Atomic theory is the scientific concept that matter is made up of tiny, indivisible particles called atoms. Each atom consists of a nucleus, containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons. This theory helps explain the composition and behavior of all substances in the universe. Developed over centuries, atomic theory has evolved from early ideas by philosophers like Democritus to modern understandings based on quantum mechanics. It provides a framework for understanding chemical reactions, the formation of molecules, and the properties of elements in the periodic table.