Development of Modern Atomic Theory

Development of Modern Atomic Theory

• Ancient Greek Philosophers: Leucippus and Democritus proposed that matter is composed of indivisible particles called "atomos".
• Lavoisier's Law of Conservation of Mass: Matter cannot be created or destroyed, only transformed.
• Dalton's Atomic Theory: Each element is made up of unique and distinct atoms with characteristic properties.
• Goldstein's Discovery: Atoms contain both positive and negative components contributing to their behavior.
• Thomson's Plum Pudding Model: Introduced the concept of electrons as negatively charged particles within a positively charged sphere.
• Rutherford's Gold Foil Experiment: Proved the existence of the nucleus, a dense, positively charged core within the atom.
• Rutherford's Conclusion: The atom is mostly empty space with a small, dense positively charged nucleus, and the negatively charged electrons are dispersed throughout the atom's space.

Ancient Greek Concepts About Atoms

• Leucippus and Democritus theorized that matter is composed of tiny particles, which they called "atomos" (meaning "indivisible").
• Their idea was groundbreaking, suggesting that matter had a fundamental building block.

Lavoisier's Contribution

• Antoine Lavoisier formulated the law of conservation of mass, stating that matter cannot be created or destroyed, only transformed.
• This law laid the foundation for understanding chemical reactions and the nature of matter.

Dalton's Atomic Theory

• John Dalton proposed that elements are distinct packets of matter, each with unique properties and behaviours.
• He emphasized that no two elements are exactly alike, leading to the concept of unique atomic identities.

The Investigation of Atomic Structure

• Goldstein suggested that atoms have both a positive and negative portion, hinting at the existence of subatomic particles.
• This idea formed the foundation for future discoveries about the structure of atoms.

Thomson's Plum Pudding Model

• Joseph John Thomson used cathode ray tubes to determine the tiny mass of cathode rays, which he identified as electrons – negatively charged particles.
• He proposed the "Plum Pudding Model," where electrons are distributed randomly within a positively charged sphere, resembling the structure of plum pudding.

Rutherford's Nuclear Model

• Ernest Rutherford conducted the famous "gold foil experiment," shooting positively charged alpha particles at a thin gold foil.
• He discovered that most of the alpha particles passed straight through the foil, indicating that atoms are mostly empty space.
• Some particles were deflected backwards, revealing the existence of a small, densely packed, positively charged region called the nucleus.
• Rutherford concluded that the nucleus is at the atom's center, and electrons occupy the vast space surrounding it.
• His model, however, didn't explain the exact location of electrons within the atom.

Key Figures in Atomic Theory

• Democritus proposed the idea that matter is made up of tiny, indivisible particles called "atomos"

• Antoine Lavoisier formulated the Law of Conservation of Mass, stating that matter cannot be created or destroyed, only transformed.

• John Dalton developed the first atomic theory, asserting that:

  • Elements are distinct packets of matter with unique properties.
  • All atoms of a given element are identical.
  • Atoms cannot be created, destroyed, or broken down into smaller particles.

Early Models of the Atom

• Goldstein hypothesized the existence of both positive and negative components within the atom.

• J.J. Thomson discovered electrons through experiments with cathode ray tubes.

  • He proposed the plum pudding model, imagining the atom as a sphere of positive charge with negatively charged electrons dispersed throughout.

The Nucleus: Rutherford's Discovery

• Ernest Rutherford conducted the gold foil experiment, firing positively charged alpha particles at a thin gold foil.
  • The results showed that most alpha particles passed straight through the foil, indicating the atom is mostly empty space.
  • A small number of alpha particles were deflected back, revealing the presence of a densely packed, positively charged center called the nucleus.
  • Rutherford's model could not explain the location of electrons.

Early Atomic Theory

• Democritus proposed that matter is made up of indivisible particles called atomos.

Key Laws and Theories

• Law of Conservation of Mass, proposed by Antoine Lavoisier, states that matter cannot be created or destroyed, only transformed.
• John Dalton expanded on Democritus’s ideas, stating that elements are distinct packets of matter with unique properties.

Atomic Structure

• Goldstein suggested that atoms have positive and negative portions.
• J.J. Thompson discovered electrons, negatively charged particles, using cathode ray tubes and proposed the plum pudding model which pictured electrons embedded in a positively charged sphere.
• Ernest Rutherford used the gold foil experiment to discover the nucleus of an atom.
  • He bombarded a thin sheet of gold foil with positively charged alpha particles.
  • Most of the particles passed straight through, indicating an atom is mostly empty space.
  • A small percentage of particles deflected backwards, revealing a concentrated positively charged center, the nucleus.
• Rutherford’s experiment didn’t determine the location of electrons within the atom.