Why Fluorine has less electron affinity than chlorine?

Fluorine has a lower electron affinity than chlorine due to its smaller atomic size and its electronic configuration.

Fluorine is the smallest halogen and has a smaller atomic radius compared to chlorine. The smaller size of fluorine means that when it gains an additional electron, the electron-electron repulsion becomes significant, leading to a less favorable electron affinity.

Fluorine already has a relatively high electron density in its valence shell due to its electronic configuration. It has seven valence electrons in its 2p orbital and needs only one additional electron to achieve a stable octet configuration. Adding another electron to an already high electron density region requires overcoming strong electron-electron repulsion, making it energetically unfavorable and resulting in a lower electron affinity compared to chlorine.

Chlorine, on the other hand, has a larger atomic size and a lower electron density in its valence shell. It has three vacant 3p orbitals and needs to gain one electron to achieve a stable octet configuration. The addition of an electron to the 3p orbital is relatively more favorable due to the larger size and lower electron density, resulting in a higher electron affinity compared to fluorine.

So, despite the general trend of increasing electron affinity across the periodic table, the specific atomic properties of fluorine and chlorine lead to the observed deviation in their electron affinity values.