Fluorine is more electronegative than sodium because it has a higher number of protons in its nucleus, which creates a stronger pull on the electrons in its outermost energy level. This results in fluorine having a greater ability to attract and hold electrons compared to sodium.
Besides having a higher number of protons, fluorine also has a smaller atomic radius and a higher effective nuclear charge, meaning that the electrons are held more tightly by the nucleus. Additionally, fluorine has one less energy level compared to sodium, making it easier for the nucleus to attract and hold onto electrons.
Electronegativity plays a significant role in determining the type of bond that will form between atoms. In the case of fluorine and sodium, fluorine's high electronegativity causes it to attract electrons from sodium, leading to the formation of an ionic bond. Electronegativity also affects the polarity of molecules and their reactivity in chemical reactions.
Electronegativity is a periodic trend and generally increases from left to right across the periodic table. It can also vary within the same element depending on its oxidation state. For example, in the case of sodium, its electronegativity is lower in its ionic form (Na+) compared to its metallic form (Na), where it is more similar to fluorine.
Fluorine's high electronegativity makes it a highly reactive element, as it seeks to gain an electron to achieve a stable octet in its outer energy level. This makes it a strong oxidizing agent and allows it to form compounds with almost all other elements in the periodic table, making it one of the most reactive elements.