Rutherford’s Atomic Model: History, Experiments, Postulates

The Rutherford atomic model is the description of the atom created by the British physicist Ernest Rutherford (1871-1937) discovered in 1911 when the atomic nucleus by the famous scattering experiments that take their name.

The idea of ​​the atom (” indivisible ” in Greek) as the smallest component of matter , was an intellectual creation born in Ancient Greece, around 300 BC Like so many other Greek concepts, the concept of the atom is developed based on logic and argumentation, but not experimentation.

The most notable atomist philosophers were Democritus of Abdera (460 – 360 BC), Epicurus of Samos (341 – 270 BC), and Titus Lucretius (98 – 54 BC). The Greeks conceived four different types of atoms that corresponded to the four elements that according to them made up matter: air, water, earth and fire.

Later Aristotle would add a fifth element: the ether that formed the stars, since the other four elements were purely terrestrial.

The conquests of Alexander the Great, of whom Aristotle was a teacher, expanded his beliefs throughout the ancient world, from Spain to India and thus, for centuries, the idea of ​​the atom was creating its own place in the world of science.

The atom is no longer indivisible

The ideas of the Greek philosophers about the structure of matter held true for hundreds of years, until an English chemist and school teacher named John Dalton (1776-1844) published the results of his experiments in 1808.

Dalton agreed that elements are made up of extremely small particles, called atoms. But he went further by stating that all the atoms of the same element are equal, have the same size, the same mass and the same chemical properties, which makes them remain unchanged during a chemical reaction.

This is the first scientifically based atomic model. Like the Greeks, Dalton continued to regard the atom as indivisible, therefore lacking in structure. However, Dalton’s genius led him to observe one of the great conservation principles of Physics:

  • In chemical reactions, atoms are neither created nor destroyed , they only change their distribution.

And he established the way in which chemical compounds are formed by “compound atoms” (molecules):

  • When two or more atoms of different elements combine to form the same compound, they always do so in defined and constant mass proportions .

The 19th century was the great century of electricity and magnetism. A few years after Dalton’s publications, the results of some experiments cast doubts among scientists about the indivisibility of the atom.

Crookes tube

The Crookes tube was a device designed by the British chemist and meteorologist William Crookes (1832-1919). The experiment that Crookes carried out in 1875, consisted of placing, inside a tube filled with gas at low pressure, two electrodes, one called the cathode and the other called the anode .

By establishing a potential difference between the two electrodes, the gas glowed with a color that was characteristic of the gas used. This fact suggested that there was a certain particular organization within the atom and that therefore, it was not indivisible.

Furthermore, this radiation produced a weak fluorescence on the wall of the glass tube in front of the cathode, cutting out the shadow of a cross-shaped mark located inside the tube.

It was a mysterious radiation known as “cathode rays”, which traveled in a straight line to the anode and was highly energetic, capable of producing mechanical effects, and was deflected towards a positively charged plate or also through magnets.

The discovery of the electron

The radiation inside the Crookes tube could not be waves, as it carried a negative charge. Joseph John Thomson (1856 – 1940) came up with the answer in 1887 when he found the relationship between charge and mass of this radiation, and found that it was always the same: 1.76 x 10 11 C / Kg., Regardless of the gas enclosed in the tube or the material used to make the cathode.

Thomson called these particles corpuscles . By measuring its mass in relation to its electrical charge, he concluded that each corpuscle was vastly smaller than an atom. Therefore, he suggested that they should be part of these, thus discovering the electron .

The British scientist was the first to sketch a graphic model of the atom, by drawing a sphere with inserted points, which due to its shape was given the nickname “plum pudding”. But this discovery raised other questions:

  • If matter is neutral, and the electron has a negative charge: where in the atom is the positive charge that neutralizes the electrons?
  • If the mass of the electron is less than that of the atom, then what does the rest of the atom consist of?
  • Why were the particles thus obtained always electrons and never of another type?

Rutherford scattering experiments: the atomic nucleus and the proton

By 1898 Rutherford had identified two types of radiation from uranium, which he named alpha and beta .

Natural radioactivity had already been discovered by Marie Curie in 1896. Alpha particles are positively charged and are simply helium nuclei, but at that time the concept of a nucleus was not yet known. Rutherford was about to find out.

