how do scientists discovered atoms if you cant see it?

Answer 1

By experiments. Don't remember how the atom was discovered but the nucleus was discovered by bombarding a thin gold foil with helium nuclei. Since these helium nuclei deviated from their path, it meant that they collided with something which was found to be the nucleus !

Answer 2

for the new elements )greater than 94) they look at the decay of the isotope to determine the original atom