History of Atoms

Pre-Quantum Mechanics Atomic Theory

Democritus and John Dalton's Theories

Democritus (460 B.C.) speculated the existence of atoms, theorizing them as indivisible and invisible to the naked eye, calling them 'atomos'.

John Dalton's theory (approx. 1800) included the Law of Definite Proportions and the Law of Multiple Proportions, presenting atoms as indivisible solid spheres.

J.J. Thompson and Ernest Rutherford's Discoveries

J.J. Thompson (approx. 1900) discovered the electron through the cathode ray tube experiment, proposing the 'raisin bun' model of the atom.

Ernest Rutherford (1911) conducted the Gold Foil Experiment, leading to the discovery of the nucleus and the atom's composition of mostly empty space.

Classical Theories of Light

James Maxwell's Discoveries

James Maxwell (mid 1800s) described light as an electromagnetic wave with continuous wavelengths forming a spectrum.

Different colors of light have specific wavelengths (e.g., red light = 656 nm) and frequencies.

The Photoelectric Effect and Planck's Contribution

Heinrich Hertz (1887) discovered the photoelectric effect, where light with high frequency emitted electrons from a metal surface.

Max Planck's experiment on blackbodies led to the conclusion that matter emits energy in discrete quantities, represented by E = nhf.

Quantum Mechanics Beginnings

Niels Bohr's Quantum Model

Niels Bohr (1913) developed a quantum model of the atom based on the wave properties of electrons.

Bohr's postulates included the concept of stationary orbits with constant energy levels and transitions between energy levels emitting or absorbing light.

Limitations of Bohr's Model

Bohr's model explains only the first 20 elements of the periodic table and provides a 2D representation of electron locations, lacking a complete quantum mechanical description.