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 they claimed to constitute 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 achievements of Alexander the Great, of whom Aristotle was professor, expanded his beliefs across the ancient world, from Spain to India, and so 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 Greek philosophers about the structure of matter held true for hundreds of years, until a chemist and school teacher of English named John Dalton (1776-1844) published the results of their experiments in 1808.
Dalton agreed that elements are made up of extremely small particles called atoms. But he went further by asserting that all atoms of the same element are the same, have the same size, the same mass, and the same chemical properties, which keep them unchanged during a chemical reaction.
This is the first science-based atomic model. Like the Greeks, Dalton still considered the atom indivisible and therefore lacking in structure. However, Dalton’s genius led him to observe one of the great principles of the conservation of physics:
- In chemical reactions, atoms are neither created nor destroyed , they just change their distribution.
And he established the way in which chemical compounds were formed by “compound atoms” (molecules):
- When two or more atoms of different elements combine to form the same compound, they always do so in definite 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 raised doubts among scientists about the indivisibility of the atom.
The Crookes tube was a device designed by British chemist and meteorologist William Crookes (1832-1919). The experiment that Crookes carried out in 1875 consisted of placing, inside a tube filled with low pressure gas, two electrodes, one called the cathode and the other called the anode .
Upon establishing a potential difference between the two electrodes, the gas glowed with a characteristic color of the used gas. This fact suggested that there was a certain specific organization within the atom and that, therefore, it was not indivisible.
In addition, this radiation produced a weak fluorescence on the wall of the glass tube in front of the cathode, cutting 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 which was diverted to a positively charged plate or also by magnets.
the discovery of the electron
The radiation inside Crookes’ tube could not be waves, as it carried a negative charge. Joseph John Thomson (1856 – 1940) found 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 closed gas in the tube or in the material used to make the cathode.
Thomson called these bodily particles . By measuring its mass in relation to its electrical charge, he concluded that each corpuscle was much smaller than an atom. Therefore, he suggested that they should be part of them, thus discovering the electron .
The British scientist was the first to sketch a graphic model of the atom, drawing a sphere with some inserted points, which, due to its shape, was called “plum pudding”. But this discovery raised other questions:
- If matter is neutral and the electron has a negative charge: in which part of 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 is the remainder of the atom?
- Why were the particles thus obtained always electrons and never of another kind?
Rutherford scattering experiments: the atomic nucleus and the proton
In 1898, Rutherford had identified two types of uranium radiation, which he called alpha and beta .
Natural radioactivity had already been discovered by Marie Curie in 1896. Alpha particles have a positive charge and are simply helium nuclei, but at that time the concept of a nucleus was not known. Rutherford was about to find out.
One of the experiments that Rutherford conducted in 1911 at the University of Manchester, with the assistance of Hans Geiger, was to bombard a thin gold leaf with positively charged alpha particles . Around the gold leaf, he placed a fluorescent screen that allowed them to view the effects of the bombing.
Studying the impacts on the fluorescent screen, Rutherford and his assistants found that:
- A very high percentage of alpha particles crossed the sheet with no noticeable deviation.
- Some deviated at very sharp angles
- And very few jumped completely back
Observations 2 and 3 surprised the researchers and led them to assume that the person responsible for scattering the rays must have a positive charge and that, because of observation number 1, this person responsible was much smaller than the alpha particle. .
Rutherford himself said it was “…like you were firing a 15-inch naval shell into a sheet of paper and the shell recovered and hit you.” Definitely, this could not be explained by Thompson’s model.
Analyzing his results from the classical point of view, Rutherford had discovered the existence of the atomic nucleus, where the positive charges of the atom were concentrated, which gave it its neutrality.
Rutherford continued with his scattering experiments. In 1918, the new target of alpha particles was gaseous nitrogen atoms.
In this way, he detected hydrogen nuclei and knew immediately that the only place these nuclei could come from was nitrogen itself. How was it possible that hydrogen nuclei were part of nitrogen?
Rutherford then suggested that the hydrogen nucleus, an element that had already been assigned to atomic number 1, was a fundamental particle. He called it a proton , a Greek word for first . So the discoveries of the atomic nucleus and the proton are due to this brilliant New Zealand.
Rutherford’s atomic model postulates
The new model was very different from Thompson’s. These were his postulates:
- The atom contains a positively charged nucleus, which although 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 on the atom is zero, as the charges on the electrons outweigh the positive charge present on the nucleus.
Rutherford’s calculations pointed to a spherical nucleus and a radius as small as 10 -15 m, the atomic radius value being 100,000 times larger, since the nuclei are comparatively very far apart: on the order of 10 -10 m .
This explains why most of the alpha particles passed through the sheet either without a hitch or barely suffered a very small deflection.
Viewed on 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, so the atom can be considered almost anything as empty space.
Due to its resemblance to a miniature solar system, it was known as the “planetary model of the atom”. The force of the electrostatic attraction between the nucleus and electrons would be analogous to the gravitational attraction between the sun and the planets.
However, there were some differences in relation to some observed facts:
- If the idea is accepted that the electron orbits around the nucleus, it turns out that it must continuously emit radiation until it reaches the nucleus, with the consequent destruction of the atom in much less than a second. Fortunately, that’s not really what happens.
- Furthermore, on occasions, the atom emits certain frequencies of electromagnetic radiation when there are transitions between a higher energy state to a lower energy state and only those frequencies and not others. How to explain the fact that energy is quantized?
Despite these limitations, because today there are models that are much more sophisticated and consistent with the observed facts, the Rutherford 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, another constituent of the nucleus, which was not discovered until 1932, does not appear.
Shortly after Rutherford proposed his planetary model in 1913, Danish physicist Niels Bohr modified it to explain why the atom is not destroyed, and we are still here to tell that story.