Atoms, Molecules, and Ions

Early Observations of Matter

The concept of atoms and the nature of matter has evolved through history, beginning with philosophical ideas and advancing through scientific experimentation.

• Ancient Greece: Philosophers such as Leucippus, Democritus, and Epicurus theorized that matter could not be infinitely divisible. The term atomos (Greek for indivisible) was introduced to describe the smallest unit of matter.

• 1700s Scientific Laws:

  • Law of Conservation of Matter: Mass is neither created nor destroyed in a chemical reaction.

  • Law of Definite Proportions: A given compound always contains exactly the same proportion of elements by weight.

Dalton’s Atomic Theory (1808)

John Dalton formalized the atomic theory, providing a scientific basis for the nature of matter.

• Postulate 1: Matter is composed of indivisible atoms.

• Postulate 2: An element is a type of matter composed of only one kind of atom.

• Postulate 3: A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions.

• Postulate 4: A chemical reaction involves the rearrangement of atoms to form new chemical combinations.

Law of Multiple Proportions

This law explains how elements can combine in different ratios to form different compounds.

• Definition: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

• Example: Carbon monoxide (CO) and carbon dioxide (CO2):

  • CO: 12 g C + 16 g O

  • CO2: 12 g C + 32 g O

  • Ratio of oxygen in CO2 to CO:

Discovery of the Electron

J.J. Thomson’s experiments with cathode ray tubes led to the discovery of the electron, a fundamental subatomic particle.

• Cathode Ray Tube: Thomson observed that cathode rays are composed of negatively charged particles (electrons) present in all matter.

• Significance: This discovery showed that atoms are not indivisible, contradicting Dalton’s postulate.

Key Terms and Concepts

• Atom: The smallest unit of an element that retains its chemical properties.

• Molecule: A group of atoms bonded together, representing the smallest fundamental unit of a chemical compound.

• Ion: An atom or group of atoms that has gained or lost electrons, acquiring a net electrical charge.

Example Table: Comparison of CO and CO2

Additional info:

• The development of atomic theory set the stage for modern chemistry, including the understanding of chemical reactions, molecular structure, and the periodic table.