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Let's pretend that in other universe we have an element with atomic number 6, like our carbon, but in this other universe the element is completely different from our carbon, like a new element with other properties.

Is this a possible scenario?

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marked as duplicate by Loong, M.A.R., ron, Klaus-Dieter Warzecha, Wildcat Jul 21 '15 at 18:12

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    $\begingroup$ Speculating about the physical and/or chemical properties of some other universe does not seem to be a very useful chemistry question. $\endgroup$ – Jon Custer Jul 21 '15 at 15:09
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    $\begingroup$ This is a very tough question to answer precisely, as you have to dig deep into nuclear physics and quantum field theory. As Jon mentions, it might be more of a physics question. I can try to make a small answer with an interesting reference, though. $\endgroup$ – Nicolau Saker Neto Jul 21 '15 at 15:21
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    $\begingroup$ One could try to imagine a universe with different underlying physical rules, and work out the implications of this on chemical properties. But, so little detail is known even about how quantum mechanics, etc. leads to the chemical properties of the elements in our actual universe, that confidence in the accuracy of such an analysis must necessarily be ... rather low. $\endgroup$ – hBy2Py Jul 21 '15 at 16:05
  • $\begingroup$ I agree with what @JonCuster said. The body should actually start from "let's pretend that there are other universes" passage, since it is a matter of warm debate are there in fact some other universes (whatever they are) or not. Before cosmologist will settle the things down the question has little sense (at best), therefore, I vote to close it. $\endgroup$ – Wildcat Jul 21 '15 at 18:12
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I admit this question is far out of my depth, but it's interesting enough that I feel I should give it a shot.

TL;DR - The question is so hard as to be almost unanswerable in detail. Things can be very different, though not arbitrarily different, and the exact line between the two is very difficult to define. There have been some mild investigations on the chemistry of toy universes, however, one of which I describe below.

The problem with thinking about other universes is that there are potentially a lot of different universes which can arise. Famously, string theory currently posits that there are something like $10^{500}$ "valid" universes, each with different details regarding physical laws and physical constants. Furthermore, universes are very complex objects in the sense that there is a ton of interrelation between everything, so even minor changes to details can create wildly different results. Not just that, but there apparently are many parameters to fiddle with, which would create wildly different periodic tables, often apparently breaking any possibility of life. This all leads to fine-tuning problems.

For example, if the attraction between two protons were about 2% larger, then every proton in the Universe would be bound in the form of a diproton nucleus, i.e. the nucleus of helium-2 ($\ce{^2He}$, which is just barely unstable in our Universe), meaning the entire periodic table would collapse to a single element. Apparently there's no very interesting chemistry in that universe! Now, what exactly would have to be tweaked in order to get a 2% stronger attraction between protons while not breaking everything else, I don't know.

So it seems thinking about chemistry in different universes is far too complicated to approach in almost any situation, but still there are some investigations. One particular case I have come across is what would happen in a universe without weak interactions.

Most of Chemistry ultimately has to do with studying different aspects of electromagnetic interactions. However, the stability of atomic nuclei depends immensely on strong interactions (aka strong nuclear force), and to a lesser extent on weak interactions (aka weak nuclear force). The authors in the article linked above analyzed the effects of taking a universe similar to ours, but removing the weak interaction. There are several differences with respect to the evolution of the universe (big bang nucleosynthesis, stellar nucleosynthesis, etc) and the abundance of the elements, but thinking just about their stability, it seems that in such a universe, a large portion of the periodic table would still exist.

