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

Atomic Theory: Historical Perspectives

• Ancient Greek Model: Early philosophers proposed that matter could be divided into indivisible units called atoms (from the Greek 'atomos', meaning indivisible).

• Modern Understanding: While atoms can be further subdivided into subatomic particles, breaking them apart destroys the identity of the original element.

• Example: Gold can be divided from ingot to dust, but the smallest unit retaining gold's properties is the gold atom. Breaking the atom yields fragments that are no longer gold.

Dalton’s Atomic Theory

Postulates of Dalton’s Atomic Theory

• All matter is made up of atoms. Atoms are the smallest units of an element that retain its chemical properties.

• Atoms of a given element are identical in mass and properties; atoms of different elements differ in these respects.

• Atoms cannot be created or destroyed in chemical reactions.

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

• Chemical reactions involve rearrangement of atoms, not their creation or destruction.

• Example: Water (H2O) forms from two hydrogen atoms and one oxygen atom combining in a fixed ratio.

Radioactivity

Discovery and Types of Radiation

Radioactivity is the spontaneous emission of radiation by certain unstable atomic nuclei. It was first observed 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.

• Example: In a field, α particles bend toward the negative plate, β toward the positive, and γ rays are unaffected.

Models of the Atom

The Plum Pudding Model (circa 1900)

• Proposed by J.J. Thomson, this model depicted the atom as a sphere of positive charge with negatively charged electrons embedded throughout, like "plums" in a "pudding."

• This model was an early attempt to explain the internal structure of the atom after the discovery of the electron.

• Limitation: It could not explain later experimental results, such as those from Rutherford's gold foil experiment.

Additional info: Later models, such as Rutherford's nuclear model, replaced the plum pudding model by introducing the concept of a dense, positively charged nucleus.