Question

For magnitude of electron gain enthalpy of chalcogens and halogens, which of the following options is correct?
A. \[Br > F\]
B. \[S > F\]
C. \[O < Cl\]
D. \[S < Se\]

Answer 1

Hint: Among Group 16 elements (chalcogens) and Group 17 elements (halogens), the negative electron gain enthalpy increases (becomes more negative) when we move from left to right and it decreases (becomes less negative) when we move down a particular group (oxygen and fluorine are two exceptions to the “down-the-group” decrease of negative electron gain enthalpy). Evaluate each option individually according to the above-mentioned trend to get the correct answer.

Complete Step by Step Solution:
Electron gain enthalpy (sometimes denoted as \[{\Delta _{eg}}H\]) can be defined as the amount of energy absorbed or released when an electron is added to an isolated, gaseous atom. As with all enthalpies, there exists a sign convention for electron gain enthalpy. When energy is absorbed by adding an electron to an atom of a particular element, the electron gain enthalpy of that element is said to be positive and if energy is released by adding an electron, the electron gain enthalpy is said to be negative.

In general, when talking about metals, the electron gain enthalpy is always positive since metals are poor acceptors of electrons. Similarly, for non-metals, electron gain enthalpy is always negative since they prefer to accept electrons. In this question, we are asked about chalcogens (Group 16 elements) and halogens (Group 17 elements). Since both are non-metals, we will be referring to the negative electron gain enthalpy only.

Among chalcogens and halogens, the electron gain enthalpy tends to increase on moving left to right and it tends to decrease when moving downwards. Two exceptions exist to the downward decreasing trend. Among chalcogens, the electron gain enthalpy increases from oxygen (\[O\]) to sulphur (\[S\]) and among halogens it also increases from fluorine (\[F\]) to chlorine(\[Cl\]). The atoms of oxygen and fluorine are smaller compared to their counterparts. Due to the small size, their valence shell electrons are present very close to each other, and this leads to inter-electron repulsion. This inter-electron repulsion makes it difficult to add an extra electron to their valence shells. Thus, the electron gain enthalpies of oxygen and fluorine are the lowest in their respective groups.

The chalcogens and halogens are shown below:

Chalcogens (Group 16)	Halogens (Group 17)
$O$	$F$
$S$	$Cl$
$Se$	$Br$
$Te$	$I$

Now, let’s look at each option individually.
Option A is incorrectbecause only chlorine has a higher electron gain enthalpy than bromine.
Option B is incorrectbecause sulphur belongs to group 16 and fluorine belongs to group 17. Electron gain enthalpy increases from group 16 to group 17.
Option C is correctbecause oxygen belongs to group 16 and chlorine belongs to group 17. Electron gain enthalpy increases from group 16 to group 17.
Option D is incorrectbecause selenium is below sulphur in group 16 and electron gain enthalpy decreases down the group.

Thus, option C is correct.

Note: When discussing electron gain enthalpy, it is crucial to be mindful of the sign convention and keep track of whether we are talking about positive electron gain enthalpy or negative electron gain enthalpy because the periodic trends will change accordingly.