Chapter 2: Atoms, Molecules, and Ions

Dalton's Atomic Theory synthesizes these observations by proposing that matter consists of indivisible atoms, that atoms of a given element are identical in mass and properties, and that chemical reactions involve recombination rather than creation or destruction of matter. This theoretical framework explains the law of conservation of mass, the law of definite proportions, and the law of multiple proportions, providing chemists with a unified explanation for how substances combine. The chapter then presents experimental evidence for atomic structure, beginning with Thomson's discovery of the electron through cathode-ray research and progressing through Millikan's quantitative determination of electron charge and Rutherford's revolutionary gold-foil experiment, which revealed that atoms possess a concentrated nucleus. Understanding atomic composition requires distinguishing protons, neutrons, and electrons, recognizing isotopes as atoms of the same element with different mass numbers, and appreciating how electron configuration determines chemical behavior. The periodic table emerges as an organizational system that groups elements by atomic number and reveals predictable patterns in chemical reactivity, electronegativity, and ion formation across metals, nonmetals, halogens, alkaline earth metals, and noble gases. To communicate chemical information effectively, students must master nomenclature conventions for ionic compounds, covalent compounds, polyatomic ions, and acids, learning systematic naming rules that allow chemists worldwide to interpret and write formulas unambiguously. These interconnected concepts—historical development, atomic structure, periodic organization, and chemical language—form the conceptual toolkit that enables all subsequent study of chemical bonding, reactions, and properties.