Fluorine is more electronegative than chlorine primarily due to its smaller atomic size and the effective nuclear charge it possesses. Electronegativity refers to the ability of an atom to attract electrons in a chemical bond, and this property is influenced by both the number of protons in the nucleus and the distance of the outer electrons from the nucleus.
Fluorine has an atomic number of 9, which means it has 9 protons in its nucleus. In contrast, chlorine has an atomic number of 17, with 17 protons. Although chlorine has more protons than fluorine, the additional electron shells in chlorine increase the distance between the nucleus and the valence electrons, which reduces the effective nuclear charge felt by these outer electrons.
Furthermore, as we move across a period in the periodic table from left to right, electronegativity generally increases. However, fluorine is at the top of group 17 (the halogens), making it the element with the highest electronegativity due to its small size and strong attraction to electrons. The close proximity of fluorine’s valence electrons to its nucleus allows it to exert a stronger pull on shared electrons compared to chlorine.
In summary, fluorine is more electronegative than chlorine because it is smaller, has a stronger effective nuclear charge relative to its valence shell, and is better positioned to attract electrons due to its place on the periodic table.