Atoms, Molecules, and Ions: Foundations of Matter

Chapter 2: Atoms, Molecules, and Ions

Introduction: Fundamental Questions in Chemistry

Chemists have long sought to answer two essential questions: "What is the world made of?" and "What holds it together?" The study of atoms, molecules, and ions provides the foundation for understanding the composition and structure of matter.

Early Concepts of Matter

Divisibility of Matter

• Ancient Beliefs: Early philosophers proposed that all matter could be subdivided into smaller and smaller pieces. The concept of the atom (from the Greek "atomos," meaning indivisible) emerged as the smallest unit that could not be further divided without losing the identity of the substance.

• Modern Understanding: While atoms can be split into subatomic particles, doing so changes the element's identity. For example, breaking a gold atom results in fragments that are no longer gold.

• Example: Gold can be divided from ingots to nuggets to dust, but at the atomic level, further division destroys its elemental identity.

Dalton’s Atomic Theory

Postulates of Dalton’s Atomic Theory

• All matter is made up of atoms. Atoms are the fundamental building blocks of matter.

• Atoms of a given element are identical. They have the same size, mass, and properties.

• Atoms cannot be created or destroyed. Chemical reactions involve the rearrangement of atoms, not their creation or destruction.

• Atoms of different elements have different properties.

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

Additional info: Modern atomic theory has refined Dalton’s ideas, recognizing isotopes (atoms of the same element with different masses) and the possibility of nuclear reactions that can change atoms.

Radioactivity

Discovery and Types of Radiation

• Radioactivity is the spontaneous emission of radiation by certain unstable atomic nuclei. Discovered by Henri Becquerel in 1896.

• There are three basic types of radiation:

  • Alpha (α) particles: Helium nuclei, positively charged.

  • Beta (β) particles: Electrons, negatively charged.

  • Gamma (γ) rays: Electromagnetic radiation, no charge.

• These types of radiation can be separated by their behavior in electric or magnetic fields.

Atomic Models

The Plum Pudding Model

• Proposed by J.J. Thomson around 1900.

• Described the atom as a sphere of positive charge with negatively charged electrons embedded throughout, like "plums" in a "pudding."

• This model was later replaced by the nuclear model after further experiments.

Example Table: Comparison of Atomic Models

Model	Main Features	Key Scientist
Plum Pudding Model	Positive sphere with embedded electrons	J.J. Thomson
Nuclear Model	Dense, positive nucleus with electrons in surrounding space	Ernest Rutherford

Additional info: The nuclear model was established after Rutherford’s gold foil experiment, which showed that atoms have a small, dense nucleus.