* Atoms are the fundamental building blocks of matter and are not considered living.

* Bacteria are single-celled organisms, much larger and more complex than viruses.

* Mitochondria are organelles found inside cells, not independent living organisms.

* Frog embryos are multicellular organisms in their early stages of development.

* Viruses are non-cellular entities that require a host cell to reproduce. While they exhibit some characteristics of life (like evolution and replication), they are not considered truly living organisms.

Therefore, viruses are the smallest entities on the list that exhibit some life-like characteristics.