Additionally, given that the weak interaction mediates the decay of neutrons in our Universe, its absence means neutrons are completely stable subatomic particles (in our Universe free neutrons decay with a half-life of around 8 minutes). This also means that neutron-rich nuclei are more stable to decay in the hypothetical weakless universe. In our Universe, deuterium ($\ce{^2_1H}$)is the most neutron-rich stable hydrogen isotope, while tritium ($\ce{^3_1H}$) is stable enough, with a half-life of about 11 years, to be chemically important (heavier hydrogen isotopes can essentially be neglected). However, in the weakless universe, it seems that even hydrogen nuclei as rich as $\ce{^6_1H}$ are indefinitely stable, and there are possibly even more neutron-rich isotopes. See the following figure:

enter image description here

(Source)

For comparison purposes, the most neutron-rich stable isotopes for each element in our Universe are $\ce{^2_1H}$, $\ce{^4_2He}$, $\ce{^7_3Li}$, $\ce{^9_4Be}$, $\ce{^11_5B}$, $\ce{^13_6C}$, $\ce{^15_7N}$ and $\ce{^18_8O}$.

Due to this added nuclear stability, the periodic table in the hypothetical weakless universe could actually have more stable elements in it. In our Universe, elements beyond about the 110th are quite unstable, being produced naturally in small amounts, if any, and they will not likely have great significance in Chemistry. However, highly neutron-rich nuclei could make ultraheavy elements more accessible, even in natural conditions such as supernovae.

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@NicolauSakerNeto has nicely covered the possible effects that a change in nuclear physics would have on the periodic table. I wanted to point out two other possibilities: a different spin for the electron, and a massive photon.

Electron spin

In particle physics, particles come in two main types: "bosons" and "fermions". The electron in our Universe is a fermion, with "spin 1/2"; however, you can in principle also have fermions with spin 3/2, 5/2, 7/2, ... Bosons, meanwhile, have spin 0, 1, 2, ... In terms of familiar quantum numbers, a particle of spin $s$ can have a quantum number $m_s$ anywhere between $-s$ and $+s$, in integer increments. So the electron in our Universe can have two possible values for $m_s$ (-1/2 or +1/2); a spin-one particle can be in one of three "spin states" (-1, 0, or +1); and so forth.

Now, bosons (unlike fermions) do not obey the Pauli Exclusion Principle; in fact, they are more likely than not to be found in the same state. (This is a consequence of something called the spin-statistics theorem.) This means that a Universe in which the electron was a boson would be a very boring universe indeed; instead of filling orbitals in a sequential way, every "electron" would be in the ground state of the atom. (This neglects interactions between the electrons themselves, but this would be a good first-order approximation.) You therefore wouldn't get unpaired electrons in orbitals like you do in our Universe; bonding between atoms, if it exists in this Universe, would be very different from our own. My guess would be that you'd effectively have a Universe full of noble gas atoms, with nothing around that wasn't monatomic.

A Universe with a spin-3/2 electron would be more interesting. These do obey the Pauli Exclusion Principle; and so every electron in an atom would need to have a different set of quantum numbers. However, you could now have four electrons in every orbital ($m_s = -3/2, -1/2, +1/2$, or $+3/2$); and so the periodic table would look very different indeed. I wouldn't want to hazard a guess as to what it would look like in the end; the filling order of the orbitals would be very different in this Universe, and it would be a hard question to determine (for example) how many electrons would go into the $2s$ orbital before the $2p$ orbitals started to fill.

Massive photon

In our Universe, the photon is a massless particle; through the magic of quantum electrodynamics, this leads to the familiar Coulomb potential for an electron orbiting a proton: $$ U(r) = - \frac{Ze^2}{r}. $$ However, if the photon had a mass, then this potential would be replaced by something called the Yukawa potential: $$ U(r) = - \frac{Ze^2}{r} \exp{\left(-\frac{mc}{\hbar}r\right)}. $$ In particular, the Yukawa potential (unlike the Coulomb potential) does not allow for bound states with any value of the angular momentum. This would again change the structure of the periodic table drastically. The filling order of the orbitals would certainly be changed. It's not even clear to me that a heavy nucleus (with atomic number $Z$) would necessarily always have $Z$ orbitals for electrons to fill, and so you could end up with the odd effect of heavy atoms tending to exist as charged ions rather than being neutral.

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