared pair of electrons towards it in a covalent bond.