Video: History of the Periodic Table

In this video, we will learn about the history of our understanding of the elements, and how that understanding was reflected in the organization and shape of the periodic table of elements.


Video Transcript

In this video, we will learn about the history of our understanding of the elements and how that understanding was reflected in the organization and shape of the periodic table of elements. How could you tell if a substance is fundamental or elemental?

Let’s take a simple cake as an example. If you break down a cake, some of the mass will be from the flour, some will be from the butter, and some from the sugar, and so on. If we want to prove that sugar isn’t fundamental or elemental, we have to try to break it up again. If we can’t, it’s probably an element. But in the modern era, we know that sugar is made up of carbon, hydrogen, and oxygen atoms.

We know about atoms and protons inside the nucleus. And we know that carbon atoms have six protons in their nuclei, oxygen atoms have eight, and hydrogen atoms have one. We know these substances are elements because no matter how hard we try, we would never get another substance more elemental than carbon, hydrogen, and oxygen. It takes tremendous amounts of energy to split atoms. And if we do that, we change the elements entirely. As our technology and understanding has improved, we’ve been able to prove that more and more substances either are or are not elements.

Long before we understood what atoms, ions, or nuclei were, there were substances in use that turned out to be chemical elements. It’s not until much more recently we were able to prove that. These were the elemental substances in use before about 1000 CE: copper, lead, gold, iron, silver, carbon, tin, sulfur, mercury, zinc, arsenic, antimony, and bismuth.

At room temperature and pressure, all of these substances were solids, apart from mercury, which was a liquid. After bismuth, it took over 600 years for the next element to be discovered. And by the end of the 1700s, another 21 substances we now know to be elements had been discovered. For the first time, elements that form gases at room temperature were identified: hydrogen, oxygen, nitrogen, and chlorine.

In 1789, the discoverer of hydrogen and oxygen as distinct elements, Antoine Lavoisier, published a chemistry textbook with one of the first formal groupings of the elements — gases, metals, nonmetals, and earths — based on their behavior. Under gases, Lavoisier identified oxygen, nitrogen, and hydrogen. But under gases, he also grouped light and heat. It may seem strange now because we’re so familiar with the definition of element. But he was trying to understand and group all the phenomena that he’d come across.

Under metals, Lavoisier only included substances we consider to be elements today. Under nonmetals, Lavoisier listed sulfur, phosphorus, and carbon and three substances we wouldn’t call elements: hydrochloric acid, hydrofluoric acid, and boric acid. All the substances Antoine put under earths are not, as we consider them, elements. It would take the discovery of electrochemistry in the coming decades to separate out these compounds.

The next revolution came in the early 1800s. The idea of an atom, something so fundamental it can never be cut in two, is much older than the 1800s. But the first scientific model of the atom wasn’t proposed until 1808 by John Dalton. At this point, 46 elements had been discovered. His simple model drew atoms of the elements as circles. These could be combined in specific ratios, giving distinct chemical compounds, something we take for granted today.

In 1814, Jöns Jacob Berzelius came up with a system that used letters rather than circles to indicate the elements, using words like “oxygen” to derive O and the Latin for iron, “ferrum,” to derive Fe. A lot of the symbols he came up with are in use today.

There were a few observations made over the next few decades that led the way toward a better understanding of the elements, such as the work of Johann Wolfgang Döbereiner in 1829 that identified triplets of elements that have patterns in their properties, like chlorine, bromine, and iodine. But the first hint of a periodic table we could recognize came for Alexandre-Émile Béguyer de Chancourtois in 1863. By this point, there were reliable estimates of the relative atomic masses for many of the elements. So we put the elements in order of relative atomic mass. After this, he noticed periodic patterns in the properties of the elements.

