By Robert M. Hazen, Ph.D., George Mason University
Order was given to the wide and growing range of chemical elements by the Russian chemist Dmitri Mendeleev. He lived from 1834 to 1907. His father was a teacher of Russian literature, and Dmitri received an excellent education from a very early age. He eventually attended university in St. Petersburg, where he studied chemistry.
Mendeleev’s Brilliant Table
Many of Dmitri Mendeleev’s earliest publications relate to the chemical analysis of minerals, and also of petroleum—materials from the Earth. That was the raw material for chemists in this period—the unusual minerals, the petroleum, all the different substances that could be got out of the ground.
It was this work, along with his interest in teaching chemistry, that led him to search for systematics in the patterns and the properties of the different chemical elements. Mendeleev knew of 63 different chemical elements. This was his data; this was the raw material of his study. And that’s when he began working on his famous table in the 1860s.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Measurable Properties of Chemical Elements
It turns out that chemical elements have a whole number of measurable properties, and this provides the basis for grouping the elements in various rows and columns, and various patterns, and so forth. There are relative weights. The relative weights can be measured by observing ratios of the weights when compounds are decomposed. When water is decomposed by electricity, for example, it produces two parts hydrogen to one part oxygen gas by volume, 1:8 by weight. So there are all these different ratios, weights, and volumes, and so forth.
By measuring the decomposition of many different compounds, the relative weights of many different elements can be determined. If hydrogen has a weight of 1, for example, Mendeleev found that sodium has a weight of 23, calcium of 40, barium of 137, for example. All the elements then can be arranged in sequence according to increasing weight.
Systematic Properties of Chemical Elements
Elements also displayed other systematic properties. During electrolysis, for example, a mineral or other compound is dissolved in acid. The two electrodes, of course, separate out these elements. And what is found is that a few elements, including oxygen and chlorine, and bromine, always go to the positive electrode. These gases can be collected. They bubble up through the acid and can be collected there. Metals are always deposited at the negative electrode. And so the metals can be collected and their properties determined.
So, for example, if silver chloride is dissolved in acid, the chlorine gas comes bubbling up off the positive side. The metal, the silver metal, is plated onto the negative anode. In fact, this is the principle for silver plating and other kinds of metal plating by electrical methods, which are called electroplating, in a general sense. It is done by using a battery and dissolved metals in the solution.
Learn more about the periodic table.
Features of Alkaline Metals
Long before Mendeleev’s work, chemists had recognized that some groups of elements showed striking similarities and patterns. This is true for their physical appearance, for their chemical properties, and other sorts of behavior. Furthermore, for each of these groups of related elements, the physical and chemical properties seemed to somehow change systematically with their weight.
The elements lithium, sodium, potassium, and rubidium, for example, are all soft, silvery metals. They’re called alkaline metals. These elements all react violently with halogens like chlorine. And a 1:1 ratio of sodium to chlorine, or potassium to chlorine, or rubidium to chlorine is obtained.
They’re all soluble in water. In fact, they all react somewhat violently to water. They also all burn rapidly to form 2:1 compounds with oxygen, for example, sodium two oxygen [Na2O], or rubidium two oxygen [Rb2O].
It turns out that lithium is the least reactive; that’s the lightest one as well. Rubidium is the heaviest, and it’s the most reactive. Despite their similarities, the relative weights of these four compounds—lithium to sodium to potassium to rubidium—are a rather odd ratio; it’s seven to 23, to 39 to 85, which seems rather arbitrary.
Alkaline Earth Metals and Their Features
Another group of elements are called alkaline earth metals. These include beryllium, magnesium, calcium, and barium. They form 1:2 compounds with chlorine, such as calcium two chlorides [CaCl2], or a 1:1 compound with oxygen, such as calcium one oxygen one [CaO]. That’s called lime, by the way.
The relative weights of these four silvery metals are nine to 24 to 40 to 88. Once again, that’s an odd sequence of numbers; but think about those numbers: nine, 24, 40, 88, to seven, 23, 39, 85. There seems to be some sort of similarity. It’s not an identical pattern, but there’s certainly something very close there. So, Mendeleev would have noticed that sort of thing as well.
Learn more about why atoms bond to one another.
Properties of Halogens and Other Elements
And then there is another group called the halogens: chlorine, bromine, and iodine. They’re all highly reactive nonmetals. They form 1:1 compounds with hydrogen, 1:1 compounds with the alkaline metals, 2:1 compounds with the alkaline earths.
They’re all gases or liquids. Their relative weights are 35, 80, 126. And those numbers—35, 80—are very similar to the 39, 85 ratio of potassium and rubidium in the alkaline metals. So once again, there seems to be some mathematical relationship there, but it’s hard to know whether that’s significant or not.
Still, other elements, such as hydrogen, carbon, and sulfur, were more difficult to group in any systematic way. There didn’t seem to be any other elements that were exactly like those, so Mendeleev had to be very puzzled when he filled this complete array of elements and tried to systematize them.
Common Questions about How Dmitri Mendeleev Organized the Chemical Elements
Dmitri Mendeleev was a renowned Russian chemist who arranged a large number of chemical elements in a table according to their various properties, such as their systematic and measurable properties.
Many of Dmitri Mendeleev‘s publications relate to the chemical analysis of minerals, and also of petroleum. His interest in this work, as well as teaching chemistry, led him to the systematic study of chemical elements. He knew about 63 elements and these elements were the raw material for designing his famous table.
Halogens such as iodine, bromine, and chlorine are a group of nonmetals in the periodic table. Halogens are highly reactive. They form compounds with hydrogen, alkaline metals, and alkaline earth metals.
