A number of subsequent experiments have proven this early estimate for the atom’s size to be amazingly accurate. One of the more elegant of these was conducted in Munich in 1912 by Max von Laue. Here, he and his students combined the wave theory of light with the concept of the atom to develop a powerful new imaging tool.
Just as Thomas Young had used the concept of wave interference to interpret the diffraction pattern of light passing through two slits, von Laue used it to interpret the diffraction pattern of X-rays reflecting from the atoms of a crystal. The atoms in a crystal, such as copper sulphate, are regularly spaced in three dimensions, just like a giant stack of cannon balls. The images that von Laue obtained after reflecting X-rays from such crystals confirmed that the spacing between atoms is approximately 1 × 10-9 m, or slightly more than the diameter of a single atom.
During X-ray diffraction from a crystal, the incoming waves reflect from the regularly spaced atoms. The X-rays that penetrate to the second row of atoms must travel a longer distance than X-rays reflected from the top row. When a whole number of wavelengths fits exactly along segments A and B, then constructive interference is observed between the outgoing waves. The length of segments A and B depends, of course, on the spacing between the atoms.
Von Laue had discovered X-ray crystallography, a field that developed quickly thereafter. Before long, scientists could use diffraction patterns to infer not only the structure of simple, regular crystals but also to gain insight into significantly more complex crystalline structures. Forty years later, Rosalind Franklin, James Watson, and Francis Crick would use this same technique to discover the double helix structure of DNA, thereby launching the field of molecular biology.
John Dalton’s “modern” atomic theory, as he called it, provided chemists with a solid foundation upon which to explain the formation of chemical compounds and their rearrangement during chemical reactions. It also provided physicists with a building block underlying the structure of matter more generally. By the late 1800s, the notion of the atom as nature’s fundamental particle was widely held.
QUANTUM QUOTE
Atoms are the minima naturae and are conceived as the first principles or component parts of all physical magnitude.
—the entry for “Atom” in the first edition of the Encyclopedia Britannica, published in 1771
Fortunately this notion turned out to be wrong. Or, at the very least, substantially incomplete. As we will see, if atoms were truly the smallest particles in nature, there wouldn’t be much quantum physics out there to explain!
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