Chapter 2 Notes: Atoms, Molecules, and Ions

Early Ideas in Atomic Theory

• All matter is composed of tiny particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.
• All atoms of a given element have identical chemical properties that are characteristic of that element.
• A compound consists of atoms of two or more elements combined in a small, whole-number ratio.
• Atoms can change how they are combined, but they are neither created nor destroyed in chemical reactions.
• Law of Constant Composition / Law of Definite Proportions: All samples of a pure compound contain the same elements in the same proportion by mass (established by the experiments of Joseph Proust).
• Atoms of one element differ in properties from atoms of all other elements.
• John Dalton (1803–1807) proposed Dalton’s Atomic Theory:
  • An element is a substance that cannot be broken down into two or more simpler substances by any means.
  • Elements are displayed on the periodic table with their symbols.

The Atomic Theory and Related Laws

• Law of Definite Proportions (Proust): Pure compounds have fixed composition by mass.
• Law of Multiple Proportions (Dalton’s Law): When two elements react to form more than one compound, the masses of one element that combine with a fixed mass of the second element are in small whole-number ratios.
  • Example derivation as used in the transcript:
  • If Compound B contains 64 g of O and 24 g of C, the ratio is $$rac{mO}{mC}=rac{64}{24}=2.666\