The section discusses the foundational concepts of atoms and molecules, including the laws of conservation of mass and constant proportions, and introduces Dalton's atomic theory. ### Medium Summary: This section delves into the historical perspectives on atoms from ancient philosophies to modern scientific understanding. It covers key principles, such as the law of conservation of mass and the law of definite proportions, as well as Dalton's atomic theory, which explains the nature of matter in terms of atoms and molecules. ### Detailed Summary: # ATOMS AND MOLECULES The study of atoms and molecules is crucial in understanding the composition of matter and chemical reactions. Philosophers from ancient India and Greece theorized about the divisibility of matter long before the science of chemistry was established. The Indian philosopher Maharishi Kanad and Greek philosophers like Democritus contributed to the concept of indivisible particles, which were later termed atoms. Lavoisier, among others, laid down the foundation for modern chemistry in the late 18th century, establishing the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. Proust further refined this with the law of definite proportions, indicating that a compound always contains elements in fixed ratios. Dalton's atomic theory, presented in 1808, synthesized these laws and proposed that matter consists of atoms—indivisible particles that combine in whole-number ratios to form compounds. Dalton's postulates provided a framework for understanding chemical reactions, emphasizing that all matter is composed of atoms with specific properties that define elements. This section also explores molecules, the smallest units of compounds, which consist of two or more atoms bonded together. The distinction between elements, compounds, and mixtures is essential for understanding the interactions and reactions that occur in chemistry.