RUTHERFORD'S EXPERIMENT: HISTORY, DESCRIPTION AND CONCLUSIONS - SCIENCE - 2025

The experiment Rutherford, carried out between 1908 and 1913 consisted bomb a thin gold film of.0004 mm thick, with alpha particles and analyze the dispersion pattern of said particles left on a fluorescent screen.

In fact, Rutherford conducted numerous experiments, refining the details more and more. After carefully analyzing the results, two very important conclusions emerged:

-The positive charge of the atom is concentrated in a region called the nucleus.

-This atomic nucleus is incredibly small compared to the size of the atom.

Figure 1. Rutherford's experiment. Source: Wikimedia Commons. Kurzon

Ernest Rutherford (1871-1937) was a New Zealand-born physicist whose field of interest was radioactivity and the nature of matter. Radioactivity was a recent phenomenon when Rutherford began his experiments, it had been discovered by Henri Becquerel in 1896.

In 1907 Rutherford went to the University of Manchester in England to study the structure of the atom, using these alpha particles as probes to peer inside such a tiny structure. Physicists Hans Geiger and Ernest Marsden accompanied him on the task.

They hoped to see how an alpha particle, which is a doubly ionized helium atom, would interact with a single gold atom, to ensure that any deviation it experienced was due solely to electrical force.

However, most of the alpha particles passed through the gold foil with only a slight deviation.

This fact was in total agreement with Thomson's atomic model, however, to the surprise of the researchers, a small percentage of the alpha particles experienced a rather remarkable deviation.

And an even smaller percentage of particles would come back, bouncing completely back. What were these unexpected results due to?

Description and conclusions of the experiment

In fact, the alpha particles that Rutherford used as a probe are helium nuclei, and at that time it was only known that these particles were positively charged. Today it is known that alpha particles are made up of two protons and two neutrons.

Alpha particles and beta particles had been identified by Rutherford as two different kinds of radiation from uranium. Alpha particles, much more massive than the electron, have a positive electrical charge, while beta particles can be electrons or positrons.

Figure 2. Detailed scheme of the Rutherford, Geiger and Marsden experiment. Source: R. Knight. Physics for Scientists and Engineering: a Strategy Approach. Pearson.

A simplified scheme of the experiment is shown in Figure 2. The alpha particle beam comes from a radioactive source. Geiger and Marsden used radon gas as the emitter.

The lead blocks were used to direct the radiation towards the gold foil and prevent it from going directly to the fluorescent screen. Lead is a material that absorbs radiation.

Subsequently, the beam thus directed, was made to impinge on a thin gold foil and most of the particles continued on their way to the fluorescent zinc sulfate screen, where they left a small light trace. Geiger was in charge of counting them one by one, although they later designed a device that did it.

The fact that some particles underwent a small deflection did not surprise Rutherford, Geiger, and Marsden. After all, there are positive and negative charges on the atom that exert forces on the alpha particles, but since the atom is neutral, which they already knew, the deviations had to be small.

The surprise of the experiment is that a few positive particles were bounced almost directly back.

Conclusions

About 1 in 8000 alpha particles experienced deflection at angles greater than 90º. Few, but enough to question some things.

The atomic model in vogue was that of raisin pudding by Thomson, Rutherford's former professor at the Cavendish Laboratory, but Rutherford wondered if the idea of ​​an atom without a nucleus and with electrons embedded as raisins, was correct.

Because it turns out that these large deflections of alpha particles, and the fact that a few are able to return, can only be explained if an atom has a small, heavy, positive nucleus. Rutherford assumed that only the electrical attractive and repulsive forces, as indicated by Coulomb's law, were responsible for any deviation.

When some of the alpha particles approach directly towards this nucleus and since the electrical force varies with the inverse square of the distance, they feel a repulsion that causes them the wide-angle scattering or the backward deflection.

To be sure, Geiger and Marsden experimented with bombarding sheets of different metals, not just gold, although this metal was the most appropriate for its malleability, to create very thin sheets.

By obtaining similar results, Rutherford was convinced that the positive charge on the atom should be located in the nucleus, and not dispersed throughout its volume, as Thomson postulated in his model.

On the other hand, since the vast majority of alpha particles passed without deviation, the nucleus had to be very, very small compared to atomic size. However, this nucleus had to concentrate most of the mass of the atom.

