2.1 Early Ideas in Atomic Theory | Chemistry: Atoms First 2e

Learning Objectives

State the postulates of Dalton’s atomic theory
Use Dalton’s postulates to explain the laws of definite and multiple proportions

Core Concepts & Principles (De-Verbosed)

The modern understanding of atomic structure began with John Dalton in 1807, building on early philosophical ideas from ancient Greeks like Democritus (who coined the term atomos for indivisible particles). Dalton formulated the first scientific atomic theory to explain the behavior of matter.

Dalton’s Atomic Theory Postulates

Matter is made of atoms: An atom is the smallest unit of an element that can take part in a chemical change.
Elements consist of identical atoms: All atoms of a given element have identical chemical properties and characteristic mass.
Different elements have different atoms: Atoms of one element differ in properties from atoms of all other elements.
Compounds form in fixed ratios: A compound consists of two or more elements combined in small, whole-number ratios.
Conservation of mass: Atoms are neither created nor destroyed during chemical changes; they are simply rearranged.

Core Mental Model: Conservation and Fixed Ratios

Chemical reactions involve rearranging atoms, not destroying or creating them.
Because compounds are built from specific whole-number combinations of atoms, they always exhibit fixed mass compositions (explaining the law of definite proportions).

Problem-Solving Routines & Methods

How to Test the Law of Multiple Proportions

1
Calculate the mass ratio of Element B to a fixed unit (e.g., 1.00 g) of Element A for each compound.
2
Divide the larger mass ratio by the smaller mass ratio.
3
Check if the resulting value is a simple, small whole number (like 2, 3, or 1.5).

Pro-Tip: Always normalize your denominator to 1.00 g of the reference element first to make comparison straightforward.

Law of Multiple Proportions Formula

\\text{Mass Ratio} = \\frac{\\text{Mass of Element B}}{\\text{Mass of Element A (fixed)}}, \\quad \\text{Ratio of Ratios} = \\frac{\\text{Ratio}_2}{\\text{Ratio}_1}

Mathematical test to show that two elements forming multiple compounds combine in small whole-number ratios.

Variables: Mass A = reference mass (g), Mass B = variable reacting mass (g)

Practice & Concept Checks

Concept Check

Does a chemical reaction converting 4 green and 2 blue reactant spheres into 4 green and 2 blue product spheres violate Dalton's atomic theory?

Concept Check

Compound A has an oxygen-to-carbon mass ratio of 1.33:1. Compound B has an oxygen-to-carbon mass ratio of 2.67:1. What law do these data support, and what does it tell you?

Key Terms & Vocabulary

Dalton’s atomic theoryAtomic Theory

A scientific model proposed by John Dalton stating that matter is composed of indestructible atoms that combine in whole-number ratios to form compounds.

Example: Explains why chemical reactions conserve mass.

atomMatter & Structure

The smallest unit of an element that can participate in a chemical change.

Example: A single copper atom (Cu).

elementMatter & Structure

A substance consisting of only one type of atom, with a characteristic mass and identical chemical properties.

Example: Pure gold (Au).

law of definite proportionsChemical Laws

The principle that all samples of a pure compound contain the exact same elements in the exact same proportion by mass.

Example: Pure water is always 11.2% hydrogen and 88.8% oxygen by mass.

law of constant compositionChemical Laws

Another name for the law of definite proportions, highlighting that a pure compound's elemental makeup never varies.

Example: Carbon dioxide always contains carbon and oxygen in a fixed mass ratio.

law of multiple proportionsChemical Laws

The principle that when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers.

Example: Carbon and oxygen combining to form CO and CO₂.