The Salt That Had No Molecules: How X-rays Revealed Extended Structures

The Salt That Had No Molecules

[1]In 1772 the French chemist Antoine Lavoisier focused sunlight through a large lens onto a diamond sealed inside a glass jar. The diamond heated, glowed and slowly disappeared, leaving behind a gas that turned limewater cloudy, the same test that carbon dioxide passes. Twenty-five years later, in 1797, the English chemist Smithson Tennant burned a weighed amount of diamond and a weighed amount of charcoal and collected the gas from each. Equal masses gave equal amounts of carbon dioxide. The conclusion was hard to accept: diamond, the hardest material anyone knew, was made of the same thing as charcoal, which is nothing but carbon. One gram of carbon from either source produces about 3.7 grams of carbon dioxide.

[2]That left a puzzle that lasted more than a century. Chemists could measure what a substance was made of, its composition, by burning it or breaking it apart. They could not see how its atoms were arranged. If diamond and graphite, the soft gray form of carbon in a pencil core, had the same composition, then the difference between them had to lie in the arrangement, the structure. But no instrument of the 1800s could show an arrangement of atoms.

[3]The instrument arrived in 1912, when the German physicist Max von Laue showed that a beam of X-rays passing through a crystal comes out as a pattern of sharp spots. The spots meant that the atoms inside a crystal are lined up in neat, repeating rows that scatter the X-rays in an orderly way. In 1913 William Henry Bragg and his son William Lawrence Bragg worked out how to read a spot pattern backward to find where the atoms sit. Among the first structures they solved were table salt and diamond.

[4]The salt result surprised many chemists. The Braggs found that sodium and chlorine alternate in every direction, like the dark and light squares of a three-dimensional checkerboard. Each sodium has six chlorine neighbors, and each chlorine has six sodium neighbors, all at the same distance. No sodium belongs to any particular chlorine. There is no small, separate particle of salt, no 'molecule' of NaCl, only a pattern that continues to the edge of the grain. The formula NaCl describes a ratio of one to one, not the contents of a particle. Some chemists objected for years; as late as 1927 the English chemist Henry Armstrong complained in the journal Nature that a salt with no molecules went against common sense. But the checkerboard model predicted the X-ray spots exactly, and it explained why salt grains tend to grow as tiny cubes. The model won.

[5]Diamond's structure explained its hardness. Every carbon atom is bonded to four neighbors spread evenly around it, and each of those neighbors is bonded to four more. The result is a single rigid network that continues in every direction. To scratch a diamond you must break strong bonds, and there is no direction in which the bonds are weak or missing.

[6]Graphite's structure was solved in 1924 by the British scientist John Desmond Bernal. Each carbon atom is bonded to only three neighbors, and together they form flat sheets of six-sided rings, like chicken wire. The sheets stack one on top of another, but no bonds join one sheet to the next, only weak attractions. The distance between sheets is about 335 picometers, while the distance between bonded atoms inside a sheet is about 142 picometers. The sheets slide apart with a gentle push, which is why a pencil leaves a mark: the paper peels off thin stacks of carbon sheets.

[7]These discoveries settled the puzzle and gave scientists a new way of thinking. The kind of atom is only the beginning. How many atoms join, and how they are connected, decides whether a substance is a tiny separate molecule, a rigid network, a stack of sliding sheets or a long chain. And the arrangement at the atomic scale leaves fingerprints at the human scale: the cube shape of a salt grain, the hardness of a diamond, the gray streak of a pencil. When you build a model with plastic balls, you are doing what the Braggs did with X-rays: showing an arrangement nobody can see.