One of the experiments that Rutherford carried out in 1911 at the University of Manchester, with the assistance of Hans Geiger, consisted of bombarding a fine gold foil with alpha particles , the charge of which is positive. Around the gold foil he placed a fluorescent screen that allowed them to visualize the effects of the bombardment.

Observations

Studying the impacts on the fluorescent screen, Rutherford and his assistants observed that:

  1. A very high percentage of the alpha particles passed through the sheet without noticeable deviation.
  2. Some deviated at quite steep angles
  3. And very few bounced all the way back

Observations 2 and 3 surprised the researchers and led them to assume that the person responsible for the scattering of the rays must have a positive charge and that by virtue of observation number 1, that person responsible was much smaller than that of the alpha particles. .

Rutherford himself said about it that it was “… as if you fired a 15-inch naval projectile at a sheet of paper and the projectile bounced back and hit you.” This definitely could not be explained by the Thompson model.

Analyzing his results from the classical point of view, Rutherford had discovered the existence of the atomic nucleus, where the positive charge of the atom was concentrated, which gave it its neutrality.

Rutherford continued his scattering experiments. By 1918 the new target for alpha particles was nitrogen gas atoms.

In this way he detected hydrogen nuclei and knew immediately that the only place from which these nuclei could come was from nitrogen itself. How was it possible that hydrogen nuclei were part of nitrogen?

Rutherford then suggested that the nucleus of hydrogen, an element that had already been assigned atomic number 1, must be a fundamental particle. He called it proton , a Greek word for first . Thus, the discoveries of the atomic nucleus and the proton are due to this brilliant New Zealander.

Rutherford’s atomic model postulates

The new model was very different from the Thompson. These were his postulates:

  • The atom contains a positively charged nucleus, which despite being very small, contains almost all the mass of the atom.
  • Electrons orbit the atomic nucleus at great distances and in circular or elliptical orbits.
  • The net charge of the atom is zero, since the charges of the electrons compensate for the positive charge present in the nucleus.

Rutherford’s calculations pointed to a nucleus of spherical shape and a radius as small as 10 -15 m, the value of the atomic radius being about 100,000 times greater, since the nuclei are comparatively far apart: of the order of 10 -10 m.

This explains why most of the alpha particles passed through the sheet smoothly or had only very little deflection.

Seen at the scale of everyday objects, the Rutherford atom would be composed of a nucleus the size of a baseball, while the atomic radius would be about 8 km. Therefore, the atom can be considered almost everything as empty space.

Thanks to its resemblance to a miniature solar system , it became known as the “planetary model of the atom.” The electrostatic attraction force between nucleus and electrons would be analogous to the gravitational attraction between the sun and the planets.

Limitations

However, there were certain disagreements regarding some observed facts:

  • If the idea that the electron orbits around the nucleus is accepted, it happens that the electron should continuously emit radiation until it collides with the nucleus, with the consequent destruction of the atom in well under a second. This, fortunately, is not what actually happens.
  • Furthermore, on certain occasions the atom emits certain frequencies of electromagnetic radiation when there are transitions between a state of higher energy to one with lower energy, and only those frequencies, not others. How to explain the fact that energy is quantized?

Despite these limitations, since today there are much more sophisticated models in line with the observed facts, Rutherford’s atomic model is still useful for the student to have a first successful approach to the atom and its constituent particles.

In this model of the atom, the neutron does not appear, another constituent of the nucleus, which was not discovered until 1932.

Shortly after Rutherford proposed his planetary model, in 1913 Danish physicist Niels Bohr would modify it to explain why the atom is not destroyed and we are still here to tell this story.

Articles of interest

Schrödinger’s atomic model .

De Broglie atomic model .

Chadwick’s atomic model .

Heisenberg atomic model .

Perrin’s atomic model .

Thomson’s atomic model .

Dirac Jordan atomic model .

Atomic model of Democritus .

Bohr’s atomic model .

Dalton’s atomic model .

References

  1. Rex, A. 2011. Fundamentals of Physics . Pearson. 618-621.
  2. Zapata, F. 2007. Class notes for the chair of Radiobiology and Radiological Protection . School of Public Health of the Central University of Venezuela.

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