Ch 2 Atoms, Molecules, and Ions: Foundations of Atomic Theory and Structure

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: $\frac{32\,g}{16\,g} = 2$

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

Compound	Mass of C (g)	Mass of O (g)	Ratio O:C
CO	12	16	1:1
CO2	12	32	2:1

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.