Dalton's Atomic Theory

Fundamental Principles

John Dalton proposed the first systematic atomic theory in the early 1800s, revolutionizing our understanding of matter and chemical reactions.

Key Postulates

  • All matter is made up of indivisible atoms
  • Atoms of the same element are identical in mass and properties
  • Different elements have different types of atoms
  • Atoms combine in whole number ratios to form compounds
  • Chemical reactions involve rearrangement of atoms

Law of Constant Proportions

A pure compound always contains the same elements combined in the same proportion by mass, regardless of its source or method of preparation.

Example: Carbon Monoxide (CO)

Carbon and oxygen combine in a fixed ratio to form carbon monoxide.

2D Visualization of CO molecule

Example: Water (H₂O)

Mass ratio of H:O=1:8\text{Mass ratio of H:O} = 1:8
Molar mass of H₂O=2(1)+16=18 g/mol\text{Molar mass of H₂O} = 2(1) + 16 = 18 \text{ g/mol}

3D Visualization of Water (H₂O) molecule with correct 104.5° bond angle

Law of Multiple Proportions

When two elements form multiple compounds, the ratio of the masses of the second element that combine with a fixed mass of the first element is always a small whole number ratio.

Example: Carbon Oxides

Carbon forms two oxides: CO and CO₂

In CO: C:O=12:16=3:4\text{In CO: C:O} = 12:16 = 3:4
In CO₂: C:O=12:32=3:8\text{In CO₂: C:O} = 12:32 = 3:8

Ratio of oxygen in CO₂ to CO = 8:4 = 2:1

Mathematical Applications

Molecular Mass Calculations

Mcompound=∑iniMiM_{\text{compound}} = \sum_{i} n_i M_i

where:

  • Mcompound = molecular mass of compound
  • ni = number of atoms of element i
  • Mi = atomic mass of element i