What is the “gold leaf experience”? Geiger-Marsden experiments explained
The Geiger-Marsden experiment, also called gold sheet experiment or α-particle scattering experiments, refers to a series of experiments from the early 20th century that gave physicists their first insight into the structure of the atomic nucleus and the physics underlying the everyday world. It was first proposed by Nobel Prize-winning physicist Ernest Rutherford.
As familiar as we are today with terms like electron, proton, and neutron, in the early 1900s scientists had very little idea about the fundamental particles that made up atoms.
In fact, until 1897 scientists believed that atoms had no internal structure and believed that they were an indivisible unit of matter. Even the label “atom” gives this impression, since it is derived from the Greek word “atomos”, which means “indivisible”.
JJ Thomson model of the atom
But that year, Cambridge University physicist Joseph John Thomson discovered the electron and disproved the concept of the unbreakable atom, according to British. Thomson discovered that metals emit negatively charged particles when illuminated with high frequency light.
His discovery of electrons also suggested that there were more elements in the atomic structure. This is because matter is generally electrically neutral; so if the atoms contain negatively charged particles, they must also contain an equivalent positive charge source to balance the negative charge.
In 1904, Thomson had suggested a “plum pudding model” of the atom in which an atom comprises a number of negatively charged electrons in a sphere of uniform positive charge, spread out like blueberries in a muffin.
The model had serious shortcomings, however, primarily the mysterious nature of this positively charged sphere. One scientist who was skeptical of this model of atoms was Rutherford, who won the Nobel Prize in Chemistry for his discovery in 1899 of a form of radioactive decay via α particles – two protons and two neutrons bound together and identical to one helium-4 core, although researchers at the time did not know it.
Rutherford’s Nobel Prize-winning discovery of α particles formed the basis of the gold foil experiment, which cast doubt on the plum pudding model. His experiment would probe atomic structure with high-speed α particles emitted from a radioactive source. He first entrusted his investigation to two of his proteges, Ernest Marsden and Hans Geiger, according to Britannica.
Rutherford reasoned that if Thomson’s plum pudding model was correct, then when an α particle strikes thin gold foil, the particle should pass through with only the smallest of deflections. Indeed, the α particles are 7,000 times more massive than the electrons which probably constituted the interior of the atom.
Gold leaf experiments
Marsden and Geiger conducted the experiments primarily in the physics laboratories of the University of Manchester in the UK between 1908 and 1913.
The duo used a radioactive source of α particles facing a thin sheet of gold or platinum surrounded by fluorescent screens that glowed when hit by the deflected particles, allowing scientists to measure the angle of deflection.
The research team calculated that if Thomson’s model was correct, the maximum deflection should occur when the α particle brushes against an atom it encounters and thus experiences the maximum transverse electrostatic force. Even then, the plum pudding model predicted a maximum deflection angle of only 0.06 degrees.
Of course, an α particle passing through extremely thin gold foil would still encounter about 1,000 atoms, and so its deviations would be essentially random. Even with this random scattering, the maximum angle of refraction if Thomson’s model were correct would be just over half a degree. The probability of an α particle being reflected was only 1 in 10^1000 (1 followed by a thousand zeros).
Yet when Geiger and Marsden conducted their eponymous experiment, they found that in about 2% of cases, the α particle experienced large deflections. Even more shockingly, about 1 in 10,000 α particles were reflected directly from the gold foil.
Rutherford explained how extraordinary this result was, comparing it to firing a 15-inch (38 centimeter) shell (projectile) at a sheet of tissue paper and bouncing it off you, according to Britannica
Rutherford’s model of the atom?
As extraordinary as they were, the results of the Geiger-Marsden experiments did not immediately cause a stir in the physics community. Initially, the data went unnoticed or even ignored, according to the book “Quantum physics: an introduction” by J. Manners.
The results had a profound effect on Rutherford, however, who in 1910 set out to determine a model of atomic structure that would replace Thomson’s plum pudding model, Manners writes in his book.
Rutherford’s model of the atom, proposed in 1911, proposed a nucleus, where the majority of the particle’s mass was concentrated, according to Britannica. Surrounding this tiny central nucleus were electrons, and the distance at which they orbited determined the size of the atom. The model suggested that most of the atom was empty space.
When the α particle approaches within 10^-13 meters of the compact nucleus in Rutherford’s atomic model, it experiences a repulsive force about a million times stronger than it would experience in the plum pudding model. This explains the wide-angle scattering observed in the Geiger-Marsden experiments.
Later, the Geiger-Marsden experiments were also instrumental; the 1913 trials helped determine the upper limits of the size of an atomic nucleus. These experiments revealed that the scattering angle of the α particle was proportional to the square of the charge of the atomic nucleus, or Z, according to the book “Quantum Physics of Matter”, published in 2000 and edited by Alan Durrant.
In 1920, James Chadwick used a similar experimental setup to determine the Z value for a number of metals. The British physicist then discovered the neutron in 1932, delineating it as a separate particle from the proton, the American Physical Society said.
What did Rutherford’s model do right and wrong?
Yet Rutherford’s model shared a critical problem with the earlier plum pudding model of the atom: orbiting electrons in both models would have to continuously emit electromagnetic energy, causing them to lose energy and would eventually wind up in the core. In fact, the Rutherford model electrons should have lasted less than 10^-5 seconds.
Another problem presented by Rutherford’s model is that it does not take into account the size of atoms.
Despite these flaws, Rutherford’s model derived from the Geiger-Marsden experiments was to become the inspiration for Niels Bohrthe atomic model of hydrogenfor which he won a Nobel Prize in Physics.
Bohr united Rutherford’s atomic model with Max Planck’s quantum theories to determine that electrons in an atom can only take on discrete energy values, thus explaining why they remain stable around a nucleus unless they emit or to absorb a photon or a light particle.
Thus, the work of Rutherford, Geiger (who later became famous for his invention of a radiation detector) and Marsden helped lay the foundations for both Quantum mechanics and particle physics.
Rutherford’s idea of firing a beam at a target was adapted to particle accelerators during the 20th century. Perhaps the ultimate example of this type of experiment is the Large Hadron Collider near Geneva, which accelerates beams of particles to nearly the speed of light and causes them to collide.
Thomson’s atomic modelLumens Chemistry for Non-Majors,.
Model of Rutherford, Britannica, https://www.britannica.com/science/rutherford-model
Alpha particle, US NRC, https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html
manners. J., et al, “Quantum physics: an introduction”, Open University, 2008.
Durrant, A., et al, “Quantum Physics of Matter”, Open University, 2008
Ernest Rutherford, British, https://www.britannica.com/biography/Ernest-Rutherford
Niels Bohr, the Nobel Prize, https://www.nobelprize.org/prizes/physics/1922/bohr/facts/
Accommodation. JE, “Origins of Quantum Theory”, Fundamentals of Quantum Mechanics (Third Edition)2018