Introduction

William Lawrence Bragg

William Lawrence Bragg
Wikimedia Commons

William Henry Bragg

William Henry Bragg
SPL, Science

Von Laue

Von Laue
Wikimedia Commons

“Our ignorance of solids is very nearly complete,” lamented physical chemist H.C. Jones in his 1913 book New Era of Chemistry. “We do not know what is the formula of solid sodium chloride or rock salt; or of solid water or ice; and we have no reliable means at present of finding out these simplest matters.”

Jones didn’t realize how quickly this was about to change.

In fact, father and son team William Henry Bragg and William Lawrence Bragg had already begun chipping away at this ignorance, by using X-ray crystallography to reveal atomic structure.

The younger Bragg had been captivated by a result reported by German physicist Max von Laue. Pass an X-ray beam through the atomic lattice of a crystal and the X-rays should diffract, von Laue had surmised, yielding a characteristic interference pattern of bright spots on a photographic plate. His collaborators collected the first such diffraction pattern from a copper sulfate crystal in April 1912. The result seemed to settle what had been a source of hot debate: X-rays behave like waves.

But explaining the origin of the pattern of spots was another matter entirely. Enter the younger Bragg. “I can remember the exact spot on the Backs [the riverside in Cambridge] where the idea suddenly leapt into my mind that Laue’s spots were due to the reflection of X-ray pulses by sheets of atoms in the crystal,” Lawrence later wrote. X-rays, he surmised, could therefore be used to reveal molecules’ atomic structure.

Interference pattern observed by von Laue and collaborators using a photographic plate. The central spot is from the unscattered X-ray beam. The dark spots result when X-rays scattering from different layers of a ZnS crystal interfere constructively.

Interference pattern observed by von Laue and collaborators using a photographic plate. The central spot is from the unscattered X-ray beam. The dark spots result when X-rays scattering from different layers of a ZnS crystal interfere constructively.
Wikimedia Commons

The idea, encapsulated in what we now call Braggs law, proved transformative. Inspired by his son’s theory, Henry built the first X-ray spectrometer, which the pair quickly put to use to record the reflections of X-rays from a series of crystals.

For example, their structure of sodium chloride and those of related alkaloid salts revealed that crystals could be made up of repeating atomic lattices rather than molecular ones. Their structure of diamond provided long-sought experimental support for the theory that carbon is tetrahedral.

The Braggs’ spectrometer.

The Braggs’ spectrometer.
Science Museum, London, Wellcome Images

“It was a wonderful time,” Laurence later recalled. “Like discovering a new goldfield where nuggets could be picked up on the ground, with thrilling new results every week.”

That is, until World War I stopped their work together. Lawrence headed off to war while Henry turned his talents to submarine detection. But the discipline the Braggs founded, X-ray crystallography, has continued to thrill during the past century, giving scientists a window on the atomic world. In honor of 2014’s International Year of Crysallography, we’ve highlighted a few of our favorite X-ray crystal structures – ones that answered pressing chemical questions of their day. We hope you’ll share your favorites at http://cenm.ag/favecrystals. –Amanda Yarnell

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