Why Fluorine has more electronegativity than Chlorine although Chlorine has more electron affinity/pull for the electrons than Fluorine ?
The question is fairly good. One might expect that if Chlorine has more pull for electrons(tendency to gain electrons) than Fluorine then it should also have more tendency to gain the shared pair of electrons of the covalent bond. But this is not true and the reason for so is explained as follows:-
Let us consider the molecule AーB in which A and B are bonded covalently. I must remember you that there are two electrons present in the bond pair which are in turn of two different origin, one comes from A and other comes from B. Now electronegativity is the tendency to attract (or gain) both the electrons of the bond pair. The attraction here is for 2 different electrons, one is of the other atom's and one is of own's atom. The strength of the pull for the 2 electrons of different origin will depend upon 2 different respective factors. The pull for the electron of the other atom will depend upon the electron affinity of the pulling atom, higher the affinity stronger will be the pull but the pull for atom's own electron will depend upon ionization energy (or you can say size, both are equally correct) of the pulling atom, the more will be the ionization energy(lesser will be the size) the stronger will be the pull.
Now you might suspect the dependency of the pull for one's own electron on the ionization energy but the dependency is genuine and right since ionization energy is the energy required to take out an electron from the outermost shell of the neutral, gaseous, isolated atom and more will be the ionization energy that means stronger will be the pull for the electrons, Therefore ionization energy is directly related to the pull of one's own electron, the more is the ionization energy stronger is the pull and vice-versa.
Now you are well learned that the electronegativity is the pull for 2 different electrons(one is own's and one is other's) and stronger the both pulls(that means more ionization energy and electron affinity) higher the electronegativity.
Now comes the comparison for Fluorine and Chlorine
Electron Affinity Ionization Energy
Fluorine - 328 1681 Kj/mole
Chlorine - 348 1251 Kj/mole
The sum of electron affinity and ionization energy for Fluorine is greater than that of Chlorine. Therefore stronger pull for 2 different electrons will be in the case of fluorine and hence fluorine will be more electronegative than Chlorine although Chlorine has more affinity/pull for the electrons than Fluorine.
For any doubts regarding this answer follow me in the comments below.
Let us consider the molecule AーB in which A and B are bonded covalently. I must remember you that there are two electrons present in the bond pair which are in turn of two different origin, one comes from A and other comes from B. Now electronegativity is the tendency to attract (or gain) both the electrons of the bond pair. The attraction here is for 2 different electrons, one is of the other atom's and one is of own's atom. The strength of the pull for the 2 electrons of different origin will depend upon 2 different respective factors. The pull for the electron of the other atom will depend upon the electron affinity of the pulling atom, higher the affinity stronger will be the pull but the pull for atom's own electron will depend upon ionization energy (or you can say size, both are equally correct) of the pulling atom, the more will be the ionization energy(lesser will be the size) the stronger will be the pull.
Now you might suspect the dependency of the pull for one's own electron on the ionization energy but the dependency is genuine and right since ionization energy is the energy required to take out an electron from the outermost shell of the neutral, gaseous, isolated atom and more will be the ionization energy that means stronger will be the pull for the electrons, Therefore ionization energy is directly related to the pull of one's own electron, the more is the ionization energy stronger is the pull and vice-versa.
Now you are well learned that the electronegativity is the pull for 2 different electrons(one is own's and one is other's) and stronger the both pulls(that means more ionization energy and electron affinity) higher the electronegativity.
Now comes the comparison for Fluorine and Chlorine
Electron Affinity Ionization Energy
Fluorine - 328 1681 Kj/mole
Chlorine - 348 1251 Kj/mole
The sum of electron affinity and ionization energy for Fluorine is greater than that of Chlorine. Therefore stronger pull for 2 different electrons will be in the case of fluorine and hence fluorine will be more electronegative than Chlorine although Chlorine has more affinity/pull for the electrons than Fluorine.
For any doubts regarding this answer follow me in the comments below.