The observed electron gain enthalpy values for fluorine and chlorine highlight an important trend within the periodic table. Despite the high electronegativity of fluorine, its electron gain enthalpy (with a negative sign) is less exothermic (less negative) than that of chlorine. Let's explore the reasons behind this phenomenon:
Smaller size of the fluorine atom: Fluorine is in the second period of the periodic table, and its atomic size is smaller compared to chlorine, which is in the third period. The smaller atomic size of fluorine results in a higher electron-electron repulsion when an additional electron is added to the already compact 2p orbital, causing less energy to be released. Therefore, despite its high electronegativity, the energy released upon gaining an electron is not as much as in chlorine. This is the correct answer.
High ionization enthalpy of fluorine: While fluorine has a high ionization enthalpy, this does not directly influence its electron gain enthalpy. The ionization enthalpy is a measure of the energy required to remove an electron, which is a separate concept from gaining an electron.
Smaller size of the chlorine atom: This is incorrect. The chlorine atom is larger than the fluorine atom, and its more spacious 3p orbital can better accommodate the additional electron with less repulsion.
Bigger size of 2p orbital of fluorine: The 2p orbital isn't bigger in fluorine; rather, it is crowded because of the small size of the atom, which increases repulsion among electrons.
In conclusion, the smaller size of the fluorine atom leads to greater electron-electron repulsion in the 2p orbital compared to the larger third-period 3p orbital in chlorine, resulting in less negative electron gain enthalpy for fluorine.
