Why is the alpha particle called a particle when it is made of four particles?

We know the alpha particle is the nucleus of [helium](https://en.wikipedia.org/wiki/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.




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"
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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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Note that calling something a particle does not imply calling it a [*fundamental particle*](http://en.wikipedia.org/wiki/Elementary_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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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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Particles can be elementary or composite. Composite particles are those made of simpler particles. Elementary particles are not made of anything else and are the building blocks of nature.


[![enter image description here](https://i.stack.imgur.com/spUQc.jpg)](https://i.stack.imgur.com/spUQc.jpg)


Even molecules are often considered particles: [List of composite particles](https://en.wikipedia.org/wiki/List_of_particles#Composite_particles)


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