How the periodic table was developed

In eighteen sixty-nine, a Russian chemist called Dmitri Mendeleev drew up a table of the elements and left some of its boxes empty. Beside the gaps he wrote what the missing elements would weigh, what they would look like and what they would react with. Nobody had ever seen any of them. Within seventeen years, three of them had been dug out of the ground, and his predictions were close enough to be uncanny. A filing system for things already known had told chemists about things nobody had found. Filing systems are not supposed to be able to do that.

This is video two of nine in The periodic table. If reading a group or a period feels shaky, The periodic table: groups, periods and position is the one to watch before this.

In Mendeleev's day nobody knew about protons. The one thing chemists could measure was how heavy an element is, compared with the others, and they called that its atomic weight. Every part of this video is for both tiers, Foundation and Higher alike. The early periodic tables put the elements in order of atomic weight, lightest first. Mendeleev did the same, and then he lined up elements that behave alike in the same column. Weight order on its own caused two problems. A few elements landed beside elements they behave nothing like. And an element nobody had discovered left no room for itself. Mendeleev made two moves to fix them. He left gaps for elements not yet found, and he swapped a few pairs out of weight order so that like stayed with like. Keep a label on each table from here on. Mendeleev's table goes by atomic weight. The modern table goes by atomic number, the number of protons. One examiner's report on a Foundation paper notes: "Very few candidates identified atomic weight as the property used to arrange the elements in Mendeleev's Periodic Table." It goes on: "Most remembered that our modern Periodic Table is arranged in order of atomic number so opted for this instead." The two rules had swapped places. Saying the label every time you name a table is the habit that keeps them apart: weight for Mendeleev, number for today.

Here is one of his swaps, and you can check it on the table you are given. Find tellurium and iodine, side by side in Period five. Read off their relative atomic masses. Tellurium is about one hundred and twenty-eight. Iodine is about one hundred and twenty-seven. In strict weight order, which would come first: tellurium or iodine? Iodine, because it is the lighter of the pair. Strict weight order puts iodine first and tellurium after it. That drops iodine into the column with oxygen and sulfur, and pushes tellurium one place to the right. Iodine reacts like chlorine and bromine. Why does that rule out the oxygen column for it? A column is meant to hold elements that behave alike. Chlorine and bromine sit in Group seven, so iodine belongs there with them, whatever its weight says. Mendeleev swapped the pair, putting tellurium before iodine. He trusted how they behaved over what they weighed. Today the reason is atomic number. Tellurium has fifty-two protons and iodine has fifty-three, so number order puts them exactly where Mendeleev put them. The weights disagree because a relative atomic mass is an average over an element's isotopes, forms of one element with different numbers of neutrons. The video on isotopes covers those. Tellurium has plenty of heavy isotopes, so its average comes out heavier, even though it has fewer protons than iodine. Think of lining a class up by height when you really wanted them by age. Mostly it matches, and now and then two people stand the wrong way round.

The gaps were the part that convinced people. Take the gap Mendeleev left in the column below silicon. He said the missing element would have an atomic weight of about seventy-two. He said its oxide would be like silicon's, one atom of the element joined to two of oxygen. In eighteen eighty-six, germanium was discovered. Its relative atomic mass is about seventy-three, and its oxide is G e O two, one germanium to two oxygens. That oxide is the clever part, because Mendeleev had never held a single grain of the element. Why could anyone predict the formula of an element's oxide from its column alone? Elements in one column behave alike, so they form compounds of the same shape. Today we know why: they have the same number of electrons in the outer shell. Mendeleev knew the pattern, not the reason. Electron shells and protons were discovered decades after his table was published. When the predictions came true, they showed the order was more than tidy filing. It matched something real about the elements, and that is why chemists came to trust the table. A gap was never Mendeleev being unsure. A hedge comes with no number attached, and his gaps came with numbers that could be checked.

Comparing the two tables is a question plenty of students handle well. One examiner's report on a Foundation paper lists "the Periodic Table and Mendeleev" among the questions that "were particularly well answered." Here is a real kind of answer to improve. A student compares the tables and writes only that the modern one has more elements in it. That is true. But it says nothing about how the two tables are built. What could they say about the order of the elements instead? Mendeleev's table is in order of atomic weight, and the modern table is in order of atomic number. A second difference is the gaps. His table had empty spaces waiting for elements, and in the modern table every space is filled. A third is a whole missing group. Group zero, the noble gases, is absent from his table, because none of them had been discovered in eighteen sixty-nine. One examiner's report on a Foundation paper records "the most popular answer being in terms of fewer elements in Mendeleev's table". It adds that "there was no reference to ordering by atomic mass or to the absence of the noble gases." Answer instead on three named lines: the order, the gaps and the missing group. Fewer elements can go in too, but never by itself. A different report marks the edge of this story: "Where candidates did not score, it was often because they misinterpreted or misread the question with discussions of Mendeleev's periodic table seen frequently in answers." This history answers one question, why the table looks the way it does. It never tells you what an element will do, so answer those questions with the chemistry instead.

See how much of the story you can rebuild, one question at a time. Argon weighs forty and potassium thirty-nine. In number order, which comes first? Argon, atomic number eighteen, before potassium, nineteen. The heavier one comes first, because the modern table goes by number and not by weight. Now, what did Mendeleev's gaps turn out to be evidence of? That his order was real. The missing elements turned up with the properties he had predicted. Next, which whole group is missing from Mendeleev's table? Group zero, the noble gases. None of them had been discovered when he drew it up. One more: a question asks why sodium fizzes in water. Does Mendeleev belong in the answer? No. His story explains why the table looks the way it does, not what sodium does in water. Those empty boxes from eighteen sixty-nine were a filing system making predictions, and atomic number is the reason it could.

That was the second of the nine videos in The periodic table, the one that comes straight after groups, periods and position. A thumbs up, if you could teach this one, drops it off your revision list. A video with no thumb is still waiting for you, so save it, give it a day and look again.

Next in the chapter: Metals and non-metals.

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