Which has the most exothermic (negative) first electron affinity?

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We start by restating the problem and listing the options clearly to compare them: Question: Which has the most exothermic (negative) first electron affinity?
Options: Se, S, Br, Cl, Kr

Now we analyze each option in terms of first electron affinity (EA1), which is the energy change when a neutral atom gains one electron to form a negative ion. A more negative (more exothermic) EA1 means the atom more readily accepts an electron.

Option Se (selenium): Selenium is a chalcogen (group 16). Its first electron affinity is negative (exothermic) but relatively moderate in magnitude compared with halogens, because adding an electron to Se partially completes a p-subshell but is less favorable than adding to a halogen. The energy release is smaller in magnitude than that of the halogens.

Option S (sulfur): Sulfur is also a group 16 element and has a negative first electron affinity, indicating some exothermicity when it gains an electron. However, its EA1 is not as large in magnitude as the halogens, and certainly not as large as the top exothermic values seen in halogens like chlorine.

Option Br (bromine): Bromine is a halogen (group 17). Halogens generally have large negative first electron affinities because gaining an electron completes a noble gas–like octet. Among the halogens listed, bromine’s EA1 is quite exothermic, but still slightly less negative than chlorine’s.

Option Cl (chlorine): Chlorine is a halogen with a well-known highly exothermic first electron affinity. It readily accepts an additional electron to achieve a stable octet, releasing a substantial amount of energy. In typical comparisons among Se, S, Br, Cl, and Kr, chlorine has the most negative EA1 because of its high effective nuclear charge and desire to complete the octet.

Option Kr (krypton): Krypton is a noble gas. Its first electron affinity is either very small and often endothermic (positive energy change) because adding an electron to a closed-shell, inert gas is energetically unfavorable. This makes Kr the least exothermic (or most endothermic) option among those listed.

Putting these pieces together, we compare the magnitudes: Se and S have negative EAs but smaller in magnitude than the halogens; Br is quite exothermic but still usually less than Cl; Kr tends to be endothermic. Among the provided options, chlorine shows the largest magnitude in the negative direction, i.e., the most exothermic first electron affinity.

In summary, by evaluating the general trend that halogens exhibit more exothermic first electron affinities than neighboring elements and noble gases, and by comparing the specific members listed, the option with the most exothermic first electron affinity is chlorine.

Which has the most exothermic (negative) first electron affinity?

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