In 1864, John Newlands proposed the law of octaves. When arranging the elements by relative atomic mass, he saw patterns of behavior that repeated every eighth element. Newlands put these related elements in columns, forming rows of seven elements each. While the beginning of his table resembles the modern version, there were lots of groupings that didn’t prove to be true. The noble gases were yet to be discovered. If they had been, we might have had a law of nonaves instead.

Five years later, 200 years of elemental discovery led to one of the most memorable episodes in chemistry history. In 1869, Dmitri Ivanovich Mendeleev published his first periodic table of elements. His table used some of the same principles as Newlands’ table. It was arranged by relative atomic mass and grouped by chemical behavior. But Mendeleev did something that every good scientist should be willing to do. He didn’t force the data to fit his theories. Instead, where there wasn’t an element with properties he expected, he left a gap. This is his initial table, where the elements are arranged vertically in order of relative atomic mass and periodically left to right. This is rotated 90 degrees compared to what we consider normal.

He used element symbols and an equal sign to connect it to its relative atomic mass. His table included predictions of elements yet to be discovered, with their estimated relative atomic masses. Some of the symbols in this table will look unfamiliar because he used Pl for palladium and Ur for uranium. Mendeleev also consciously went against the arrangement by relative atomic mass when it made more sense based on the chemical character of the elements. So the positions of tellurium and iodine were reversed. Tellurium behaved more like the elements oxygen, sulfur, and selenium. And iodine behaved more like bromine, chlorine, and fluorine.

However, the table wasn’t perfect. There was still some uncertainty, particularly as elements got heavier. Mendeleev released a revised version in 1871, arranging elements horizontally by relative atomic mass and periodically top to bottom, giving us the groups and periods we’re more familiar with. This version of the table is slightly simplified compared to the original print.

The unknown elements below boron, aluminum, and silicon were labeled eka boron, eka aluminum, and eka silicon. Eka, meaning one in Sanskrit, indicates the element one below the given element. So eka boron is one below boron. And it would’ve been below in the original layout.

Eka boron was discovered eight years later and named scandium. Eka aluminum was discovered in 1875 and was named gallium. And eka silicon was discovered in 1886 and named germanium. Mendeleev’s predictions came true. So we could admire the courage of Mendeleev to go against the established science of the time and introduce gaps.

There was still a problem with the table. There were certain elements that were reversed with respect to their order by relative atomic mass, like tellurium and iodine. Revisions to Mendeleev’s table occurred as new elements were discovered. But a breaking point in the fundamental principle was reached in 1913.

With the discovery of the noble gases and more accurate measurements of the relative atomic masses of copper and nickel, more swapped pairs had emerged. Tellurium and iodine had been joined by argon and potassium and cobalt and nickel. The existence of the nucleus inside atoms and protons inside the nucleus had only being demonstrated two years before by Ernest Rutherford, Marsden, and Geiger.

With more accurate relative atomic masses, more evidence of a problem, and awareness of the proton, Henry Moseley overrode the fundamental organizing principle for Mendeleev’s periodic table of elements. Instead of organizing elements by relative atomic mass, Moseley proposed the template for the modern periodic table, with elements arranged by atomic number, the number of protons in the nuclei of the element.

Atomic number and relative atomic mass are roughly proportional across the elements. So it’s understandable why relative atomic mass worked so well. Under this system, elements like tellurium and iodine, which in the old system seemed to be swapped, actually followed the correct order, sitting in their right group and increasing in atomic number in a natural fashion.

The relative atomic mass is roughly equivalent to the number of protons and neutrons in an atom of an element, while the atomic number is equal to the number of protons in an atom or ion of an element. The number of protons in an atom or ion is what primarily determines its chemical behavior. But neutrons only add mass. Because of the range of isotopes and isotopic abundances for an element, the relative atomic mass of an element is not as conclusive a predictor of chemical behavior as atomic number is.

Now, it’s about time we had some practice.

The periodic table is an example of a model. It allows scientists to make predictions by highlighting patterns in the properties of elements. The discovery of new elements allowed scientists to fill gaps and correct mistakes in the original periodic table. Which of the following words best describes the model used to construct the original periodic table?

