Why are atoms particles?

Question

The Oxford English Dictionary definition of particle is as follows:

"A component of the physical world smaller than the atom."

I read an article in NewScientist and it said

"...all particles from the atom to the electron..."

Now I'm assuming that since NewScientist is one of the leading science magazines, albeit slightly more of a simple read than others, it wouldn't make a mistake this obvious, so therefore am assuming that an atom is referred to as a particle in physics but I'm slightly sceptical. So:

1. Is an atom a particle?
2. If so, why? It doesn't fit the definition of a particle and I am sure that I have been told on multiple cases that a particle is the smallest form of composite matter.

And after all, how can a particle be made up of particles?

This brings me to a third question, so long as the answer to question 1 is "no":

1. Does that mean that the proton and neutron aren't 'particles', but called something else? (only answer this question if question 1 has an answer of "no").

Answer 1

In practice, I would say a particle is anything that can be treated as a single, bound, small object. This means that whether an object is considered a particle depends on how you're working with it.

For example, an atomic nucleus is a particle if you're doing spectroscopy, because the structure of the nucleus is irrelevant to the changes in energy levels of the electrons and so you can basically consider it a charged point. But that same nucleus would not be considered a particle in an ion-ion collider, because the structure of the nucleus is highly relevant there.

More generally: quarks, gluons, protons, neutrons, atoms, molecules, cells, grains of dust, raindrops, baseballs, satellites, planets, stars, and even galaxies can all be considered particles in an appropriate context.

It doesn't fit the definition of a particle

This is an example of why you should never trust a general-purpose dictionary to give the appropriate definition for a term used in a technical field like physics.

I have been told on multiple cases that a particle is the smallest form of composite matter.

That's not necessarily true; as I've said, it's context-dependent. However, that is the definition implied in the term "particle physics" (which is really meant to mean something like "fundamental particle physics").

Answer 2

It seems you are trying to say that the word particle only has one possible meaning. And if you decide for yourself that that is true, then you have to stick with it. But in general, a "particle" can have various meanings. I can say I have a "particle of dust", which is certainly a particle. I can also refer to a "fundamental particle" like an electron, which, as of now we believe is not a "composite particle", meaning it is not made up of constituents. Consider the nucleons, or other hadrons, these "particles" are made up of more fundamental particles called quarks, which we believe are actually elementary. So, sure an atom is a particle, but it's a composite object, however it's composite nature only reveals itself in particular instances. I guess maybe don't try and force the word "particle" into only one strict definition.