What is the "Gold Leaf Experiment"? Geiger-Marsden experiments explained (2023)

The Geiger-Marsden experiment, also known as the Geiger-Marsden experimentgoldThe leaf experiment, or α-particle scattering experiments, refers to a series of experiments from the early 20th century that gave physicists their first glimpse into the structure of the atomic nucleus and the physics underlying everyday life. It was first proposed by Nobel Prize-winning physicist Ernest Rutherford.

Although terms like electron, proton and neutron are familiar to us today, in the early 20th century, scientists had very little idea of ​​the basic particles that made them up.Atom.

In fact, until 1897 scientists believed that atoms had no internal structure and believed that they were an indivisible unit of matter. Even the term 'atom' gives this impression, as it derives from the Greek word 'atomos', which means 'indivisible'.

JJ Thomson-Model of atoms

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But that year, University of Cambridge physicist Joseph John Thomson discovered the electron, disproving the idea that atoms are not fissile.British(opens in new tab). 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 normally electrically neutral; Therefore, if atoms contain negatively charged particles, they must also contain an equivalent source of positive charge to balance the negative charge.

Por 1904 Thomson(opens in new tab)he had proposed a "plum pudding" atomic model, in which an atom consists of a series of negatively charged electrons in a sphere of uniformly positive charge, scattered like blueberries on a scone.

However, the model had serious flaws, most notably the mysterious nature of this positively charged sphere. One scientist skeptical of this atomic model was Rutherford, who won theNobel Prize in Chemistryfor his discovery in 1899 of a form of radioactive decay by α particles: two protons and two neutrons bound and identical to oneHelium-4 core, even if researchers at the time didn't know about it.

Rutherford's Nobel Prize-winning discovery of α particles formed the basis of the gold leaf experiment, which cast doubt on the plum pudding model. Her experiment would study atomic structure using high-speed α particles emitted by a radioactive source. He initially turned his research over to two of his protégés, Ernest Marsden and Hans Geiger,a Britannica(opens in new tab).

Rutherford reasoned that if Thomson's plum pudding model were correct, when an α particle strikes thin gold foil, it should pass through the particle with only the smallest deflections. Because α particles are 7,000 times heavier than electrons, which probably make up the interior of the atom.

Experiments with gold leaf.

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Marsden and Geiger carried out the experiments between 1908 and 1913 mainly at the Physical Laboratories of the University of Manchester in Great Britain.

The duo used a radioactive source of α particles instead of thin gold foil orplatinumsurrounded by fluorescent displays that light up 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 would occur when the α particle brushed against an atom it encountered and thus experienced the maximum electrostatic lateral force. Even in this case, the plum pudding model predicted a maximum angle of deflection of just 0.06 degrees.

Of course, an α particle passing through extremely thin gold foil would still encounter about 1,000 atoms, so its deflections would essentially be random. Even with this random scattering, if Thomson's model were correct, the maximum angle of refraction would be just over half a degree. The chance of an α particle being reflected was only 1 in 10^1000 (1 followed by a thousand zeros).

However, when Geiger and Marsden performed their eponymous experiment, they found that the α particle underwent large deflections about 2% of the time. Even more shocking was that approximately 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) grenade (projectile) at a piece of tissue paper, which bounces off you.a Britannica(opens in new tab)

Rutherford-Modell des Atoms?

As extraordinary as they were, the results of the Geiger-Marsden experiments did not immediately cause a stir in the physics community. At first the data went unnoticed or even ignored,according to the book(opens in new tab)"Quantum Physics: An Introduction" by J. Manners.

However, the results had a profound impact on Rutherford, who in 1910 set out to determine a model of atomic structure that would replace Thomson's plum pudding model, Manners wrote in his book.

Rutherford's atomic model, presented in 1911, proposed a nucleus in which most of the mass of the particle was concentrated,a Britannica. Electrons resided around this tiny central nucleus, 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 gets within 10^-13 meters of the compact nucleus in the Rutherford atomic model, it experiences a repulsive force about a million times stronger than in the plum pudding model. This explains the wide-angle scattering observed in the Geiger-Marsden experiments.

The later Geiger-Marsden experiments were also seminal; He1913 exams(opens in new tab)helped to determine the upper limits of the size of an atomic nucleus. These experiments showed that the scattering angle of the α particle was proportional to the square of the charge on the atomic nucleus, or Z.according to the book(opens in new tab)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 series of metals. The British physicist discovered the neutron in 1932 and defined it as a separate particle from the proton.disse a American Physical Society(opens in new tab).

What did Rutherford's model do right and wrong?

However, Rutherford's model shared a critical problem with the earlier plum pudding model of the atom: orbiting electrons in both models would have to continually emit electromagnetic energy, which would cause them to lose energy and eventually spiral into the atomic nucleus. In fact, the electrons in Rutherford's model should have lasted less than 10^-5 seconds.

Another problem with Rutherford's model is that it doesn't take into account the size of atoms.

Despite these shortcomings, Rutherford's model derived from the Geiger-Marsden experiments became the inspiration fornils bohratomic model ofhydrogen, for which he won aNobel Prize in Physics.

Bohr combined Rutherford's atomic model with Max Planck's quantum theories to establish that the electrons in an atom can only take on discrete energy values, which explains why they remain stable around a nucleus unless they emit or absorb a photon or a particle of light.

Thus, the work of Rutherford, Geiger (who later became famous for his invention of a radiation detector), and Marsden helped lay the groundwork for both.quantum mechanicsand particle physics.

Rutherford's idea of ​​shooting beams at a target was adapted for particle accelerators in the 20th century. Perhaps the most recent 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 bounces them off.

additional resources

• See a modern reconstruction of the gold leaf experiment by Geiger-Marsdenscience behind the scenes(opens in new tab)Yexplained by particle physicist Bruce Kennedy(opens in new tab).
• More information about the Bohr model(opens in new tab)of the atom that would eventually replace Rutherford's atomic model.
• Rutherford's protégé Hans Gieger eventually became famous for inventing a radioactive detector, the Gieger counter.SciShow explains how they work(opens in new tab).

bibliography

Thomson's atomic model(opens in new tab), Chemical lumens for no greater,.

Rutherford-Modell, Britannica,https://www.britannica.com/science/Rutherford-modelo

alpha particle, US NRC,https://www.nrc.gov/reading-rm/basic-ref/glossary/alpha-particle.html

Maneiras. J., et al., "Quantum Physics: An Introduction", Open University, 2008.

Durrant, A., et al., "Quantum Physics of Matter", Open University, 2008

Ernst Rutherford,British,https://www.britannica.com/biography/Ernest-Rutherford

Niels Bohr, the Nobel Prize,https://www.nobelprize.org/prizes/physics/1922/bohr/facts/

A house. JE, "Origins of Quantum Theory",Fundamentals of Quantum Mechanics (3rd Edition), 2018