Question
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Which of the following has the least electron affinity?
A: $Na$
B: $Ca$
C: $K$
D: $F$

Answer
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Hint: Electron affinity is defined as the amount of energy released when an extra electron is added to a neutral atom in its gaseous state. Adding an extra electron neutral atom becomes a negative ion.

Complete step by step answer:
Electron affinity is the amount of energy released when an extra electron is added to an atom. Electron affinity is inversely proportional to the size of an atom. Because if the size of the atom will be larger then the hold of the nucleus on the outermost shell will be less. As a result, when an electron comes, the nuclear pull of that atom will not be much effective in gaining an electron. So we will have to provide some extra energy in order to add that electron in the valence shell of that atom. It depends upon the number of valence electrons as well. If an atom has less valence electron it will tend to lose those electrons rather than gaining one more electron. These are major factors affecting electron affinity. Fluorine has $7$ electrons in its outermost shell. It needs only one electron to complete its octet. So it will readily gain one electron and acquire a stable electronic configuration. Due to which high amount of energy is released. Calcium has two electrons in its outermost shell. So it will tend to lose those electrons not to gain one extra electron. This means electron affinity of calcium is less than that of fluorine. So fluorine is definitely not the answer (as we have to find an element with the least electron affinity). Sodium and potassium both have one electron in their outermost shell. These atoms will tend to lose their electron. Calcium has two electrons to lose but sodium and potassium have only one electron to lose. This means electron affinity of calcium will be greater than that of sodium and potassium. So calcium is also not the answer. Among sodium and potassium electrons affinity of potassium is least as the size of potassium is greater than the size of sodium (more will be the size less will be the holder of the nucleus).

So the correct answer is option c that is $K$.

Note:
Electron affinity is the least for noble gases. As they have a stable electronic configuration. Their octet is complete. So it is difficult to add extra electrons as it will make stable electronic configuration unstable.