Question

Which of the following elements has the highest electron-gain enthalpy?
A. Oxygen
B. Fluorine
C. Chlorine
D. Neon

Answer 1

Hint: Remember that the electron-gain enthalpy of an element is the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous anion. For instance, the energy released in the process to create anion E- for an element E.

Complete step-by-step solution:
To find out the answer of this question, we need to observe the trend in electron-gain enthalpy both across a period, through a group and in noble gases.
The atoms of noble gases have completely filled subshells, as a result of which there is no room in their valence orbitals and the additional electron has to be placed in an orbital of the next higher shell.
As a result, energy has to be supplied to add on additional electrons therefore making their ionisation enthalpy positive.
Thus, we can eliminate Neon as a possible answer since Neon is a noble gas.
The electron gain enthalpy becomes less negative as we move down a group because as we move down a group, both the atomic size and the nuclear charge increase. But the effect of the increase in atomic size is much more pronounced than the nuclear charge.
Electron gain enthalpy becomes more and more negative from left to right in a period because as we move across a period from left to right the atomic size decreases and the nuclear charge increases resulting in increased attraction to the nucleus for the incoming electron.
Hence, electron gain enthalpy becomes more and more negative in a period from left to right.
Therefore, by the above information. Fluorine should have the highest electron gain enthalpy as it is the halogen element of the 2nd period.
However, it is Chlorine that possesses the highest electron gain enthalpy as due to Fluorine's extremely small size there is a strong electron-electron repulsion in its 2p subshell thereby making it not as accepting of an incoming electron as Chlorine is.
Therefore, as a result of all of this analysis, the answer to this question is c) Chlorine.

Note: While the knowledge of electron gain enthalpy and its trends across the periodic table is incredibly important to answer this question, however the distinction between the chemical properties of Fluorine and Chlorine that tip the scales in the favour of the latter are incredibly unique and a thorough knowledge of those is required to correctly solve this question.