Here's why:

* Atomic mass: Atomic mass is the average mass of an atom of an element.

* Zirconium has an atomic mass of approximately 91.224 atomic mass units (amu).

* Indium has an atomic mass of approximately 114.818 amu.

Since zirconium has a lower atomic mass than indium, one atom of zirconium is lighter than one atom of indium.