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I confused that if different isomers are identical particles or not?

If yes, how about $^4$He and D$_2$ (these two are not isomers but contain the same number of protons, neutrons and electrons)?

If no, how about same molecular in different electronic energy level, how about same electronic energy level but different vibration or rotation energy?

And I want to know why? I confused about these composite particles.

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    $\begingroup$ en.wikipedia.org/wiki/Nuclear_isomer discusses nuclear isomers, answer below touches on chemistry. Which are you imagining here? $\endgroup$
    – Jon Custer
    Jun 23 at 12:39
  • $\begingroup$ @JonCuster +1 I didn't think of it, since the article linked in the OP is on chemistry. $\endgroup$ Jun 23 at 13:30
  • $\begingroup$ @RogerVadim - indeed, but then the example seemed to concern the equivalence of 2 protons and 2 neutrons in different 'configurations'. Confusing to be sure. $\endgroup$
    – Jon Custer
    Jun 23 at 15:46

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In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space.[

That is, isomers are molecules that consist of the same atoms, but differently arranged, as, e.g., glucose, fructose and saccharrose (which are sometimes all referred to as different isomers of glucose).

$^4He$ and $D_2$ are not isomers - one is a helium atom, while the other is a hydrogen molecule (although with the same number of protons and neutrons). Also, one should not confuse isomers with isotopes, which are the same chemical element with different number of neutrons.

Isomers are especially common in inorganic compounts. A particular type of isomers, known as chiral molecules, are mirror reflections of each other. Although, in principle, these possess the same chemical properties, they behave differently vis-à-vis otehr chiral molecules. Notably, biological systems have preferred chirality, and failure to appreciate this fact has resulted in a pharma scandal that entered many textbooks.

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Isomers are compounds that contain exactly the same number of atoms, i.e., they have exactly the same empirical formula, but differ from each other by the way in which the atoms are arranged. So He and D2 are not isomers

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I feel like you have confused isotopes with isomers.

Isotopes are atoms that have the same number of positively charged protons but different number of neutron, hence the almost identical electrons behavior around the nucleus, which is all chemistry is about. (Isotope actually means the same place in latin, which denotes the idea that they occupy the same place in the periodic table of CHEMICAL elements.)

As the result compound made of different isotopes have almost the same chemical properties. That's why isotopic labeling is used to study the mechanisms of chemical reactions.

But a slight difference is there nonetheless. Heavier isotopes react at slower rates in general. That's why the heavy water D$_2$O is toxic, as it slows down your metabolism, but only lethal in quite high dosage, like 50% of all the water in your body.

As far as nuclear physicists are concerned atoms with different compositions of neutrons are totally different as well. That's why there is another term nuclides to emphasize they are different species.

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