What are all atoms?
The fundamental structure of matter.
We now know that they are themselves composed of yet smaller particles, but those particles to not really exist (long-term) apart from their inclusion in an atom.
The atom gets its name from early Greek philosophers. The concept of a tiny “building block” of nature is a very old one espoused in ancient Greece and India. “Modern chemistry” beginning in the 1800’s experienced the first direct attempts to isolate atoms. See also: