Today I was reading the book titled "World of Physics Volume II by Jefferson Weaver"; a great book that contains Nobel speeches and information on some of the greatest contributions in science, but more on that later. Anyways back to the topics, in that book in the chapter regarding the Scattering Experiment by Lord Rutherford, in it he coined the terms as Alpha(α), Beta(β) and Gamma(γ). But in the paper he called them as Alpha, Beta and Gamma Rays. While I discarded it as irrelevant, in my further reading the terms used were as α and β particles while γ Rays. That was what burned my curiosity : Why α and β as particles while γ as rays?
Now when I looked up for definition of a particle I found this
"... a particle is a small localized object to which can be ascribed several physical properties such as volume or mass..." - Wikipedia
And the answer to question is the mass of the three elements.
Alpha (α) : Alpha particles are made of 2 protons and 2 neutrons.
This means that they have a charge of +2, and a mass of 4.
Beta (β) : Beta particles are made of 1 electron. Beta particles have a charge of minus 1, and a mass of about 1/2000.
Gamma (γ) : Gamma are not particle rather they are a bust of energy i.e. they are in wave form. Thus it has neither any charge nor any mass.
Hence Gamma as called rays while the other as particles.