The electron affinity of Fluorine is less than that of Chlorine because: A.The incoming electron enters the $3p$ orbital in fluorine. B.The incoming electron enters the $2p$ orbital in fluorine and $3p$ orbital in chlorine. C.The electron density in fluorine is higher. D.Fluorine has lower ionisation potential than chlorine.
Hint:Electron affinity is the energy liberated when an atom accepts an electron. Fluorine belongs to the second period and chlorine belongs to the third period. Both are the same group elements.
Complete answer: Fluorine and chlorine are group $17$ elements and they are called halogens. They need only one electron to attain noble gas electronic configuration. And they are highly electronegative elements. Hence they always have a tendency to accept electrons. Electronegativity of fluorine is higher than chlorine, but electron affinity of fluorine is lower than chlorine. The valence shell for fluorine is \[2p\] and the valence shell of chlorine is $3p$. Both of them carry seven electrons in their valence shell. $3p$ is larger than $2p$. Hence it can accommodate the seven electrons easily. Also it can readily accept the extra electron to complete the octet. Hence electron affinity of chlorine is high. But $2p$ is very compact to accommodate seven electrons. Hence addition of one more electron to this subshell is difficult. So the tendency to accept electrons is low for $2p$ shells. Hence electron affinity of fluorine is low. This is why electron affinity of fluorine is less than that of chlorine.
Note: Electronegativity is the tendency to attract electrons. But it should be noted that higher electronegativity cannot guarantee a high value of electron affinity. Due to the higher electronegativity of fluorine, we confuse that fluorine has more electron affinity than chlorine. But it is not so.