A century of science behind the modern periodic table

To continue our story of the periodic table, we must consider that for much of the 1800s chemists worked in a strange state of indeterminacy. While the concept of an atom had been proposed in 1808 by John Dalton, the actual existence of discrete atomic entities was fiercely debated. For the most part, the idea of atoms was considered an accounting trick which allowed chemists to discuss chemical compounds and properties. Not everyone agreed.

Stanislao Cannizzaro’s suggestion for the organization of the elements based on atomic weight was met by some with derision. After all, with no proof atoms existed, what did these weights actually measure?

It was against this back drop that Dmitri Mendeleev, Lothar Meyer, John Newcomb, and others worked to understand the relationships between the elements. As mentioned last week, Mendeleev is given credit for finally formulating and publishing the first periodic table.

In part, this is because he recognized there were elements missing from his table allowing him to make predictions about their chemical and physical properties.

For example, one of the elements he called eka-silicon which literally translates as “beyond silicon.”

In 1871, he predicted a gray metallic substance with an atomic weight of 72 and a density of 5.5 grams per cubic centimetre.

When germanium was discovered in 1886, it was indeed a gray metallic substance with an atomic weight of 72.59 and a density of 5.47 grams per cubic centimetre.

Mendeleev was also able to predict the formulas and chemical properties of the oxide, chloride and sulfide while describing how to isolate the pure element accurately.

If Mendeleev had only been able to do this with one element it might have been an amazing guess but he actually predicted just as accurately the chemical properties of a number of elements missing from his table.

As each empty slot was filled in, confidence grew in the periodic system of the elements. And the atomic theory gained traction.

It was another 35 years, give or take, before the last bastions fell and science finally accepted atoms as physical entities.

And it required a shift in our understanding of the universe.

Throughout the 19th century, a number of experiments left scientists perplexed. For example, in the 1860s, Robert Bunsen and Gustav Kirchhoff developed a spectroscope which allowed them to identify elements within a flame by their emission lines. Using this technique, they were able to discover cesium, rubidium and a mystery element found in the spectroscopic signature of the sun which they called helium.

Emission lines from elements were a problem. What were these lines?

Why did they have specific and seemingly fixed positions in the spectrum? What was the relationship which gave rise to the line spacing? These questions remained unanswered for a number of decades although the relationship was resolved to be an inverse square of a numerical series.

In the 1890s, Henri Becquerel noted the decomposition of atoms and J.J. Thompson measured the size of the electron. Atoms appeared to be neither indestructible nor the smallest piece of matter. Electrons by any standard are roughly 2,000 times smaller by mass than the hydrogen atom.

Around the same time, Sir William Ramsay discovered a whole new column for the periodic table.

He had been conducting work on liquefied air and noted a fraction which persistently remained gaseous. Careful separation of the gas led to the discovery of Argon.

Further work, allowed Ramsay and coworkers to isolate all of the “inert” gases – helium, neon, krypton and xenon. Eventually radon joined the family.

Confirmation of helium as present in our atmosphere and in the sun was a major step forward in astrophysics, eventually leading to our understanding of nuclear fusion.

In 1894, Mac Planck turned his attention to the black-body problem which had been troubling physicists.

In 1900, he answered the problem by proposing Planck’s law which is familiar to anyone who has done high school physics. Essentially it says the energy of a photon is proportional to its frequency. The proportionality constant is known as Planck’s constant.

All of this work led to an understanding of atoms as discrete particles. Planck’s work argued even light existed as discrete units. Photons of light were quantized.

The final step in the process was realized by Albert Einstein who wrote a paper in 1905 pointing out the statistical distribution of velocities for atoms and molecules in a gas provided an explanation for Brownian motion. Here was a physically observable manifestation of atoms as real, discrete entities capable of manifesting in the macroscopic world.

How is this related to the periodic table? It to a century for scientists to accept atoms as real entities but the questions then became “what are atoms composed of?” and “how does one element differ from another?”

This leads us to the world of quantum mechanics next week.