jan 1, 1911 - Ernest Rutherford discovered the nucleus.
The nucleus is the tiny central core of an atom that is made up of protons and neutrons. He used the gold foil experiment and observed a small portion of the particles bouncing off the foil and very large angles. After his observations he proposed that the atom was mostly empty space but a small region of positive charge in the middle which he called the nucleus. His atomic model was correct, just missing a piece, but ended up being known as the nuclear model. It became what other scientists used when studying the atom and gave them a foundation to build upon.
Added to timeline:
The Atom vs the Telephone