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What are the differences between the alpha particle and helium 2+ ion? Or are they the same? Do differences arise because of the process by which they are formed?

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    $\begingroup$ They are the same think: $\ce{^4_2He^{2+}}$ nuclei. Guess you would call them $\alpha$ particles when the source is the radiactive decay of a heavy atom. $\endgroup$
    – PAEP
    Commented Oct 14, 2023 at 12:18
  • $\begingroup$ It is similar as the difference between proton and "naked" H+, or a beta particle and an electron. The same thing but different context. $\endgroup$
    – Poutnik
    Commented Oct 14, 2023 at 13:15
  • $\begingroup$ Wikipedia: Alpha particles have a typical kinetic energy of 5 MeV (or ≈ 0.13% of their total energy, 110 TJ/kg) and have a speed of about 15,000,000 m/s, or 5% of the speed of light. $\endgroup$
    – Karsten
    Commented Oct 14, 2023 at 14:03

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This is a good thoughtful question. Alpha particles and $\ce{He^{2+}}$ have the same composition but their origins are different. There are no "helium nuclei" units or packets in a large radioactive nucleus. Suppose someone hands over two red apples to you, one from America and one from Japan, then just by looking at them you will not be not able tell their farm origins. However, the key point is that if you to measure the energies of alpha particles, then one might say something about the source. Once alpha particle is out of the nucleus of a heavier nucleus, it is no different chemically or composition wise than a doubly ionized helium atom.

Historically nobody knew that alpha particles have the same composition as the helium nuclei until an elegant experiment was done. Rutherford and his students passed alpha particles through a thin sheet into an empty glass tube. After a while, they passed electric current through the glass tube, lo and behold, the spectrum of the "material" was identical to helium gas. Hence the connection is made.

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  • $\begingroup$ Thanks for differentiating them. $\endgroup$ Commented Oct 18, 2023 at 6:19
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  • All alpha particles are $\ce{^4He}$ nuclei, but not necessarily vice versa. (and not all $\ce{He^2+}$ ions belong to helium-4 as alpha particles do, as permeakra noted.)
  • Alpha particles have been named so historically as particles of the unknown, high energy radiation named alpha, observed at some heavy elements, originally for uranium minerals like uraninite $\ce{U3O8}$, later for newly discovered radium, radon, polonium, thorium and others.
  • Alpha particles are product of an alpha decay of some unstable atomic nuclei. $\ce{^{$x$}_{$y$}A -> ^{$x-4$}_{$y-2$}B + ^4_2He}$
  • $\ce{^4He}$ nuclei may have origin of being initially an alpha particle, or they can be product of helium ionization.
  • Alpha particles are called so while they keep substantial part of their original high kinetic energy in order of $\ce{MeV}$(while typical thermal particle energies are in order of $\ce{0.01 eV}$) . Their mean flight length until becoming neutral helium atoms is typically fractions of a milimeter in condensed matter or $\pu{1-2 cm}$ in air.
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Helium 2+ ion is any $\ce{He^{2+}}$ ion, meaning both $\ce{^3_2He^{2+}}$ and $\ce{^4_2He^{2+}}$ ions qualify as well as heavier ones.

In contrast, $\alpha$-particle means specifically $\ce{^4_2He^{2+}}$, usually in context of nuclear decay or nuclear and particle physics.

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  • $\begingroup$ @Poutnik I meant exactly what I wrote. Isotopes up to He-10 are characterised, though only He-6 and He-8 live long enough to speak about (absence of) chemical properties. $\endgroup$
    – permeakra
    Commented Oct 16, 2023 at 14:06
  • $\begingroup$ Yeah, I remember them. I just did not have them on mind, mentioning them myself elsewhere. $\endgroup$
    – Poutnik
    Commented Oct 16, 2023 at 14:09

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