Arguably, the very first periodic table came in 1863 from Alexandre-Émile Béguyer de Chancourtois, who put elements on a spiral on a piece of paper. Elements were arranged on the cylinder left to right by relative atomic mass and arranged vertically using the spiral by chemical behavior. However, this version isn’t generally considered a traditional table.

Traditionally, the original periodic table is that of Dmitri Ivanovich Mendeleev in 1869, which arranged elements bottom to top by relative atomic mass and left to right in periodic chemical behavior. In 1871, Mendeleev produced a revised version where periods went top to bottom and groups went left to right, more like our modern periodic table. The key feature that distinguished Mendeleev’s system from previous systems was that he left gaps using existing data to predict the properties of unknown elements. This made Mendeleev’s table a very good model because it allowed for accurate prediction. New elements likes scandium, gallium, and germanium were discovered later and inserted naturally into the gaps, fitting the predictions very closely.

Now, let’s have a look at the question. We need to look at five words and find the one that best describes the model used in the original periodic table. These three organizing principles constitute the model used to make the table. Now, it would be perfectly accurate to say that Mendeleev’s table was wrong. There were lots of things that have since being changed.

But the question isn’t just asking for any description. We’re looking for the best description, one that does justice to the great work that it was. So the original table was wrong in some respects. But it was also correct in many respects. It would also be fair to say that Mendeleev’s tables were fundamentally flawed because they use relative atomic mass rather than atomic number as we use today.

However, based on the data of the time, tellurium and iodine were the only pair of elements that seemed out of sequence. Tellurium had a higher relative atomic mass, but its chemical behavior meant it fit better if it was before iodine rather than after. What would be unfair is to call Mendeleev’s tables unscientific because they reflected insight into the data available at the time.

The fact that Mendeleev left gaps suggested by the data and the fact that he switched round tellurium and iodine despite it not fitting the relative atomic mass principle suggests that he was genuinely thinking about what he was doing. He didn’t want to just make the data fit his theory.

The last word that we could apply sensibly to Mendeleev’s tables is simply “incomplete.” It was made before we understood atoms in any more detail and before we understood protons and their impact on chemical behavior. Out of all the answers, this is the fairest. While there were wrong and correct and a flawed aspects to the table, it was a step in the right direction, a decisive turn in our understanding of the elements.

As with many scientific models and theories, development happens in stages. And we don’t necessarily need to discard a model just because it’s not perfect. So of the five words we’ve been given, the one that best describes the model used to construct the original periodic table is “incomplete.”

Let’s have a look at the key points. The pure chemical substances we now identify as being elements were isolated at different times in history. After enough elements were discovered, they were arranged by relative atomic mass. And when scientists looked at how the elements behaved, they noticed periodic patterns in chemical behavior. Models, in the form of periodic tables, were made to predict properties of undiscovered elements, particularly those of Mendeleev around 1870.

After minor flaws in Mendeleev’s tables, relative atomic mass was replaced by atomic number as the primary organization principle of modern periodic tables. Over the last 100 years, although many elements have since been discovered, the fundamental organizing principles of our periodic table haven’t really changed. Over the last 350 years, the number of identified elements has skyrocketed from 14 in the late 1600s to 118 in the modern day.

In 1789, Antoine Lavoisier identified four groups of substances that he considered elements: gas, metal, nonmetal, and earths. The gases, metals, and nonmetals contained a number of substances we consider elements today. In 1808, John Dalton proposed the first scientific model of the atom. And in 1869, Dmitri Mendeleev laid the foundations of our modern periodic table with his first draft. And lastly, in 1913, two years after Rutherford demonstrated the existence of the proton, Henry Moseley rearranged the periodic table according to atomic number, fixing minor issues with Mendeleev’s original design. Since then, elements have been added to the periodic table into the gaps of atomic numbers.

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