"I think that it is very interesting that one can see the [psi] functions of Schrödinger’s wave mechanics by means of the X-ray study of crystals. This work should be continued experimentally. I believe that much information regarding the nature of the chemical bond will result from it."
Linus Pauling. Letter to A. A. Noyes. 1926.

"One day, late in the day...I had an idea. It was the basic idea of hybrid orbitals. I was trying to understand why the carbon atom is tetrahedral, forms bonds directed towards the four corners of a tetrahedron. Even as early as 1924, I had made a model of methane, in which I said the four outer electrons of the carbon are in orbits directed towards the corners of a tetrahedron.... When quantum mechanics came along, a result was confirmed that had been accepted earlier, that the four outer electrons in the carbon are of two different kinds.... I thought 'the basic principles of quantum mechanics permit us to combine these functions from the Schrödinger equation in another way.' And I said to myself: 'Let’s suppose that I look just at the distribution in various directions, and not worry about the difference in the radial distribution for those.' This permitted rather simple calculations to be made in a straightforward manner. The first result I got was that the best bonds that the carbon atom can form are directed towards the four corners of a tetrahedron. So, in 1931, I had a simple theory of the tetrahedral carbon atom and an explanation of a great bit of organic chemistry."
Linus Pauling. AHQP (Archive for the History of Quantum Physics), volume 6. March 27, 1964.

"For Schrödinger, the atom was an oscillating system -- like the string of a musical instrument -- for which there exist a number of modes of oscillation (fundamentals and overtones) which are interpreted as the atom's energy states. Schrödinger's wave equation impressively provided -- without any additional assumptions -- the right values for spectral lines of the hydrogen atom."
Armin Hermann. Werner Heisenberg 1901-1976, pg. 36. 1976.

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