This document discusses the structure of atoms. It states that all matter is made up of atoms, which are the smallest particles that can engage in chemical reactions. Atoms have a central nucleus containing positively charged protons and uncharged neutrons, and electrons orbiting the nucleus. Rutherford discovered the nuclear model of the atom through deflecting alpha particles with a gold foil. The document also discusses Bohr's model of electron shells and energy levels in atoms, as well as the composition of protons, neutrons, and electrons that define atomic number and mass number.