"Particle" is used at all scales, most usually subatomic, but also microscopic and macroscopic. A car can be viewed as a particle, so I'm not sure it's so wrong

"Particle" is used at all scales, most usually subatomic, but also microscopic and macroscopic. A car can be viewed as a particle, so I'm not sure it's so wrong to label a group a molecules as "particles". Also, the author was only referring to particles when trying to explain the concept of thermal radiation where he is actually speaking of subatomic particles, not of molecules.