Question

Would you expect the second electron to gain enthalpy of $O$ as positive, more than negative or less negative than the first? Justify your answer.
What is the basic difference between the terms electron gain enthalpy and electronegativity?

Answer 1

Electron gain enthalpy is defined as the amount of energy released when an electron is added to an isolated gaseous state. During addition of an electron, energy can neither be created nor destroyed. Electronegativity measures the tendency of an atom to attract a shared pair of electrons.

Complete step by step answer:
The second electron gain enthalpy of $O$ is positive as explained below:
When an electron is added to $O$ atom from ${O^ - }$ ion, energy is released. Thus, first electron gain enthalpy of $O$ is negative.
$O\left( g \right) + {e^ - }\left( g \right) \to {O^ - }\left( g \right)$
$\Delta eg{\text{ }}H = - 141KJ/mol$
But when another electron is added to ${O^ - }$ to form ${O^{2 - }}$ ion energy is absorbed to overcome the strong electrostatic repulsion between the negatively charged ${O^ - }$ ion and the second electron being added. Thus, the second electron gain enthalpy of oxygen is positive.
${O^ - }\left( g \right) + {e^ - }\left( g \right) = {O^{2 - }}\left( g \right)$
$\Delta eg{\text{ }}H = + 780KJ/mol$
Both the electron gain enthalpy and electronegativity refers to the tendency of the atom to attract electrons. Whereas electron gain enthalpy refers to the tendency of an isolated gaseous atom to accept an additional electron to form a negative ion. Electronegativity refers to the tendency of an atom of an element in a molecule to attract the shared pair of electrons towards it in a covalent bonded molecule.

Additional information:
1. The fluorine atom has the highest electron affinity or electronegativity. Electronegativity follows the trend as fluorine has higher electronegativity than oxygen $\left( O \right)$, followed by Nitrogen then chlorine.
$F > O > N > Cl > C > H$
2. Halogens, on the other hand, have very high negative values of electron gain enthalpy as they acquire stable configuration of noble gas in addition to one electron.

Note:
$Bi$ and $Mg$ (considered to be exceptions) have positive electron affinity (electronegativity) because of the energy difference between the $S$ and $P$ subshells.