Balanced chemical reaction: 2Na(s) + F₂(g) → 2NaF(s) + energy.
This is exothermic reaction (energy or heat is released), because reactants have higher energy than products of reaction.
In ionic salt sodium fluoride (NaF), fluorine has electronegativity approximately χ = 4 and sodium χ = 1 (Δχ = 4 - 1; Δχ = 3), fluorine attracts electron and it has negative charge and sodium has positive charge.