Watsisname wrote:A-L-E-X wrote:Source of the post Wat, are some of the elements we call "artificial" able to exist for short periods of time when stars explode?
Yes, absolutely, and kilonovae (when two neutron stars merge together) are best at both producing and ejecting these heavy elements (especially with atomic mass numbers greater than about 140) into the universe.
For context, let's first look at where the artificial elements lie on the periodic table. Courtesy of wikipedia,Technetium (Tc) with atomic number 43 was first made artificially, as this element has no long-lived isotopes to be found in nature (its longest half-life is about 4.2 million years.) Besides Technetium, the artificial elements have atomic numbers greater than 95. Can supernovae make these elements? You bet. They do so by what's called the "r-process", which is where prolific numbers of free neutrons are available to be rapidly absorbed (hence the "r" in the name) to build larger nuclei. There is a good review of this process within neutron star mergers which is freely available to read here: Neutron Star Mergers and Nucleosynthesis of Heavy Elements (annualreviews.org). Here's one of the most useful figures:
This shows the relative abundances of the nuclei synthesized by neutron star mergers as a function of their atomic mass, and compares to the relative abundances seen in the solar system. In fact, these kinds of simulations are what motivate the graphic you showed earlier. This is how we figure out how much of these elements come from different processes in the universe.Unfortunately, the simulations referenced here cut off above atomic mass numbers of about 240 (remember atomic mass number A, means total number of protons + neutrons, while atomic number Z is just the number of protons and defines the element). So we don't really see the upper tail of elements synthesized here. However, I found a paper published late last year with newer simulations, and while the paper is not free to view, I'll share some of its figures. First is the relative abundance of nuclei, as a function of the mass number, produced by the merger according to three different models. The abundances are also shown at different times after the merger, and you may notice how the heaviest nuclei decay away.Here we can see the r-process nucleosynthesis is creating nuclei with mass numbers well above 300! These are enormous nuclei! For perspective, on the periodic table, the average mass number of element 118 (Oganesson) is 294. However, there is a caveat: because these nuclei are formed by the r-process, they are far more neutron rich than stable nuclei with the same mass number. So they have fewer protons than we would expect for their mass numbers if we just glanced at a periodic table. We can see that more easily in the next figure, where the stable nuclei are represented by black squares.
From this we can see that the r-process nuclei with atomic masses of 280 should correspond roughly to atomic number 95. The ones with atomic masses of 300 should have about 100 protons, and those with atomic masses of 330 should have about 110 protons. So these models are showing that neutron star mergers should be very effective at synthesizing the heaviest elements, even well into what we consider "artificial" elements. The reason we call them "artificial" is not that nature can't make them, but rather that they decayed long before they could be incorporated into Earth and found by humans.
140?! I didn't even know we had names for elements that high! I remember back when I was in 7th grade, for the fun of it, I memorized all the chemical elements in order and back then the list only went to 109. When I was memorizing them, it seemed like they had a nice sense of rhythm and rhyme to them
I also remember reading about an "island of stability" and that when we discovered element 126 we'd find it had some special properties (more stable than the ones surrounding it), I wonder if that'll turn out to be true?
How common are neutron star mergers, Wat, are they much less common than supernova explosions?
I loved those graphs and your explanation... at first I was thinking that anything above 244 would be transplutonium but reading your explanation it looks like atomic masses in those graphs correspond to a lower than expected atomic number. But still being able to get atomic numbers of 110 and even slightly higher (looks like there is a precipitous drop off around atomic mass 340 or so) is pretty amazing! I remember the two elements that always used to perplex me in HS were technetium and promethium, because those were the only two elements below atomic number 95 considered to be "artificial" I take it that neutron star mergers can make these two elements too? I also heard of natural nuclear reactors found in caves in Africa, can these generate some of these higher atomic number elements too, or only up to plutonium?
The other element that perplexed me was bismuth, as in some places I read it was the heaviest stable element, but in others I read that lead was the heaviest stable element and that bismuth had a very long half life. I think the latter turned out to be true.
Also you mentioned element 118, I noticed its placement on the periodic table, is this a noble gas? If so it would be the heaviest gaseous element!
Element 118 (Oganesson)