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We know the alpha particle is the nucleus of helium. It contains four subatomic particles - two protons and two neutrons. The protons and neutrons are further made of particles called up and down quarks. So why call an alpha particle a particle. And similarly, why do we say electrons, protons, neutrons etc. are particles, even though, they are made of particles?

I can understand the case of electrons, protons, and neutrons as when they were discovered and named, quarks were not discovered. But the helium nucleus is a different case.

I reckon talking about an English word's meaning changes in the frame of reference. We do not call a quark an object, although, an object could be any real or virtual thing. So in chemistry, I don't think calling an alpha particle a particle is a good practice. I do understand there are dust particles and all but we mention them only while talking about the macroscopic world. Not so in the case of chemistry. I hope you get my point.

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also known as 'alpha radiation', just the terminology that stuck I think - will be interesting to see an answer to this –  user4076 Mar 7 at 10:34
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I think electrons are elementary particles. It still isn't known whether they actually have some volume or not. –  tschoppi Mar 7 at 11:18
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@AwalGarg done and done and done - always a good thing to learn something new –  user4076 Mar 7 at 11:29
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@Amaterasu good thing. The answers below are really intriguing. –  Awal Garg Mar 7 at 11:30
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@AwalGarg wouldn't you mind to adjust the title of your question ? A pair of four particles yields 8 particles.... –  mannaia Mar 8 at 10:50

4 Answers 4

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Actually, the alpha particle contains two protons and two neutrons. It is what is emitted from the nucleus under alpha decay, giving off what was first classified as alpha radiation by Rutherford at the end of the 19th century (very late 1800's).

In 1932 the neutron was discovered. Only then did it become clear that the nuclei of atoms contain more than just some positively charged particles (which we know as protons).

So there is this historic reason for the naming of the alpha particle.

"Particle"

The word particle does not only pertain to elementary particles (and in [quantum] chemistry, we stop at the level of nuclei as particles, everything that goes deeper is nuclear physics). There are smoke particles, dust particles, etc.

There is also the wave-particle duality. In principle, since we can observe this "alpha radiation" from radioactive processes, there must be a particle that belongs to the wave. With electromagnetic radiation, this is the photon, and in alpha radiation it is the alpha particle.

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Thnx for pointing out the mistake. Updated the question - my bad. I reckon talking about an English word's meaning changes in the frame of reference. We do not call a quark an object, although, an object could be any real or virtual thing. So in chemistry, I don't think calling an alpha particle a particle is a good practice. I do understand there are dust particles and all but we mention them only while talking about the macroscopic world. Not so in the case of chemistry. I hope you get my point. Still, grateful for your answer. –  Awal Garg Mar 7 at 11:35
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Well, maybe then this question is more suited for English.SE than Chemistry.SE. As I understand, your question is: "Why do we still use 'alpha particle' in the light of the evidence that it is made up of four elementary particles?" –  tschoppi Mar 7 at 13:24
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I'm not participating at English.SE, it just struck me that the question you're trying to ask has more to do with the usage of English than actual chemistry. –  tschoppi Mar 7 at 13:31
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To expand on the wave-particle duality comment, there are commercial surface analysis instruments that operate by observing the diffraction of helium from a surface. It's about the largest particle that can be reliably observed interfering in a 2-slit type experiment (ignoring things like Bose-Einstein condensation and superfluidity). –  craigim Mar 7 at 22:43
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@craigim Actually, some rather remarkably large molecules have been shown to exhibit diffraction pattens in double-slit experiments, as expected of a wave. A while ago, the experiment was performed successfully with $\ce{C_{60}}$, and recently with molecules about 50% larger. Last year another type of quantum interference experiment worked with a molecule containing over 800 atoms! –  Nicolau Saker Neto Mar 8 at 12:17

Note that calling something a particle does not imply calling it a fundamental particle. Anything can be a particle (a particle of dust, a particle of sand, a particle of cheese, etc). However, only non-subdivisible particles (particles with no further inner structure) are called fundamental (electrons, quarks, neutrinos, photons, etc).

We used to not know what radiation actually was, so we classified them broadly into three groups according to the way their trajectories bent in an electric or magnetic field, and the three main types found were given an effectively placeholder name (alpha, beta, gamma). The names stuck, even though we now know their real identities.

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Thnx for your answer. I am not saying that particle only implies for fundamental particle. But in the microscopic world of chemistry, a particle, in my view, should not be made up of particles. Do we call an atom a particle? No. Because it does not satisfy the condition of being a particle, i.e. its not only divisible, but in-fact, made of further small particles which are complete entities themselves. I hope you get my point. Still, grateful for your answer. –  Awal Garg Mar 7 at 11:41
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I think it's completely arbitrary whether you want to call them particles or not. We don't usually call things particles in chemistry because in common use it has a very general meaning when used informally (of course physics usually regards particles as specific entities). In chemistry, we don't call atoms particles because we have a more specific way to refer to them (atoms!). We do sometimes use the term particle in chemistry, such as in nanoparticles, which are composed of many atoms themselves. –  Nicolau Saker Neto Mar 7 at 11:47
    
Hmm, that nano-particles' thing is really interesting. –  Awal Garg Mar 7 at 11:50

The alpha particle is more "particle-like" than many other nuclei because it is so tightly bound together, thereby making it much more difficult to break into its constituent parts. The stability of an alpha particle is a result of the number of neutrons and protons in the nucleus. Nuclei that have complete nuclear shells have a larger binding energy per nucleon, giving rise to so-called magic numbers of nucleons (2, 8, 20, 28, 50, 82, 126) that result in particularly tightly bound nuclei. In an alpha particle, the two protons form one complete nuclear shell and the two neutrons form a second complete nuclear shell. This makes the alpha particle doubly magic. The stability and simplicity of the alpha particle makes it energetically favorable to be produced from the fusion of lighter particles and to be emitted in the radioactive decay of heavy nuclei.

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Particle denotes a logical partition of components of a system in a manner which retains an accurate model (on some particular scale) of a physical system while greatly simplifying analysis of the system.

Your example of the dust particle is analogous to the alpha. To someone studying dust interactions, they might create a useful abstraction by ignoring the microstructure of the dust particle and just regard the particle as a single unit. In the case of analysis of radiation transport of alpha particles in a shielding application, the internal structure of the particle is useful and important, but only to the extent that it affects the external microscopic properties of the particle, such as its reaction cross sections, charge, and kinetic energy.

If for almost all intents and purposes nothing is lost, and much is gained, by treating some system as a particle, then it is useful to do so.

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