Influences on the model of the atom

The results greatly surprised Rutherford, who declared at a conference in Cambridge: “… it's like when you shoot a 15-inch cannonball at a sheet of tissue paper and the projectile bounces directly towards you and hits you”.

Since these results could not be explained by Thomson's atomic model, Rutherford proposed that the atom was made up of a nucleus, very small, very massive and positively charged. The electrons remained in orbits around him, like a miniature solar system.

Figure 3. Rutherford's atomic model on the left and Thomson's raisin pudding model on the right. Source: Wikimedia Commons. Left image: Jcymc90

This is what the nuclear model of the atom shown in Figure 3 on the left is all about. As electrons are very, very small too, it turns out that the atom is almost everything…. empty! Therefore, most of the alpha particles pass through the sheet hardly deflected.

And the analogy with a miniature solar system is very accurate. The atomic nucleus plays the role of the Sun, containing almost all the mass plus the positive charge. Electrons orbit around them like planets and carry a negative charge. The assembly is electrically neutral.

About the distribution of electrons in the atom, Rutherford's experiment showed nothing. You might think that the alpha particles would have some interaction with them, but the mass of the electrons is too small and they were not able to significantly deflect the particles.

Disadvantages of the Rutherford model

One problem with this atomic model was precisely the behavior of the electrons.

If these were not static, but orbiting the atomic nucleus in circular or elliptical orbits, driven by electrical attraction, they would end up rushing toward the nucleus.

This is because the accelerated electrons lose energy, and if that happens, it would be the collapse of the atom and matter.

Fortunately this is not what happens. There is a kind of dynamic stability that prevents collapse. The next atomic model, after Rutherford's, was Bohr's, which gave some answers as to why atomic collapse does not occur.

The proton and the neutron

Rutherford continued to do scattering experiments. Between 1917 and 1918, he and his assistant William Kay chose to bombard gaseous nitrogen atoms with the highly energetic alpha particles from bismuth-214.

He was surprised again, when he detected hydrogen nuclei. This is the equation of the reaction, the first artificial nuclear transmutation ever achieved:

The answer was: from the same nitrogen. Rutherford had assigned hydrogen the atomic number 1, because it is the simplest element of all: a positive nucleus and a negative electron.

Rutherford had found a fundamental particle that he named a proton, a name derived from the Greek word for first. In this way, the proton is an essential constituent of every atomic nucleus.

Later, around 1920, Rutherford proposed that there must be a neutral particle with a mass very similar to that of the proton. He called this particle a neutron and it is part of almost all known atoms. Physicist James Chadwick finally identified it in 1932.

What does a scale model of the hydrogen atom look like?

The hydrogen atom is, as we have said, the simplest of all. However, it was not easy to develop a model for this atom.

Successive discoveries gave rise to Quantum Physics and a whole theory that describes phenomena on an atomic scale. During this process, the atomic model also evolved. But let's take a look at the question of sizes:

The hydrogen atom has a nucleus made up of one proton (positive) and has a single electron (negative).

The radius of the hydrogen atom has been estimated at 2.1 x 10 ^-10 m, while that of the proton is 0.85 x 10 ^-15 m or 0.85 femtometers. The name of this small unit is due to Enrico Fermi and is used a lot when working at this scale.

Well, the quotient between the radius of the atom and that of the nucleus is of the order of 10 ⁵ m, that is, the atom is 100,000 times larger than the nucleus!

However, it must be borne in mind that in the contemporary model, based on Quantum Mechanics, the electron envelops the nucleus in a sort of cloud called an orbital (an orbital is not an orbit) and the electron, on an atomic scale, is not punctual.

If the hydrogen atom were enlarged - imaginatively - to the size of a football field, then the nucleus composed of a positive proton would be the size of an ant in the center of the field, while the negative electron would be like a kind of ghost, scattered throughout the field and surrounding the positive core.

The atomic model today

This "planetary type" atomic model is very ingrained and is the image that most people have of the atom, since it is very easy to visualize. However, it is not the accepted model today in the scientific field.

Contemporary atomic models are based on Quantum Mechanics. She points out that the electron in the atom is not a negatively charged dot that follows precise orbits, as Rutherford envisioned.

Rather, the electron is scattered in areas around the positive nucleus, called atomic orbitals. From him we can know the probability of being in one state or another.

Despite this, Rutherford's model represented an enormous advance in the knowledge of the interior structure of the atom. And it paved the way for more researchers to continue to refine it.

References

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