Answer is: hydrogen and chlorine.
Covalent bond is bond between nonmetals. Hydrogen (H) and chlorine (Cl) are nonmetals.
Hydrogen and chlorine will form hydrogen chlorine in chemical reaction:
H₂(g) + Cl₂(g) → 2HCl(g).
Other pairs will form ionic bonds, because one pair is metal (sodium, copper and zinc) and another pair is nonmetal (chlorine and bromine).
Ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).
