- Atoms are the building blocks of matter and the smallest individual parts that make up elements, consisting of subatomic particles like protons, electrons, and neutrons. - Theories of atomic structure have evolved over time from early Greek philosophers' ideas of basic elements to J.J. Thomson's discovery of electrons and Rutherford's discovery of the dense atomic nucleus at the center, surrounded by orbiting electrons. - Modern atomic theory incorporates concepts like Bohr's planetary model of electron orbits and specific energy levels within atoms.