Atoms where discovered through the movement of tiny particles visible in air and water.The movements of these particles seemed strange in till people arrived to the conclusion that even smaller particles (atoms) that made up the air and water where bouncing off of each other and the particles (Brownian motion) causing the bigger particles to move the way that they do. Einstein used the math behind Brownian motion to prove the existence of atoms. Einstein also discovered the estimated size of atoms based on the observations of how much visible particles moved through Brownian motion.
For a Long time, people thought that atoms where the smallest known particles of the universe. Oh, but they were wrong. Atoms just so happened to be composed of smaller particles called “subatomic particles”. These particles are called electrons, neutrons, and protons.
Electrons are negatively charged particles found in the atoms electron cloud outside of the nucleus. Electrons are extremely small compared to protons and neutrons. Imagine the eraser of your pencil being the size of an electron. That would mean that the size of the protons and neutrons would be about the equivalent of the classroom.
Protons are positively charged particles found inside the atom’s nucleus. Bound to the protons by the strong force is the neutrons, a neutrally charged particle. These two subatomic particles are composed of even smaller particles called quarks. Quarks are even smaller the tiny existence called electrons. Quarks are for now considered the smallest known particles in the universe ( this is without considering particles that might possibly make up electrons: leptons, heptons, and neutrinos, whose existence is still being debated).
That my friends is what is smaller than the atom.