## Chapter 51: Interference Phenomena (Max von Laue 1879 – 1960, Walter Friedrich 1883 – 1968, Paul Knipping 1883 – 1935)

After X-rays had been discovered by Roentgen in 1895, a considerable controversy arose as to their nature. They were unresponsive to electromagnetic forces. But X-rays were scattered from small particles in suspension the way light is scattered in a dusty medium. This and other observations convinced Max von Laue that X-Rays were electromagnetic waves. But X-rays could not be refracted or diffracted the way visible light is because of much shorter wavelength. Laue introduced a very ingenious idea: to use the regular array of atoms in the lattice structure of a crystal as the grating. Laue worked out the theory of this type of diffraction. He received the Nobel Prize for this work in 1914.

## Chapter 52: Bragg’s Law (William Henry Bragg 1862 – 1942, William Lawrence Bragg 1890 – 1971)

W. L. Bragg examined the results of X-ray diffraction from Laue’s experiments done in 1812. He proceeded to show that the Laue pattern could be analyzed in a much simpler way. Bragg adopted the view that the incident X-rays consisted of a continuous spectrum extending over a wide range of wavelengths. He supposed that the atoms of the exposed crystal act as a diffraction center and radiate secondary waves, as if they are reflected from a plane made of a number of atoms. The intensity increased with the density of atoms in that plane. The research by the Braggs showed that the X-ray emission spectrum of an element is characteristic of that element, and that X-rays can be used as a powerful and precise means of crystal analysis. The 1915 Nobel Prize was awarded to the Braggs for their contribution to crystal analysis.

## Chapter 53: Atomic Number (Antonius Van der Broek 1870 – 1926)

Antonius Van den Broek was the first to suggest that the number of charges in an element’s atomic nucleus is exactly equal to the element’s place on Mendeleev’s periodic table. The number of the place of an element in the periodic table at that time was not thought by most physicists to be a physical property. Simultaneous development of α-particle scattering theory and the X-ray scattering measurements suggested that the number of electrons per atom is equal to half the atomic weight of the atom. It was not until the work of Henry Moseley working with the Bohr model of the atom with the explicit idea of testing Van den Broek’s theory, that it was realized that atomic number was indeed a purely physical property (the charge of the nucleus) which could be measured, and that Van den Broek’s original guess had been correct. Henry Moseley, in his paper on atomic number and X-ray emission, mentions only the models of Rutherford and Van den Broek.

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MAIN POINTS

1. X-rays are not deflected by electrical or magnetic fields.
2. X-rays are scattered from small particles in suspension the way light is scattered in a dusty medium.
3. X-rays can excite atoms in a body to emit fluorescence radiation
4. They are electromagnetic waves with much shorter wavelength.
5. Ingenious idea of using the atoms forming the lattice structure of a crystal as a diffraction grating.
6. Incident X-rays consist of a continuous spectrum extending over a wide range of wavelengths.
7. X-ray emission spectrum of an element is characteristic of that element.
8. X-rays can be used as a powerful and precise means of crystal analysis.
9. X-ray scattering measurements suggested that the number of electrons per atom is equal to half the atomic weight of the atom.
10. To each possible intra-atomic charge corresponds a possible element.
11. Mass deficit was accounted by protons neutralized by electrons in the nucleus.

THEORY
The number of charges in an element’s atomic nucleus is exactly equal to the element’s place on Mendeleev’s periodic table.

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