150th Anniversary of the Periodic Table of the Elements

Happy New Year, and welcome to another year of Breakfast Bytes. This year is the 150th anniversary of the periodic table of the elements. That's the table that would have been on the wall somewhere when you took any chemistry lessons. There's one below. It's officially (as decided by the United Nations) the International Year of the Periodic Table (also the International Year of Indigenous Language).

I'm putting this in the first post of the year because I've not been able to find out exactly when the original paper was published. It was Mendelejew, D. (1869). "Über die Beziehungen der Eigenschaften zu den Atomgewichten der Elemente". Zeitschrift für Chemie: 405–406.

The key sentence in that paper is:

If one arranges the elements in vertical columns according to increasing atomic weight, such that the horizontal rows contain analogous elements, also arranged according to increasing atomic weight, one obtains the following table, from which it is possible to derive a number of general deductions...

The Periodic Table

At the time, nobody had a clue about subatomic particles. We now know that the table is actually organized in increasing order of atomic number (the number of protons and electrons), although generally, this order is the same as atomic weight (including neutrons, but they weren't discovered until 1932). Of course, nobody knew anything about quantum mechanics back then either. A modern description of atomic orbitals deals with wave equations and energy, and the probability of finding electrons at certain energy levels. I'm going to stick to the classical model since it is more comprehensible.

As you scan the table from left to right, the reason for the gaps is that the orbitals can only hold a certain number of electrons. So Hydrogen (atomic number 1) has one electron in the inner shell, Helium (atomic number 2) has 2 electrons in the inner shell, and then it is full. The next element is Lithium, which like all subsequent elements has two electrons in the inner shell and a third one in a second shell. Beryllium is next with two electrons in that shell. But it is not full at two electrons, that shell will hold 8, so it is Sodium (Na), at atomic number 11 which starts the third shell with just one electron. It is in the same column as Lithium because all the elements in that column have one electron in their outer shell and end up having similar chemical properties. Indeed, I have seen chemistry described as the science of the outer shell of electrons, probably by a physicist on the basis of Rutherford's remark that all science is either physics or stamp-collecting. Once you get to the Lanthanides (rare earths) higher up the table, the outer shell fills before the inner shell, leading to very similar chemical properties despite the different atomic numbers, since they have the same number of electrons in the outer shell.

Mendeleev, with no knowledge of electrons, reverse engineered the table by noting which elements had similar properties, as you can see from the quote from his paper. Most impressively, he predicted the existence and properties of some of the holes in his table, which have subsequently been filled. Of special significance for semiconductors, one of those elements he called Ekasilicon (for some reason, he used Sanskrit prefixes, and eka means one). We now call it Germanium (named after his country by the German who finally discovered the actual mineral).

There are a number of stories of how he discovered the table, the most reasonable of which is that he wrote the first chemistry textbook and while he was preparing it, he noticed the way that periodically elements would have very similar properties, although apparently, he claims the idea came to in a dream.

If you want to explore the table, there is a truly remarkable interactive version.

Dmitri Mendeleev

Dmitri Mendeleev didn't have a very promising start in life:

Born in Siberia in 1834 as one of anywhere between 11 and 17 children—biographical accounts differ, as infant mortality rate in the era was devastatingly high—he was immersed in tragedy from an early age. His father was a professor of fine arts, philosophy, and politics, but grew blind and lost his teaching position. His mother became the sole breadwinner, working at a glass factory. When Dmitri was thirteen, his father died. Two years later, a fire destroyed the glass factory.

Things improved and he got to his father's alma mater, Saint Petersburg University, in the city that was still the capital of Russia (it only moved to Moscow after the Russian Revolution in 1917—to find out why it is called the October Revolution even though it is in November, see my post Doomsday in 1900 Was a Wednesday). By the age of 20 he was publishing academic papers. He wrote the standard textbook Principles of Chemistry when he was 27. He received tenure at 32. The work on the periodic table was published when he was 35. He died in 1907 aged 72.

Element 14

We are largely in the element 14 business. That would be silicon. It is the second most abundant element on earth (after oxygen). But much of the periodic table is used in semiconductor manufacturing. I suspect that like me, you'd never even heard of element 72, Hafnium, before it turned out to be key in HKMG transistors. The most important part for the semiconductor industry are the orange elements above. Semiconductors are one of:

• individual elements in orange such as silicon or germanium;
• compounds of elements in orange such as GaAs (Gallium Arsenide)
• alloys of elements in orange, such as aluminum gallium arsenide.

Next Generation Interconnect

When Aluminum (Al) became too thin to carry the currents needed in semiconductors, we switched to copper (Cu). This was far from a trivial switch. The method of fabrication had to change completely. Further, copper was historically regarded as a terrible contaminant and so impossible to use. Even today, if you ever have a wafer in one of those single wafer carriers, it will have a stripe of orange tape on it if the interconnect fabrication has started, and it must never go in the front end of line.

Copper is an intractable material. The reason we don't use copper is NOT because we haven't tried over the years.

That was a quote from Gordon Moore himself in 1997 (the year in which, coincidentally, copper interconnect was first announced).

But copper is becoming too thin, not so much because of the copper itself, but because copper requires a liner. Unfortunately, the liners need to be a certain thickness to both be effective and manufacturable, meaning that most of the space is taken up with liner and there is little room for the copper itself. So you need to learn that Ru is Ruthenium, Co is Cobalt, Rh is Rhodium.

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