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I'm referring to the fact that there is one element that corresponds to one proton in an atom, a second that corresponds to two protons, up to, I believe, an element with 115 protons. These elements are listed in the periodic table discovered by Mendeleev in the 19th century, or modern expansions thereof.

By "out of order," I'm referring to a situation where elements were not discovered consecutively by number of atoms, e.g. where an element with 103 protons was discovered before an element with 102 protons, and again with 109 protons before 108, according to this table. I use the qualification "after 1950", that is, the "modern era," because this would have given scientists time to sort out Mendeleev's Table after it gave us a road map to atoms. Perhaps more to the the point, the scientific process is a lot more organized than it was in Mendeleyeev's time.

What were the circumstances of these out-of-order discoveries? Isn't it exponentially harder to discover elements with 103 protons than 102 or 101, etc.? Or wouldn't scientists concentrate on creating the atom with the "next" number of protons, e.g. 116 using advanced technology nowadays? Because this is a process that is different from "randomly" coming across atoms commonly found in nature.

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    $\begingroup$ You can find a table of the elements which you can sort by year at the Wikipedia article Timeline of chemical element discoveries $\endgroup$ – MaxW Feb 8 '18 at 22:42
  • $\begingroup$ @MaxW: I rephrased (and expanded) the question. Thanks for your help. $\endgroup$ – Tom Au Feb 8 '18 at 22:54
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    $\begingroup$ Elements weren't discovered in order before 1950. Why would you expect that they would be afterwards? $\endgroup$ – Zhe Feb 9 '18 at 2:52
  • $\begingroup$ Could you reword your title? The word “been” is confusing me. $\endgroup$ – Melanie Shebel Feb 9 '18 at 6:56
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    $\begingroup$ Why would scientists concentrate on finding the next one? If everybody tries to find the same thing, there'll be a lot of wasted effort. $\endgroup$ – David Richerby Feb 9 '18 at 11:23
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The major reason is arguably nothing to do with chemistry (unless you count nuclear physics as nuclear chemistry).

The big issue is that nuclear stability isn't linear with atomic number. Some configurations of protons and neutrons are more stable than others (for reasons that would require quite a lot of nuclear physics to explain). Nuclei with even numbers of both protons and neutrons tend to be more stable, for example. There are particular magic numbers of both that impart some extra stability. And Nuclei containing odd numbers of protons and even numbers of neutrons are less stable than nuclei containing even numbers of protons and odd numbers of neutrons.

The basic takeaway of these observations is that the stability doesn't change in a simple fashion with atomic number (the number of protons). Nuclei with odd numbers tend to be less stable, for example. This means that the easiest ones to make are sometimes not the next element but the next but one element. Or something beyond that. There is even a theory that says there is an island of stability where we will discover some unusually stable heavy elements somewhere slightly beyond the list we currently have.

So there is not reason to expect that we should discover new elements in the order of atomic number.

More detail here.

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  • $\begingroup$ It's also a lot easier to bombard nuclei with alpha particles than with just protons. $\endgroup$ – OrangeDog Feb 9 '18 at 12:32
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    $\begingroup$ @OrangeDog But simple bombardment often doesn't work. Most larger transuranic were created by using much larger "bullets" than alpha particles. $\endgroup$ – matt_black Feb 9 '18 at 13:22
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    $\begingroup$ I know, but reading your answer people would assume the early discoveries went in 2s because they were more stable, rather than because people were using alpha particles to create them. $\endgroup$ – OrangeDog Feb 9 '18 at 13:24
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    $\begingroup$ @TomAu except "The big issue is that nuclear stability isn't linear with atomic number." isn't the right answer. The actual reason concerns how science is conducted and how new elements are synthesized. It's levineds' answer that is correct. $\endgroup$ – OrangeDog Feb 9 '18 at 16:03
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    $\begingroup$ @OrangeDog Even if all heavy elements were made by alpha bombardment (which as noted elsewhere is incorrect), if nothing else was involved you'd just have an 'odd target' and an 'even target' and could alternate between them going up 1 at a time. $\endgroup$ – Dan Neely Feb 9 '18 at 17:08
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The synthetic trans-uranic elements (the "modern era" elements as you call them) are synthesized by bombarding a certain isotope of one element with a certain isotope of another element with a lot of energy in order to get nuclear fusion. The reason they are "out of order" is that the building blocks of these elements have to be very specific isotopes (extra rich in neutrons for example, somewhat abundant for another). This limits the possible sources considerably and so the heavy elements synthesized (and hence discovered) earlier were those where there was a convenient pair of smaller elements to combine to form the larger ones. You don't just bombard each successive element with protons to make the next one.

A lot of the early discoveries were made by bombarding sources with alpha particles, so there are several discoveries that are increases by 2 protons (e.g. Cm->Cf, Am->Bk). Indeed, the table you linked gives more examples of the pairs of elements used to make the heavy actinides and beyond.

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    $\begingroup$ Or bombard with neutrons (e.g. in a nuclear reactor) and wait for beta-decay to turn them into protons. $\endgroup$ – OrangeDog Feb 9 '18 at 16:27

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