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

Learning Objectives

State Dalton’s atomic theory five postulates and modern limits (isotopes and subatomic particles)
Explain the laws of conservation of mass, definite proportions using mass ratios, and multiple proportions comparing masses at a fixed mass

Core Concepts & Principles

An atom is the smallest particle of an element that can enter into a chemical combination. Dalton’s atomic theory consists of five fundamental postulates:

Matter is composed of indivisible atoms.
All atoms of a given element are identical in mass and properties.
Atoms of different elements have different properties.
Compounds are formed by combinations of atoms of multiple elements in small, whole-number ratios.
Atoms are neither created nor destroyed during chemical changes, but are rearranged.

Modern Limits of Dalton's Theory

Two key postulates are modified today: (1) Atoms of the same element can differ in mass because of isotopes (atoms of the same element with different neutron counts, but essentially the same chemical properties); (2) atoms are not indivisible because they consist of subatomic particles (protons, neutrons, and electrons).

Conservation and Composition Laws

Conservation of mass: Because atoms are conserved and only rearranged during chemical reactions, the total mass of matter remains constant.
Law of definite proportions (also known as the law of constant composition): All samples of a pure compound contain the exact same elements in identical proportions by mass (e.g., isooctane always maintains a carbon-to-hydrogen mass ratio of $5.33:1.00$ ).
Law of multiple proportions: When two elements react to form more than one compound, a fixed mass of one element reacts with masses of the other element in a ratio of small whole numbers.

Core Mental Model: Discrete Atomic Ratios

Atoms combine in fixed, whole-number numerical ratios. This microscopic reality explains why macroscopic mass ratios in pure compounds are constant and why mass ratios between different compounds formed by the same elements yield small whole numbers.

Problem-Solving Routines & Methods

Analyzing the Law of Multiple Proportions

1
Calculate the mass of element B per 1 g of element A (fixed mass) for each compound.
2
Divide the mass ratio of the second compound by the mass ratio of the first compound.
3
Verify that the resulting value is a ratio of small whole numbers (e.g., 2:1).

Pro-Tip: Ensure both compound ratios use the exact same denominator (1 g of the fixed element) before comparing them.

Multiple Proportions Comparison

\frac{\text{Mass Ratio Comp 2}}{\text{Mass Ratio Comp 1}} = \text{Small Whole Number Ratio}

Compares mass ratios of two elements forming multiple compounds to confirm whole-number atomic ratios.

Variables & Constants

\text{Mass Ratio}

=mass of variable element per 1 g of fixed element

Practice & Concept Checks

Concept Check

A copper-chlorine compound has 0.558 g of chlorine per gram of copper, while a second compound has 1.116 g of chlorine per gram of copper. What law does this demonstrate, and what is the ratio of these mass ratios?

Concept Check

Why do modern scientists modify Dalton’s postulates that atoms are indivisible and that all atoms of a given element have identical masses?

Key Terms & Vocabulary

Dalton’s atomic theoryAtomic Theory and Laws

set of postulates that established the fundamental properties of atoms

law of constant compositionAtomic Theory and Laws

(also, law of definite proportions) all samples of a pure compound contain the same elements in the same proportions by mass

law of definite proportionsAtomic Theory and Laws

(also, law of constant composition) all samples of a pure compound contain the same elements in the same proportions by mass

law of multiple proportionsAtomic Theory and Laws

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