April 2019 Limerick Project

Atomic Structure

“Gold foil and a question cerebral
Caused the plum pudding model’s upheaval.
Protons, neutrons— contents
Of the nucleus dense—
Proved a fly in the atom’s cathedral.”

The April 2 limerick summarizes Ernest Rutherford’s “gold-foil experiment,” completed in the early 1900s at the University of Manchester.  The Rutherford Group’s experiment was a crucial step towards the modern understanding of how positively charged protons, neutral neutrons, and negatively charged electrons are arranged in the structure of an atom.  (This has always been one of my favorite “science stories,” especially due to the poetic language Rutherford and his colleagues employed in recounting the experiment.)   

“Gold foil and a question cerebral/ Caused the plum pudding model’s upheaval.” 
Prior to the gold-foil experiment, one theory of atomic structure was called the “plum pudding” model: an atom was predicted to exist with negatively charged electrons scattered through the uniform, positively-charged volume of the atom, as raisins are scattered throughout a plum pudding.  

Ernest Rutherford’s research group investigated atomic structure further by devising an experiment: shooting particles through a thin piece of gold foil, then examining where these particles landed.  If the “plum pudding” model were accurate, the particles would travel directly through the foil, essentially in a straight line. However, the data defied this prediction: some particles were deflected sharply, at random angles, from running into something dense within the atoms!  These data rendered the plum pudding model obsolete.      

“Protons, neutrons– contents/ Of the nucleus dense–/
Proved a fly in the atom’s cathedral.”  

Rutherford described the finding:
“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

He and his group ultimately determined that the “something” that some particles were hitting was the atom’s nucleus, a minute-but-massive volume wherein the atom’s protons and neutrons were gathered.  In Rutherford’s nuclear model of the atom, the dense nucleus accounts for nearly all of the mass of the atom, as well as all of the atom’s positive charge; the negatively charged electrons surround the tiny nucleus in a cloud of mostly empty space. Rutherford characterized the size of the nucleus compared to the atom as “a gnat in Royal Albert Hall”; others pursuing similar investigations restated this metaphor as “a fly in the cathedral.”