Protons determine the identify of an element because the number of protons does not change during chemical reactions.
Protons do not determine chemical properties. Valence electrons determine the chemical properties of an element. Chemical properties are characteristics of a material that can only be observed when the material undergoes a chemical reaction, which results in a chemical change. During chemical reactions, atoms share or transfer electrons to get 8 valence electrons, which makes them stable, except for hydrogen, lithium, and beryllium, which react to get 2 valence electrons, which makes them stable.