Ernest Rutherford was a prominent chemist/physicist at the turn of the 20th century. He performed a famous experiment whereby atomic particles were passed through gold foil to measure their mass and size. He gave his theory of the structure of the atom in 1911 that stated that the majority of the atom's mass is located in a compact, positively charged area in the center of the atom. This central positively charged core (which we now term the nucleus) was surrounded by empty space and negative charge, although Rutherford was not specific about this part of the atom. In 1913, Neils Bohr built upon the Rutherford model with is own model of the atom (sometimes called the Rutherford-Bohr model). He kept the positively charged nucleus part the same but he further defined the negatively charged space surrounding it as electrons of tiny comparative mass that orbit the central core in defined rotational orbits similar to the solar system (planets around the Sun). Bohr's model incorporated the then nascent quantum theory in that the distance between electron orbits was fixed, or quantized, and electrons traveling from one orbit to another absorbed or emitted these quanta of energy.