Answer:
The idea that everything is made of atoms was pioneered by John Dalton (1766-1844) in a book he published in 1808. He is sometimes called the "father" of atomic theory, but judging from this photo on the right "grandfather" might be a better term.
For almost 100 years after this, most scientists thought of atoms as solid structures, until in 1897 J.J.Thompson discovered that atoms contained even smaller particles called electrons. So the modified atomic structure now had electrons embedded in an atom, like M&Ms in icecream.
It was therefore a shock in 1906 when Rutherford and Gieger announced that when they fired alpha particles at a thin sheet of gold foil, most of the alpha particles passed straight through it, but a few bounced back. As Rutherford said "It was almost as if you fired an artillery shell at a piece of tissue paper and it came back and hit you!" The alpha particles were detected by flashes of light on a screen.
This experiment showed that most of an atom is empty space, but there is a very small, dense nucleus in the center which has almost all the mass of the entire atom. Later Rutherford discovered positively charged protons in the nucleus of the atom. Rutherford was well-known for his interesting phrases, like this one:
"All science is either physics or stamp-collecting!"