# 1: The Dawn of the Quantum Theory

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"A profound change has taken place during the present century in the opinions physicists have held on the mathematical foundations of their subject. Previously they supposed that the principles of Newtonian mechanics would provide the basis for the description of the whole of physical phenomenon and that all the theoretical physicists had to do was suitably to develop and apply these principles. With the recognition that there is no logical reason why Newtonian and classical principles should be valid outside the domains in which they have been experimentally verified has come the realization that departures from these principles are indeed necessary. Such departures find their expression through the introduction of new mathematical formalisms, new schemes of axioms and rules of manipulation, into the methods of theoretical physics." **P. A. M. Dirac, "Quantum Mechanics" (1930).**

- 1.1: Blackbody Radiation Cannot Be Explained Classically
- All bodies emit thermal radiation spanning a broad range of wavelengths. • The amount and peak wavelength of the radiation depends on the temperature of the body, but not on its composition. • The higher the temperature, the more radiation is emitted and the shorter (or bluer) the wavelength of the bulk of the radiation.

- 1.2: Quantum Hypothesis Used for Blackbody Radiation Law
- Energy is quantized in some systems, meaning that the system can have only certain energies and not a continuum of energies, unlike classical mechanics.

- 1.3: Photoelectric Effect Explained with Quantum Hypothesis
- Einstein's theory of the photoelectric effect made the claim that electromagnetic radiation had to be thought of as a series of particles, called photons, which collide with the electrons on the surface and emit electrons when absorbed. This theory ran contrary to the belief that electromagnetic radiation was a wave and thus it was not recognized as correct until 1916 when Robert Millikan experimentally confirmed the theory

- 1.4: The Hydrogen Atomic Spectrum
- Gases heated to incandescence were found to emit light with a series of sharp wavelengths. The emitted light analyzed by a spectrometer appears as a multitude of narrow bands of color. These so called line spectra are characteristic of the atomic composition of the gas. One such set of lines in the hydrogen ATOM emission are the Balmer lines, in which a phenomenological relatioshop between frequency and an integer of unknown origin.

- 1.5: The Rydberg Formula and the Hydrogen Atomic Spectrum
- The Rydberg formula is used to describe the wavelengths of spectral lines and was formulated by the Swedish physicist Johannes Rydberg. The Rydberg formula explains the different energies of transition that occur between energy levels. When an electron moves from a higher energy level to a lower one, a photon is emitted. The Hydrogen atom can emit different wavelengths of light depending on the initial and final energy levels of the transition.

- 1.6: Matter Has Wavelike Properties
- Matter waves are often referred to as de Broglie waves and have wavelengths (λ) to its momentum, p, through the Planck constant, h: λ = h/p .

- 1.7: de Broglie Waves can be Experimentally Observed
- An electron, indeed any particle, is neither a particle nor a wave. Describing the electron as a particle is a mathematical model that works well in some circumstances while describing it as a wave is a different mathematical model that works well in other circumstances.

- 1.8: The Bohr Theory of the Hydrogen Atom
- The model we will describe here, due to Niels Bohr in 1913, is an early attempt to predict the allowed energies for single-electron atoms. It is observed that excited hydrogen atoms emit light at only discrete wavelengths. Bohr's model was a non-phenomenological (based on basic physics principles) that predicts the discrete nature of the spectral of Rydberg's formula and decomposes the Rydberg constant into the fundamental constants of the universe.

- 1.9: The Heisenberg Uncertainty Principle
- The Heisenberg Uncertainty Principle is a fundamental theory in quantum mechanics that defines why a scientist cannot measure multiple quantum variables simultaneously. The principle asserts a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables, such as position x and momentum p, can be known.

- 1.E: The Dawn of the Quantum Theory (Exercises)
- These are homework exercises to accompany Chapter 1 of McQuarrie and Simon's "Physical Chemistry: A Molecular Approach" Textmap.

Thumbnail: The Photoelectric effect require quantum mechanics to describe accurately (CC BY-SA-NC 3.0; anonymous via LibreTexts).