Chapter 2: Atoms and the Atomic Theory

Atoms and the Atomic Theory begins by examining the cornerstone principles of stoichiometry, including the law of conservation of mass, the law of definite proportions, and the law of multiple proportions, which served as the empirical foundation for Dalton’s atomic theory. The narrative then shifts to the pivotal experiments in physics that unraveled subatomic structure, such as J.J. Thomson's characterization of the electron using cathode-ray tubes and Robert Millikan's determination of elementary charge via the oil-drop experiment. The transition from the "plum-pudding" model to the nuclear atom is highlighted through Ernest Rutherford’s alpha-particle scattering experiments, which identified the dense, positively charged nucleus containing protons and neutrons. Further exploration covers the concept of isotopes—atoms of the same element with varying mass numbers—and the behavior of ions formed through the gain or loss of electrons. The text introduces the periodic table as a sophisticated organizational tool, categorizing elements into periods and groups such as alkali metals, halogens, and noble gases, while distinguishing between metals, nonmetals, and metalloids. Finally, the chapter establishes the essential quantitative bridge between the microscopic and macroscopic worlds through the mole concept, utilizing Avogadro’s constant and molar mass to facilitate calculations involving atomic count, mass, and chemical amount.