Na is a cation (positive) and F is a anion (negative) this is an ionic bond. In a ionic bond the cation loses its valence electrons to give them to the anion. In this case Na will give its valence electron to F so F can complete its octet:)
Analytical Chemistry. The salt which in solution gives a pale green precipitate with sodium hydroxide solution and a white precipitate with barium chloride solution is : Iron (III) sulphate.