An atom is defined as the smallest unit of ordinary matter that retains the chemical properties of an element. Its structure comprises a dense nucleus, which contains positively charged protons and neutral neutrons, surrounded by negatively charged electrons orbiting in discrete energy levels. The specific number of protons determines the atomic number, which uniquely identifies the element. Atoms interact through the exchange and sharing of electrons, forming chemical bonds that allow them to combine in various ratios to create molecules. These molecular interactions govern the diverse physical properties of all known substances, from gases to complex solids, and are the foundation of all chemistry and physics.