Christmas Pudding and the Atom

Screenshot 2022-12-16 at 10.37.14What we now call Christmas Pudding is not what it used to be. In medieval times, they knew it as Christmas pottage, made from hulled wheat boiled in milk along with spices and sugar. It evolved into a more recognisable pudding in Victorian times when it was commonly called figgy or plumb pudding. Amongst the fans of Victorian plumb pudding, as we will see in a moment, was the British Physicist, Joseph John Thomson (1856-1940).

It was in the latter stages of Victoria’s reign, in 1897, when Thomson discovered the electron. Until that time, many believed atoms were just tiny solid objects, but Thompson’s discovery showed atoms were constructed from electrically charged particles. Since atoms have no overall charge, and electrons are negatively charged, clearly there had to be an equal number of positive charges so they cancelled each other out. At the start of the 20th century, how these charges might be arranged within the atom was the subject of much scientific debate.

Perhaps because of his love of all things Christmas, Thomson came up with what he called the “plumb pudding model of the atom”. He surmised electrons were like  negatively charged plumbs, all held together in an atomic matrix analogous to a positively charged pudding. Thomson’s plumb pudding was the accepted atomic model for several years, until Hans Geiger (1882-1945) of Geiger counter fame, and Ernest Marsden (1889-1970) tried to measure the size of Thomson’s plumb pudding. Geiger and Marsden worked for Ernest Rutherford (1871-1937), First Baron Rutherford of Nelson at Manchester University in the UK. Together, they fired positively charged atomic particles (known as alpha particles) into gold foil and measured the angle of deflection as they passed through. The angle, they postulated, should be proportional to the size of the pudding component of the atom.

There has been much speculation as to why Rutherford came up with the idea, but he asked Geiger and Marsden to check if any alpha particles were bouncing off the gold foil, rather than passing through it. There was nothing in the plumb pudding model to suggest this was even possible, but when Geiger and Marsden conducted the experiment, to their astonishment, they found about one in eight thousand bounced back the way they came. Rutherford said of the discovery, “it was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to hit you.”

It was clear the alpha particles bounced off some concentrated spot of positive charge inside the atom in a metaphorical game of atomic tennis. Rutherford called this concentrated spot of positive charge the nucleus and postulated it contained positively charged particles called protons. And so the plumb pudding atomic model died and the solar system model was born; the positive nucleus was analogous to the sun and negative electrons were like planets in orbit around it. This has since been modified with quantum models, but we still use the solar system model today as a simplified description of the inner workings of the atom.

So as you enjoy your festive dinner this year, give a thought to Joseph John Thomson and how his thoughts of Christmas pudding led to modern atomic theory.

Merry Christmas everyone.

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