Over 2300 years ago the pre-Aristotelian Greeks sat about arguing the nature of matter. The names Democritus (460 BCE) and Leucippus are often associated with the following 'thought experiment'.
Suppose you had a bit of copper foil. You tear it into two pieces. One of the pieces you discard - the other you tear into two pieces. You repeat the process until the remaining pieces are too small to manipulate with your fingers. You devise some tools to handle the remaining copper and continue to divide it into smaller and ever smaller bits. The question is; could you ever divide the copper foil down to the size that you had a single 'bit' of copper that could not physically be divided any further no matter what tools you had on hand?
Democritus and Leucippus argued you could, and called the itty-bitty bits atoms, which, according to my Greek dictionary is a bit of a self-defining term: atomos means indivisibility due to smallness. Hey, maybe they made the word up. However, theirs was the minority opinion of the time.
In 1803 (CE), John Dalton, a secondary teacher in Manchester, England who loved nothing more than to go out and record observations on the local weather, and whose most recent previous publication had been on elements of English grammar, published a paper suggesting that the smallest particle of any kind of matter was the atom, that atoms were sort of like very tiny billiard balls, and atoms of one kind bonded with atoms of another kind by means of very tiny hooks. He thus renewed, however indirectly and inadvertently, Democritus' concept of the nature of matter. He also did not receive a lot of support initially.
Today we accept the model of the atom with a nucleus placed at the centre. The nucleus contains positively charged protons and uncharged neutrons (with an 'e' between the 'n' and the 'u') and almost all of the atom's mass. The number of protons determines which element the atom is an atom of. Around the nucleus (without an 'e' between the 'n' and the 'u') swirl the negatively charged electrons. In an atom there is one electron for each proton, and thus no overall electrostatic charge; atoms are neutral. We will look into this whole discussion more deeply when we study radioactivity ('nuclear reactions' to be more correct).
So it turns out that Democritus was half right. You can get down to the smallest bit of copper. He was wrong in thinking it could not be broken down further. But if an atom is broken down further, into its protons, neutrons and electrons, it is no longer copper.
Were you actually to divide a piece of copper foil till you got to one atom, then disassemble that atom into its component parts, then do the same with a piece of aluminium foil, there would be no difference between a proton from the copper atom and one from the aluminium atom. They would be identical, indistinguishable. (The same could be said of their neutrons and electrons, but why belabour the point?)