Definition of an atom
Rip any piece of the world apart, but you may use only the energies available to the ancients – horses, flames, and lightning. When you cannot rip the smaller pieces apart any longer (to produce only neutral particles) then those last (neutral) particles are ATOMS.
I would start students off by asking “If I rip this piece of matter into two pieces, and I weigh the two pieces, is the sum of those weights more than, the same as, or less than the weight of the original piece?” Hopefully, your students will assent that “It is the same.” [If not, you may want to generate a lab here. However, with unskilled students and not very good weighing apparatus, you may not be able to “prove” that the weights are exactly the same.]
Ask the question again. If I rip each small piece into two parts, and I weigh these pieces, is the sum of the weights of all the pieces more than, the same as, or less than the weight of the original piece?”
Again. Again., etc.
And when appropriate, ask “Is there some last piece that I cannot rip apart?”
[Here you are following the line of reasoning that Democritus used, in approximately 500 BC, to argue that there must be a “last particle” (a = “not”, tomos = cut”; together the word “atom”), but Democritus never had any experimental data to support his assertion.]
1. Note that I have avoided using the words “proton”, “neutron”, “electron”, “nucleus”, etc. My Physical Chemistry students had great difficulty in defining an atom without describing (true) things about the structure of an atom. I have tried to develop a description of atoms as the “last particle” and to deflect all discussion – at this point – about the internal structure of that particle.
2. Note also that the phrase “using only the energies available to the ancients – horses, flames, and lightning” allows for chemical changes – breaking bonds, forming ions, etc – but does not allow energies large enough to cause nuclear reactions. The idea here is to present atoms as building blocks of our common materials, and to guide the discussion away from changing the blocks.