The only base that does not have a metal atom in its molecule is ammonia (NH₃).

Here's why:

* Bases are substances that can accept protons (H⁺ ions).

* Metal hydroxides (like NaOH, KOH) are common bases because they readily release hydroxide ions (OH⁻) in solution.

* Ammonia, while acting as a base, does not contain a metal atom. It's a compound of nitrogen and hydrogen. It acts as a base by accepting protons to form ammonium ions (NH₄⁺).