The atom is the smallest particle characteristic of an element. The difficulty in observing the atom stimulated many scientists to propose atomic models to help understand and study their structure and behavior.
As such, the observation of atoms is impossible to the naked eye, and only recently is that we have the technology available to visualize an atom.
Although the original idea of the existence of atoms arose in Ancient Greece in the fifth century BC. of C. thanks to Democritus, the first model of the atom saw light only in the nineteenth century.
Atomic model of Dalton
Studying the laws of gases, the English meteorologist John Dalton (1766-1844) proposed the first atomic theory. According to him, the atom was the smallest part of matter, which could no longer be divided.
The way to represent the atom was like a solid sphere, similar to a billiard ball. In fact, Dalton and those who supported his theory, carved wooden balls of different sizes, simulating atoms of different elements. For the time, the existence of the electron and the proton was completely unknown, so Dalton’s model persisted for almost a century.
Atomic model of Thomson
In 1897, the English physicist Joseph John Thomson (1865-1940), working with vacuum tubes, was able to show the deflection of cathode rays in an electric field. At that time, it was accepted that the cathode rays were streams of negatively charged particles.
In 1891, the Irish physicist George Johnstone Stoney (1826-1911) suggested the name of electron for the substance that produced electricity. In his honor, Thomson called the particles he discovered electron.
Thomson’s ideas are summarized below:
- Protons and electrons are particles with equal but opposite charges.
- In a neutral atom, the charge is zero, since the number of negative electrons is equal to the number of positive protons.
- An atom has the shape of a sphere with a radius of 0.00000001 cm, where protons and electrons are randomly distributed.
- The mass of the electrons is not taken into account because of their insignificance, so the mass of the atom is equal to the mass of the protons.
This is how Thomson suggested that the atom was a solid sphere of material positively charged with negative electrons nailed, like raisins in a cake or pudding.
Atomic model of Perrin
The French physicist Jean Perrin (1870-1942) published in 1901 what would be the first model based on the planetary system. Radioactivity could be explained as the diminution of the electrical attraction of the atomic sun by the most external electrons (the Neptunes of the system, as Perrin called them).
However, this model was no more than a simple sketch, and Perrin showed no interest in continuing his study. In fact, Perrin won the Nobel Prize in Physics in 1926 for his work in the movement of particles in fluids.
Interestingly, in 1924 Perrin was sworn to the thesis of Louis de Broglie, where he showed the wave properties of electrons.
Atomic model of Nagaoka
The Japanese physicist Hantaro Nagaoka (1865-1950) proposed in 1903 an atomic model with electrons orbiting in circles around a large positive central mass. His investigations were published in English in 1904.
According to Nagaoka, the particle system was similar to the Saturn system. This consisted of:
- A large number of particles of equal mass arranged in circles that repel each other;
- A central mass positively charged that attracts the other negatively charged particles, with the consequent formation of rings.
This configuration could explain the recently discovered radioactivity phenomena, and the spectra of light emission of the elements.
Atomic model of Rutherford
It was up to a brilliant student of JJ Thomson, the New Zealand physicist Ernest Rutherford (1871-1937), to solve the problem of the structure of the atom in 1911, in England.
Taking advantage of the discovery of radioactivity in 1896, Rutherford and his students, Hans Geiger and Ernest Marsden, used alpha radioactive particles of high speed and energy, bombarded chemical elements and calculated the deviation angle (dispersion) of the particles.
If the atom was like the model proposed by Thomson, the alpha particles would pass through the element and the deviation would be minimal. Instead, they observed that some particles bounced. This could only be explained if the atom had a very small and condensed nucleus.
- There is a small dense region positively charged, called a nucleus.
- The mass of the atom is approximately equal to the mass of the protons and electrons.
- The protons inside the nucleus are concentrated in the center of the atom, and the electrons distributed randomly around them.
Rutherford then proposed that the atom was like the solar system where the nucleus was the Sun and the electrons were the planets orbiting it.
Atomic model of Bohr
The planetary model of the atom had problems: if the electrons orbited freely around the nucleus, they would lose energy and collapse at some point within the nucleus.
Niels Bohr (1885-1962) went to the University of Manchester in England to study with Rutherford. This young Danish physicist invented in 1913 the atomic model that would dethrone the model proposed a few years before by his professor.
Bohr used the ideas of Max Planck and Albert Einstein and postulated that electrons could have a certain amount of energy. Arranged the electrons in circular orbits with a specific amount of energy. He also explained that if an electron jumps from a high energy orbital to a lower one, this would produce a photon, which also resolved the phenomenon of absorption spectra of the elements.
The postulates of Niels Bohr are summarized as follows:
- The electrons in an atom move stably at a certain distance from the nucleus with a definite energy. This is what is called the steady state.
- The electrons in each steady state follow a circular path or orbit. Each orbit is called an “energy level” or “layer”.
- When the electron is in the steady state, it does not produce light (photon). However, when it lowers its energy level, it emits a photon.
- Stationary levels, or layers, are named with the letters K, L, M, N, and so on.
The postulates of Bohr led to represent the atom as the layers or rings of an onion. However, the Bohr model did not explain atoms with more than one electron.
The quantum mechanical model of the atom is the accepted model today. The three physicists who contributed to the knowledge of the modern atom were Werner Heisenberg (1901-1976), Louis de Broglie (1892-1987) and Erwin Schrödinger (1887-1961).
In this case, the electron behaves like a standing wave and no longer speaks of orbits but of electronic clouds. Electronic clouds are spaces around the nucleus where the electron can probably be found.
Here each electron has a specific address reflected in the quantum numbers, which are four:
- Main quantum number: the energy level n = 1 (K), 2 (L), 3 (M), 4 (N) …
- Secondary quantum number: the sub-layer l = s, p, d, f.
- Magnetic quantum number: the orbital m = x, y, z.
- Quantum spin number: the spin type of the electron s = +1/2, -1/2.
In this sense, there are no two electrons that have the same quantum numbers. This is known as the Pauli exclusion principle , thanks to the Austrian physicist Wolfgang Pauli (1900-1958).
What’s new in the atom?
In 1932, James Chadwick (1891-1974) discovered the neutron, an elusive and difficult to detect subatomic particle. The neutron is found in the nucleus of all atoms, except hydrogen. It has no charge and its mass is a little larger than that of the proton.
In 1970, Albert Victor Crewe (1927-2009) photographed the uranium and thorium atoms using a scanning electron microscope.
Nowadays it is known that the atom is not only composed of electrons, protons and neutrons. These in turn are formed by elementary particles known as bosons and fermions.
The standard model is a mathematical model that groups the elementary particles and explains the forces that govern them. The large hadron collider is the technology that physicists currently use to study these particles.