Atoms, Molecules, and Ions: Laws of Chemical Combination

Atoms, Molecules, and Ions

Introduction

This section introduces fundamental concepts in atomic theory and the laws that govern the combination of elements to form compounds. These principles are foundational to understanding chemical reactions and the nature of matter.

The Law of Constant Composition

Definition and Explanation

• Law of Constant Composition (also known as the Law of Definite Proportions): A given chemical compound always contains its component elements in a fixed ratio by mass and does not depend on its source or method of preparation.

• This law was first stated by Joseph Proust in 1799.

Example: Oxides of Nitrogen

• Consider several samples of an oxide of nitrogen:

• Sample 1: 1.00 mg N, 1.14 mg O

• Sample 2: 1.00 mg N, 2.29 mg O

• Sample 3: 1.00 mg N, 2.86 mg O

• Each specific oxide of nitrogen will always have the same mass ratio of nitrogen to oxygen, regardless of the sample size or source.

Key Point: The composition of a pure compound is always the same.

The Law of Multiple Proportions

Definition and Explanation

• Law of Multiple Proportions: When two elements (A and B) form more than one compound, the masses of element B that combine with a fixed mass of element A are in the ratio of small whole numbers.

• This law was formulated by John Dalton in 1803.

Example: Oxides of Nitrogen

• Suppose nitrogen and oxygen form several oxides:

• Oxide 1: 1.00 mg N, 1.14 mg O

• Oxide 2: 1.00 mg N, 2.29 mg O

• Oxide 3: 1.00 mg N, 2.86 mg O

• The ratios of the masses of oxygen that combine with a fixed mass of nitrogen (1.14 : 2.29 : 2.86) are simple whole number ratios (approximately 1:2:2.5, which can be further simplified depending on the compounds).

Key Point: Elements can combine in different ratios to form different compounds, but these ratios are always small whole numbers.

The Law of Conservation of Mass

Definition and Explanation

• Law of Conservation of Mass: In a chemical reaction, matter is neither created nor destroyed. The total mass of reactants equals the total mass of products.

• This law was established by Antoine Lavoisier in 1789.

Example: Formation of Water

• 2H2(g) + O2(g) → 2H2O(l)

• 0.134 g H2 + 1.066 g O2 → 1.200 g H2O

• The total mass before and after the reaction is the same (0.134 + 1.066 = 1.200 g).

Key Point: Mass is conserved in all chemical reactions.

Visual Representations of Chemical Laws

Molecular Models

• Water (H2O): Two hydrogen atoms bonded to one oxygen atom.

• Hydrogen (H2): Two hydrogen atoms bonded together.

• Oxygen (O2): Two oxygen atoms bonded together.

These models help visualize the conservation of atoms and mass during chemical reactions.

Summary Table: Laws of Chemical Combination

Law	Statement	Example
Constant Composition	Each compound has a fixed ratio of elements by mass.	Water is always 11.2% H and 88.8% O by mass.
Multiple Proportions	Elements combine in small whole number ratios to form different compounds.	CO and CO2: 1.33 g O per 1 g C in CO, 2.66 g O per 1 g C in CO2 (ratio 1:2).
Conservation of Mass	Mass is conserved in chemical reactions.	2H2 + O2 → 2H2O (mass of reactants = mass of products)

Additional info: The images and diagrams referenced in the materials illustrate the application of these laws in real-world and laboratory settings, such as mass spectrometry analysis of breath and molecular models of compounds.