How was the nucleus of an atom discovered?

1 Answer
Jul 28, 2015

Answer:

How was the nucleus of an atom discovered? By means of the classic gold foil experiment of Marsden, Rutherford, and Geiger, 1909, at the University of Manchester.

Explanation:

Before this experiment, it was thought that matter had a plum pudding structure (see J. J. Thomson: "positive ions in a sea of electrons"). Marsden (under Rutherford's and Geiger's direction) fired alpha particles at a thin gold plate. (FYI, gold is extremely malleable: it can be hammered out to very thin surfaces, only a couple of atoms thick as it was in this experiment.)

Most of the alpha particles passed straight through the plate; some were deflected; a few of the alpha particles bounced straight back at the generator. This last result was completely unexpected given the thickness of the sheets. In Rutherford's words,

" It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you. "

Rutherford could only account for this phenomenon by proposing a central atomic nucleus, which contained all the positive charge, and most of the mass. Thus began the nuclear age: matter was composed a central, heavy nuclear core about which electrons circulated.