Is the Atom Fundamental?

Around 1900, people thought of atoms as tiny balls.

However, the fact that atoms could be categorized in terms of their similar chemical properties (as in a periodic table) suggested that atoms are not fundamental.
Moreover experiments which "looked" into an atom using particle probes indicated that atoms had structure and were not just permeable balls.
These experiments helped scientists determine that atoms had a positive, dense nucleus and a cloud of electrons (e).