10. Forms of the periodic table

‘Forms of the periodic table’ discusses some of the different periodic tables that have been published and asks whether there is an ‘optimal’ periodic table. Meaningful differences only occur when one changes the order of the elements. Examples include moving the positions of hydrogen and helium to better reflect their chemical properties, or the shifting of elements to form triads of similar behaviour. Whether an optimal periodic table exists depends ultimately on one’s philosophy. Realists believe that the repetition of chemical properties is an objective fact, and so strive for the most accurate depiction of it. Instrumentalists believe that such depictions are simply a human construct and not ultimately important.

4. Steps towards the periodic table

‘Steps towards the periodic table’ outlines the different versions of the periodic table that were devised in the 1860s following the Karlsruhe conference in Germany, which introduced standardized atomic weights. De Chancourtois actually discovered periodicity, but his ideas did not gain traction for many years. Newlands devised the law of octaves, but the musical analogy garnered ridicule from academics. Odling made important discoveries in periodicity, but lacked enthusiasm for the idea. Hinrichs used his wide range of interests to postulate that the ratios of atomic size could explain each element’s unique spectral fingerprint. Meyer made huge strides in discovering periodicity, but, due to a misplaced table, did not get the recognition he deserved.

1. The elements

‘The elements’ outlines the history of the study of elements. Ancient Greeks had just four elements—earth, water, air, and fire—which they believed possessed the shape of the platonic solids. They also believed that the four elements each possessed abstract qualities, a belief which later evolved into the concept of periodicity. Some elements have been known for millennia. Others have required technological advancements in order to be isolated. Elements have been named by a variety of means, which has caused controversy among discoverers. Alchemists applied symbols to elements based on associated planets and celestial bodies. This was replaced by the lettered system we use today.

7. Electronic structure

‘Electronic structure’ examines the work that went into determining the properties of electrons in atoms. Quantum theory was devised by Max Planck in 1900, and was applied to hydrogen atoms by Niels Bohr in 1913. Bohr hypothesized that electrons existed in set shells around a nucleus and then extrapolated this theory, using chemical rather than physical observations, to other elements. G. N. Lewis hypothesized a cube model of electron arrangement around the nucleus. Despite this not being correct, Lewis concluded that chemical bonding was due to the pairing of electrons, an idea still central to modern chemistry. Finally, Charles Bury determined that electron shells around a nucleus did not need to be filled in a particular order.

6. Physics invades the periodic table

‘Physics invades the periodic table’ assesses the impact of key discoveries in physics on the understanding of the periodic table. Ernest Rutherford provided evidence for the nuclear structure of atoms, and also determined that the charge of an atom is equal to half its atomic weight. Anton van den Broek linked this principle to the number of protons in a nucleus, thus devising the notion of atomic number. Henry Moseley quantified this principle, and used it to show exactly how many elements would fill the gaps in the periodic table. Radioactive experiments created new forms of elements with different weights but the same charge, which Frederick Soddy identified as isotopes.

5. The Russian genius

‘The Russian genius: Mendeleev’ covers the life and work of Dmitri Mendeleev, who not only discovered the periodic system, but also recognized that it pointed to the periodic law. Mendeleev was rejected by Moscow University on race grounds, so studied at St Petersburg before moving to Germany. Mendeleev differed from rival chemists in that he rejected the notion of the unity of matter, Prout’s hypothesis, and the notion of triads. In a single day in 1869, he sketched out an entire periodic table. This table made many accurate predictions, but also made some mistakes. Inert gases, when first isolated in 1894, struggled to fit into Mendeleev’s periodic table, but in 1900 they were successfully accommodated.

2. A quick overview of the modern periodic table

‘A quick overview of the modern periodic table’ explains the arrangement of elements in the periodic table, and introduces the concept of periodic law. Elements were originally ordered by their equivalent weight, but this was superseded by atomic weight, and then atomic number. There are many versions of the periodic table, but all obey periodic law, which states that after certain regular, but varying intervals, the chemical elements show an approximate repetition in their properties. Developments in physics, especially quantum mechanics and relativity, have changed the way we think about elements and periodicity. The number of known elements has increased to 118 as the result of the synthesis of artificial elements.

9. Modern alchemy

‘Modern alchemy: from missing elements to synthetic elements’ explores the science behind the synthesis of elements. Rutherford and Soddy were able to transmute elements by bombarding nuclei with particles. This only worked up to calcium, and it took the invention of the cyclotron in the 1930s to enable the transmutation of heavier elements. This enabled the gaps in the periodic table to be filled. The twenty-six elements after uranium in the periodic table do not always follow periodicity, so they were excised from the main body and put into their own section. The high speeds of electrons in atoms can produce relativistic effects, but the chemical periodicity of elements is a remarkably robust phenomenon.

8. Quantum mechanics

‘Quantum mechanics’ shows how Bohr’s quantum theory was a stepping stone for the development of quantum mechanics. Bohr’s quantum theory worked well in single electron systems, but not in multi-electron systems. Quantum mechanics allowed the development of Schrödinger’s equation, which could theoretically predict the determination of electron energy levels in any system. The advantage of quantum mechanics over quantum theory lies in its treatment of electrons as waves. This allowed Schrödinger to apply mathematical boundary conditions to his equation and quantize the energy levels of electrons. Further work by Heisenberg showed that electrons are spread around a spherical shell. New developments on atomic configurations by Eugen Schwarz are also discussed.

3. Atomic weight, triads, and Prout

‘Atomic weight, triads, and Prout’ discusses the shift in chemical thinking from qualitative analysis to quantitative analysis. John Dalton, drawing on the work of Antoine Lavoisier and Benjamin Richter, was the first to put forward the ideas of modern atomic theory. Avogadro realized that gases could exist as diatomic molecules, and Von Humboldt and Gay-Lussac postulated that these molecules reacted in a ratio. Prout also realized that atomic weight rose in multiples of the weight of hydrogen. Döbereiner introduced the concept of triads of similarly reacting elements, but developments of this system were flawed. It was Kremers who first started to relate elements that were chemically dissimilar, thus paving the way for periodicity.