Would you expect the second electron to gain enthalpy of oxygen as positive, more negative or less negative than the first one? Give reason.

Hint: When the process takes place at constant pressure, the heat absorbed or released is equal to the change in the enthalpy. Enthalpy is sometimes referred to as heat content. Etymologically, the word "entropy" is derived from the Greek word "turning" and "enthalpy" is derived from the Greek word "warming."

Complete answer:
Oxygen belongs to a group of 16 p block elements. It's the first member of Group 16. Non-metal & most abundant in earth crust (47 per cent)
When an electron is added to a gaseous atom, energy is released to form a negative ion. This enthalpy change called the first electron gain enthalpy is thus negative.
\[{{\text{O}}_{(g)}} + {{\text{e}}^ - } \to {\text{O}}_{(g)}^ - \]
But when an electron is applied to the uni negative ion, it encounters repulsion from the anion as a result of which energy is supplied that overcomes the repulsion. Therefore, the addition of the second electron requires energy, because of which enthalpy is positive for the second electron gain.
\[{{\text{O}}^ - }_{(g)} + {{\text{e}}^ - } \to {\text{O}}_{(g)}^{2 - }\]
While oxygen atoms with six valence electrons allow two electrons to become stable, the formation of \[{O^ - }\]anion by an electron gain provides repulsion to the second electron because of which energy is to be supplied, rendering the second electron gain enthalpy positive.

Note: The chemical element with the symbol O and the atomic number 8 is oxygen. In the periodic table, it is a member of the chalcogen group, a highly reactive non-metal, and an oxidizing agent that readily forms oxides both with most elements and with other compounds.