How the nucleus was discovered — Geiger Marsden Gold Foil Experiment

Intishar Alam Misbahul
3 min readDec 17, 2020

After the creation of the Plum Pudding model of atom, many scientist around the globe were baffled as they got contradictory results when studying the atomic structure. It was not until around 1911 when a scientist by the name of Ernest Rutherford did a breakthrough experiment that changed the field of chemistry and gave birth to nuclear physics.

The Plum-Pudding model described the atom to be a ‘pudding’ of positive charge with ‘plums’ of negatively charged electrons randomly embedded around the pudding.

Another way to imagine this is a blueberry/chocolate muffin, where the muffin represents the positive charge of the atom and the berries/chocolate pieces represent the electrons being randomly found around and in the muffin.

Photo by LµÐwïg on Unsplash

Rutherford wanted to study the atomic structure so he had his students, Hans Geiger and Ernest Marsden, to conduct a series of experiments. The experiments involved having a radioactive source of alpha particle in a container with a hole revealing a path to a thin gold foil surrounded by a fluorescent screen as detector, which would glow if the alpha particles hit it.

What they expected was that the alpha particles being heavy and positively charged should pass straight through the thin gold foil. The gold foil was used because gold is highly malleable so can be made to be thin sheets. As they expect the alpha particle to go straight pass, the foil had to be extremely thin(A thin gold foil has approximately 1000 atoms).

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Now, since according to the Plum Pudding model, the atom is a sphere of positive charge with electrons embedded on it, even though like charges repel, an alpha particle is a heavy positive particle which should get a bit deflected if not just go straight.

What the result showed confused the two scientist.

When the alpha particles were shot, bombarding the gold atoms, most alpha particles went straight through and some slightly deflected as they expected. But, they noticed some were deflected as if they hit something. And some rebounded almost towards the opposite direction.

What they observed were:

  • Most alpha particles went straight through
  • Some got deflected more then they should have
  • Few rebounded back

Rutherford seeing the result deducted that the Plum-Pudding model was incorrect. Observing the results he concluded that the atom was in fact mostly empty, explaining how most passed straight through. If it was indeed a sphere of ‘light’ positive charge, the experiment would have gone as expected.

But the alpha particles that were deflected must have hit something that then repelled them. As few got deflected nearly straight back, that must mean they hit the ‘something’ directly. That ‘something’ must have been the positive charge of the atom, and the results showed that it was not spread out but ‘concentrated’ in the centre which explained why some deflected or rebounded.

Therefore, he proposed the atomic model that we were taught in elementary school which earned him the title of ‘The Father Of Nuclear Physics’. The nuclear atomic model.

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Intishar Alam Misbahul

Currently 17. Loves to learn. Doing AS level. Using medium to write easy-to-understand essays/stories regarding science and study tips.