Atomic Theory, History

The History of Atomic Theory

• Overview of the evolution and major contributors to atomic theory throughout history.

Investigating Atoms and Atomic Theory

• Objectives for students:

  • Describe the particle theory of matter.

  • Use the Bohr model to differentiate among protons, neutrons, and electrons (charges, relative masses, locations).

  • Compare the Bohr atomic model to the electron cloud model regarding representation accuracy of the atom's structure.

Understanding Models

• Definition and examples of models.

• The importance of models in explaining scientific concepts.

• The iterative nature of models as they adapt to new information.

Atomic Models Overview

• Familiarity with the Bohr model:

  • Nucleus orbited by electrons in different energy levels.

  • Models utilize known concepts to elucidate unfamiliar observations.

Key Historical Figures

• Introduction to scientists who contributed to atomic theory.

Early Concepts of Atomic Theory

• Origin in 400 BC with atomic models:

  • Initial models depicted the atom as a solid 'billiard ball.'

Democritus and the Concept of Atoms

• Greek philosopher who questioned the divisibility of matter more than 2400 years ago.

• Proposed that matter could not be infinitely divided and coined the term "atomos."

  • Characteristics of atoms according to Democritus:

    • Small, hard particles of various shapes and sizes.

    • Infinite in number and in motion, capable of combining.

• Democritus' theory remained largely ignored for over 2000 years due to the influence of Aristotle and Plato.

Dalton’s Atomic Theory

• In 1803, John Dalton revitalized atomic theory through experiments:

  • All elements consist of indivisible and indestructible atoms.

  • Atoms of the same element are identical; those of different elements are different.

  • Compounds are formed by the joining of atoms from two or more elements.

• His theory laid the groundwork for modern chemistry.

Mendeleev and the Periodic Table

• In 1869, Dmitri Mendeleev organized elements into the periodic table based on atomic mass, revealing periodic properties and patterns.

• Predicted unknown elements by leaving gaps in his table, known as Mendeleev’s Periodic Law.

Discovery of Subatomic Particles

• 1886: Eugen Goldstein discovers protons using cathode ray tubes, identifying them as positively charged particles within atoms.

Thomson’s Plum Pudding Model

• In 1897, J.J. Thomson proposed the "Plum Pudding" model:

  • Atoms consist of a positively charged substance with negatively charged electrons (corpuscles) scattered within.

• Demonstrated the existence of subatomic particles, emphasizing atomic divisibility.

Milliken and Electron Mass

• 1909: Robert Milliken measured the mass of an electron using specialized equipment, contributing to our understanding of atomic structure.

Rutherford’s Gold Foil Experiment

• 1908: Ernest Rutherford conducted an experiment shooting positively charged particles at a thin gold foil:

  • Most particles passed through; some deflected, suggesting a dense, positive nucleus.

• He concluded that atoms are mostly empty space with a concentrated nucleus containing positive charge.

Rutherford's Model of the Atom

• Proposed that electrons orbit a dense nucleus, containing all positive charges while negatively charged particles are scattered.

Bohr Model Enhancements

• In 1913, Niels Bohr improved atomic theory by incorporating energy levels:

  • Electrons occupy defined orbits around the nucleus.

Modern Wave Model of the Atom

• Evolution of atomic models based on wave mechanics:

  • Electrons do not follow fixed paths but exist in probabilistic regions surrounding a nucleus.

  • This model accounts for the energy of electrons influencing their probable locations.

Electron Cloud Model

• Developed in the 1920s by Schrödinger and Heisenberg, this model suggests:

  • Atoms consist of a dense nucleus surrounded by electrons in varying energy levels, forming an "electron cloud."

  • Electrons are not in random motion but restricted by energy states.

James Chadwick’s Discovery

• In 1932, Chadwick discovered the neutron, enhancing our understanding of atomic structure and nuclear fission.

Enrico Fermi and Nuclear Reactions

• 1942: Fermi discovered that fission of uranium atoms could produce massive amounts of energy, contributing to the Manhattan Project during WWII.

Summary Timeline of Atomic Theory

• 400 BC: Greek Model (Democritus)

• 1803: Dalton Model

• 1897: Thomson’s Plum Pudding Model

• 1911: Rutherford's Model

• 1913: Bohr Model

• 1920s: Wave Model and Electron Cloud Model.

Key Contributions of Thomson, Rutherford, and Bohr to Atomic Theory

J.J. Thomson (1897)

• Proposed the "Plum Pudding" model where atoms consist of a positively charged substance with negatively charged electrons (corpuscles) scattered within.

• Demonstrated the existence of subatomic particles, emphasizing atomic divisibility.

Ernest Rutherford (1911)

• Conducted the Gold Foil Experiment, shooting positively charged particles at a thin gold foil.

• Most particles passed through while some were deflected, suggesting a dense, positive nucleus.

• Concluded that atoms are mostly empty space with a concentrated nucleus containing positive charge.

• Proposed that electrons orbit this dense nucleus.

Niels Bohr (1913)

• Improved the atomic theory by incorporating energy levels into the model of the atom.

• Proposed that electrons occupy defined orbits around the nucleus rather than random paths, leading to the Bohr model of the atom.