By Ron B. Davis Jr., Georgetown University
Russian chemist Dmitri Mendeleev gets more than the lion’s share of the credit for first proposing the periodic table. But the advancement of science involves the participation of many people, over many generations—and the development of the periodic table is one of the greatest examples of science as a collaborative effort over generations.
Discovery of Hydrogen
The story of the periodic table begins over 100 years before Mendeleev’s table was ever proposed. That was when English chemist Henry Cavendish, without knowing it, set the stage for the shattering of a 2000-year-old idea that water was an element.
In 1766, he collected what turned out to be one of the elemental components of water, hydrogen gas, from a reaction between iron filings and acid. From there, discoveries of what would soon be recognized as other elements began to pour in; Nitrogen was discovered in 1772, and oxygen was discovered in 1774.
In 1781, Cavendish went on to show that when this new gas came into contact with a flame, it burned in air to form water. This tipped off French chemist Antoine Lavoisier that this new gas must be something simpler than water—an element that combined with others in the air to form water. And because the burning of this new element GEN-erated water, Lavoisier named it HYDRO-GEN.
So the discovery of hydrogen, a substance simpler than water, marked the beginning of a revolution in chemistry.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
List of Elements
Free from the confines of the classical element system, Lavoisier turned much of his attention to identifying and compiling a revolutionary new list of elements. Substances that had been identified within the past century or two, like cobalt, nickel, manganese, and phosphorus now made more sense as elements. Even medieval discoveries like zinc, arsenic, and bismuth turned out to be elements.
Lavoisier helped birth a realization that all of the matter in the world was defined not by a set of four classical elements, but a much longer list of substances more fundamental than anything proposed by ancient scholars.
Instead, Lavoisier categorized his list of known elements, based on their properties, into gases, nonmetals, metals, and earths. The old idea of earth, water, and air as elements would eventually give way to the phases of matter: solid, liquid, gas, all of which any element could take on under the right conditions.
Armed with the knowledge Lavoisier provided, and the invention of better scientific tools, eager scientists across Europe started adding to the list of known elements at a feverish rate. Even before 1800, previously unknown metals, starting with molybdenum, tungsten, and zirconium were discovered, and the discoveries continued with titanium, chromium, and beryllium.
Organized Map of Elements
The first tentative steps toward creating a more organized map of all these elements, rather than just a list, took place in 1829. At that time, just about 50 elements had been identified.
A German chemist by the name of Johann Wolfgang Döbereiner pointed out some remarkable relationships among small groups of elements.
For example, he noticed that chlorine, bromine, and iodine have certain chemical similarities; each of these elements is highly toxic as a pure element. Yet all three form non-toxic salts when mixed with metals like sodium. Döbereiner called these ‘salt-forming elements’.
Triads: Small Groupings of Elements
Döbereiner was able to identify several other small groupings of elements that seemed to share similar properties. Moreover, using only the elements known at the time, it always seemed that there were three elements in each group.
Sulfur, selenium, and tellurium all form acids when combined with water. Lithium, sodium, and potassium were another group of three. They can all form salts with chlorine, bromine, and iodine. They also combine with oxygen in a two-to-one ratio of atoms. By contrast, he noticed oxygen combined in a one-to-one ratio for calcium, strontium, and barium.
Döbereiner thought that he was onto something. He named his groups ‘triads’. But that wasn’t all.
He also noted that when comparing the atomic masses of the elements in each triad, their atomic masses seemed to be evenly spaced out. The middle element always seemed to have an atomic mass close to the average of the other two.
However, there were also some inconsistencies when comparing one triad to another.
Inconsistencies in Triads
Manganese, iron, and cobalt are three elements that have similar properties, and manganese plus cobalt’s masses average out to about that of iron. Nickel, copper, and zinc did as well. But these elements’ masses are much closer to one another. So, these groups are related, but it’s not the same sort of ‘triad’.
Our modern periodic table makes it easy to see why Döbereiner was flummoxed. We now know that manganese, iron, and cobalt, as well as nickel, copper, and zinc, are trios of metals, located next to each other, on the same row, rather than stacked vertically, in the same column.
To make things worse, a system based on groups of three elements already had a chink in its armor. Chemists had long suspected the existence of a fourth element that seemed to act much like chlorine, bromine, and iodine. The name ‘fluorine’ had already been coined in 1812, and chemists like Humphrey Davy literally made themselves sick while trying, unsuccessfully, to isolate it from the very dangerous compound that we would call hydrofluoric acid.
So Döbereiner’s triads idea, while a great start, was just that. It was only the first step on a two-century journey to today’s periodic table.
Common Questions about the Development of the Periodic Table
Nitrogen was discovered in 1772, oxygen was discovered in 1774, and hydrogen was discovered in 1781.
Antoine Lavoisier was a French chemist who categorized his list of known elements, based on their properties, into gases, nonmetals, metals, and earths.
German chemist Johann Wolfgang Döbereiner was able to identify several other small groupings of elements that seemed to share similar properties. Moreover, using only the elements known at the time, it always seemed that there were three elements in each group. He, thus, named his groups ‘triads’.
