He Rutherford's experiment Allowed a group of scientists to discover that each atom has a positively charged nucleus.
Ernest Rutherford , Was a New Zealander physicist and chemist. He focused on the study of radioactive particles and made several investigations that allowed him to win the Nobel Prize in Chemistry in 1908. Under the direction of Rutherford, Hans Geiger and Ernest Marsden, helped create the atomic model in the laboratories of the University Of Manchester.
One of the earliest atomic theories is that formulated by Thomson , The discoverer of the electron. He believed that the atoms were spheres with a positive charge, and that the electrons were distributed in it.
Thomson's theory said that if an alpha particle collided with an atom, this particle would pass through the atom. This would be affected by the electric field of the atom according to this model.
At this time, protons and neutrons had not been discovered. Thomson could not prove its existence and its model was not accepted by the scientific community.
To demonstrate the existence of Thomson's theory, Rutherford, Geiger, and Marsdendonated an experiment in which they bombarded alpha particles, made with helium gas cores, against a sheet of metal. If the Thomson model were to work, the particles should pass through the sheet of metal with little deviation.
Development of the Rutherford experiment
First Prototype
The first design prototype of the experiment, made in 1908, was explained by Geiger in an article titled On the Particle Dispersion by Matter .
They built a glass tube almost two meters long, at one end there was a radio source, and at the opposite end a phosphorescent screen was placed. In the middle of the tube, a kind of funnel was placed for the alpha particles to pass through.
The process followed was to pass the alpha particles through the slit to project the light beam on the phosphorescent screen. By pumping all the air from the tube, the image obtained was sharp and corresponded to the middle opening of the tube. When the amount of air in the tube was lowered, the image became more diffuse.
Then, to see what trajectory the particles would follow if they hit something or pierced it, as Thomson's theory maintained, a gold leaf was inserted into the slot.
This demonstrated that air and solids caused a dispersion of the particles reflected in the phosphorescent screen with more diffuse images.
The problem with this first prototype is that it only showed the result of the dispersion, but not the path that the alpha particles followed.
Second prototype
Geiger and Marsden published an article in 1909 in which they explained an experiment to demonstrate the movement of alpha particles. In a Diffuse Reflection of the Alpha Particles It is explained that the experiment tries to find out that the particles move at angles of more than 90 degrees.
They created a second prototype for the experiment, where a conical shaped glass vessel was created. They set up a lead plate, so that the alpha particles struck with it, and to see its dispersion, a fluorescent plate was placed behind.
The problem with the configuration of this device is that the particles avoided the lead plate, bouncing off the air molecules. They tested by placing a sheet of metal and saw on the fluorescent screen that there were more punches from the particles.
It was shown that metals with a higher atomic mass reflected more particles, but Geiger and Masden wanted to know the exact number of particles. But the experiment by having radio, and radioactive materials, could not be accurate.
Third prototype
Article The dispersion of α-particles by matter Of 1910 explains the third experiment that Geiger designed. Here he was already focused on measuring the angle of dispersion of the particles, depending on the material in which they come into contact.
This time, the tube was airtight, and the mercury pumped radon-222 to the fluorescent screen. With the help of microscope The flashes that appeared on the fluorescent screen were counted.
The angles following the particles were calculated and the conclusions reached that the deflection angles increase with the larger atomic mass of the material and that it is also proportional to the atomic mass of the substance.
However, the most likely deflection angle decreases with velocity and the probability of deflecting more than 90 ° is negligible.
With the results obtained in this prototype, Rutherford calculated the dispersion pattern mathematically. Through a Mathematical equation It was calculated how the sheet should disperse the particles, assuming that the atom has the positive electric charge at its center. Although the latter was only considered a hypothesis.
The equation developed was:
Where, is = the number of alpha particles falling on the unit area with a deflection angle
- R = the distance of the point of incidence of the alpha rays on the dispersion material
- X = the total number of particles falling on the dispersion material
- N = the number of atoms in a unit volume of the material
- T = the thickness of the sheet
- Qn = the positive charge of the atomic nucleus
- Qα = the positive charge of the alpha particles
- M = the mass of an alpha particle
- V = the velocity of the alpha particle
Final Prototype
With Rutherford's model of equations, an experiment was attempted to demonstrate what was being postulated, and that atoms had a nucleus with a positive charge.
The designed equation predicted that the number of scintillations per minute (s) to be observed at a given angle (Φ) should be proportional to:
- Csc 4 Φ / 2
- Sheet thickness t
- Magnitude of the central load Qn
- 1 / (mv 2 ) 2
In order to demonstrate these four hypotheses four experiments are created, which are explained by the article The laws of the deflection of the α particles by large angles Of 1913.
To test the proportional effect to csc 4 Φ / 2, they built a cylinder on top of a turntable, on a column. The column bombarding the air and the covered microscope with a fluorescent screen allowed to observe the particles that deviated until 150º, reason why the hypothesis of Rutherford was demonstrated.
To test the hypothesis of the thickness of the sheet, they mounted a disk with 6 holes covered with sheets of varied thickness. It was observed that the number of flashes were proportional to the thickness.
They reused the disk of the previous experiment to measure the dispersion pattern, assuming that the charge of the nucleus was proportional to the atomic weight, measured if the dispersion was proportional to the atomic weight squared.
With the obtained flashes, divided by the air equivalent, and then divided by the square root of the atomic weight, they found that the proportions were similar
And finally, with the same disc of the experiment, they were placing more mica discs to retard the particles, and with an acceptable range of error, they showed that the number of scintillations was proportional to 1 / v 4 , As Rutherford had predicted in his model.
Through the experiments they proved that all the Rutherford hypotheses were fulfilled so that the Atomic Model of Rutherford was determined. In this model, finally published in 1917, it is postulated that atoms have a positively charged central nucleus.
If the central nucleus of the atom is the one with the positive charge, the rest of the atom will be empty with the electrons orbiting around it.
With this model it was shown that atoms have a neutral charge and that the positive charge found in the nucleus is counteracted by the same number of electrons orbiting around them.
If we remove electrons to the atom, they will then have a positive charge. Atoms are stable, since the centrifugal force equals the electric force, keeping the electrons in place
References
- CUÉLLAR FERNÁNDEZ, Luigi; GALLEGO BADILLO, Rómulo; PÉREZ MIRANDA, Royman. The atomic model of E. Rutherford. Science Teaching , 2008, vol. 26.
- BOHR, Niels. The Rutherford Memorial Lecture 1958 Reminiscences of the Founder of Nuclear Science and Some Developments Based on his Work. Proceedings of the Physical Society , 1961.
- JUSTI, Rosaria; GILBERT, John. History and philosophy of science through models: some challenges in the case of 'the atom'. International Journal of Science Education , 2000, vol. 22.
- COHEN-TANNOUDJI, Claude, et al. Atom-photon interactions: basic processes and applications . New York: Wiley, 1992.
- AGUILERA, Damarys, et al. Conceptual models of university students on atomic structure based on the experiments of Thomson, Rutherford and Bohr / Conceptual models of university students on atomic structure based on experiments of Thomson, Rutherford and Bohr. Journal of Science Education , 2000, vol. 1, paragraph 2.
- DE LA LLATA LOYOLA, María Dolores. Inorganic chemistry . Editorial Progreso, 2001.
- TORRES, Amalia Williart. Historical experiment: discovery of the atomic nucleus: Rutherford's experiment. 100 UNED , 2003, No 6, p. 107-111.