Our understanding about the atom have changed with changes in atomic theories over time.
Our understanding of the atom as the smallest indivisible particle of a substance that can take part in a chemical reaction dates back to Democritus.
However, our understanding of the atom as "indivisible" was revised when J.J Thompson discovered electrons and protons as subatomic particles.
Going forward, Rutherford in his famous gold foil experiment established the fact that the atoms contains a positively charged nucleus which houses most of the mass of the atom. He introduced a planetary model of the atom in which electrons move round this positively charged nucleus.
However, we know from electromagnetism that an accelerating charge radiates energy hence Rutherford's model was quickly replaced by the Bohr model in which electrons were proposed to occupy orbits of specific energy. The atomic spectra was explained on the basis of transition between these orbits also known as energy levels.
However, Heisenberg's uncertainty principle makes it clear that the position of a particle can not be known with precision. Combining this with De Broglie's idea of matter waves, Erwin Schrodinger arrived at the modern cloud model of the atom in which electrons are treated as waves and an orbital is regarded as a region in space where there is a high probability of finding the electron.
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