What happens when sodium reacts with fluorine?

Answer 1

Sodium is a reactive metal since it has "outer shell" electrons that it would like to get rid of. Fluorine, by contrast, is highly electronegative - it badly wants electrons, and will seek those electrons from any other chemical entity that has loosely held electrons. The net result is that sodium has a net negative charge, and fluorine has a net positive charge, and the two elements migrate toward each other in a non-covalent bond known as an ionic bond, wherein they distribute the charge difference between them forming the salt "sodium fluoride."