What is more reactive Na ( sodium) or As ( arsenic )?

Arsenic would be more reactive.

Explanation:

Electronegativity (tendency of an atom to attract a bonding pair of electrons) increases as you move up and to the right of the periodic table.

Elements further to the right of the table have more valence electrons, meaning they are closer to creating the full ring (8 electrons) and therefore are more reactive.

Arsenic is more further up and to the right than Sodium.