Atoms, Molecules, and Atomic Theory: Foundations of Chemistry

Atomic Theory of Matter

Historical Development of Atomic Theory

The concept of the atom originated with ancient Greek philosophers such as Democritus, who proposed that matter is composed of indivisible particles called "atomos." However, it was not until the eighteenth and nineteenth centuries that experimental evidence led to the development of a scientific atomic theory, most notably by John Dalton in the early 1800s.

• Atomic Theory: The idea that all matter is made up of tiny, indivisible particles called atoms.

• John Dalton: Developed the first modern atomic theory based on experimental laws.

Fundamental Laws Leading to Atomic Theory

• Law of Constant Composition: Also known as the law of definite proportions, this law states that a given compound always contains the same proportion of elements by mass. For example, water (H2O) always contains two hydrogen atoms for every oxygen atom, regardless of the sample source.

• Law of Conservation of Mass: The total mass of substances present after a chemical reaction is the same as the mass before the reaction. This law, discovered by Antoine Lavoisier, underpins the principle that atoms are neither created nor destroyed in chemical reactions.

• Law of Multiple Proportions: 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. For example, carbon and oxygen form both CO and CO2, but the ratio of oxygen atoms in these compounds is a simple whole number (2:1).

Dalton’s Atomic Theory

Postulates of Dalton’s Atomic Theory

Dalton’s atomic theory provided a systematic explanation for the laws of chemical combination. The main postulates are:

1. All matter is composed of extremely small particles called atoms.

2. All atoms of a given element are identical in mass and properties, but atoms of different elements differ in mass and properties.

3. Atoms cannot be created, destroyed, or transformed into atoms of another element by chemical reactions.

4. Atoms of different elements combine in simple whole-number ratios to form compounds.

Example: In nitrogen dioxide (NO2), one nitrogen atom combines with two oxygen atoms, always in the same ratio.

Visual Representation of Dalton’s Theory

• Atoms are represented as spheres of different colors or sizes for different elements.

• Compounds are combinations of these spheres in fixed ratios.

Significance and Limitations

• Dalton’s theory explained the laws of constant composition and conservation of mass.

• Later discoveries (such as subatomic particles and isotopes) showed that atoms are divisible and that atoms of the same element can have different masses.

Key Terms and Concepts

• Atom: The smallest unit of an element that retains its chemical properties.

• Element: A substance made up of only one kind of atom.

• Compound: A substance composed of two or more different elements chemically combined in fixed ratios.

• Isotope: Atoms of the same element with different numbers of neutrons (not covered in detail in these slides, but important for later study).

Summary Table: Laws Leading to Atomic Theory

Law	Statement	Discoverer	Relation to Dalton’s Theory
Constant Composition	Compounds have a definite, fixed composition by mass.	Joseph Proust	Postulate 4
Conservation of Mass	Mass is neither created nor destroyed in chemical reactions.	Antoine Lavoisier	Postulate 3
Multiple Proportions	Elements can combine in different ratios to form different compounds, with ratios of small whole numbers.	John Dalton	Postulate 4

Example Problems

• Law of Constant Composition: If a 10 g sample of water contains 8.89 g of oxygen and 1.11 g of hydrogen, what is the mass ratio of hydrogen to oxygen? Solution: $\frac{1.11}{8.89} = 0.125$ (or 1:8 by mass)

• Law of Multiple Proportions: Carbon and oxygen form CO and CO2. In CO, 12 g of carbon combine with 16 g of oxygen. In CO2, 12 g of carbon combine with 32 g of oxygen. The ratio of oxygen masses is $\frac{32}{16} = 2:1$.

Additional info: Later sections of this chapter will cover the discovery of subatomic particles, atomic structure, isotopes, and the periodic table.