Question

what discovery led j.j. thomson to decide that atoms were not indivisible, but are actually composed of smaller parts?
thomson used a scanning tunneling micrograph to capture images of atoms.
thomson used a beam of negatively charged particles.
thomson used only the experimental results of other scientists.

Explanation:

J.J. Thomson discovered electrons through cathode - ray tube experiments using a beam of negatively - charged particles, which showed atoms were made of smaller parts. Scanning tunneling micrographs came much later and he didn't just rely on others' results.

Answer:

Thomson used a beam of negatively charged particles.