# Atomic models

The atom is the smallest characteristic particle of an element. Difficulty observing the atom spurred many scientists to come up with atomic models to help understand and study its structure and behavior.

As such, the observation of atoms is impossible with the naked eye, and it is only recently 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 5th century BC. of C. thanks to Democritus, the first model of the atom saw light only in the 19th century.

Dalton’s Atomic Model

Dalton represented the atom as a solid sphere.

Studying the gas laws, the English meteorologist John Dalton (1766-1844) proposed the first atomic theory. According to him, the atom was the smallest part of matter, the one that could no longer divide.

The way to represent the atom was as a solid sphere, similar to a billiard ball. In fact, Dalton and those who supported his theory carved balls out of wood of different sizes, simulating atoms of different elements. At the time, the existence of the electron and proton was completely unknown, so Dalton’s model persisted for almost a century.

Thomson’s Atomic Model

In Thomson’s atomic model the electrons are embedded in a positively charged mass.

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, cathode rays were accepted as streams of negatively charged particles.

In 1891, Irish physicist George Johnstone Stoney (1826-1911) suggested the electron name for the substance that produced electricity. In his honor, Thomson named the particles he discovered electron.

Thomson’s ideas are summarized below:

• Protons and electrons are particles with equal charges but of opposite sign.
• In a neutral atom, the charge is zero, since the number of negative electrons is equal to the number of positive protons.
• An atom is shaped like 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 due to 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 stuck negative electrons, like raisins in a cake or pudding.

However, the idea of ​​a positively charged solid atom was not upheld. This model does not present neutrons either.

Perrin’s Atomic Model

Perrin suggested that atoms were made up of positive suns surrounded by small negative planets, such as the solar system.

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 decrease in the electrical attraction of the atomic sun by the outermost 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 on the movement of particles in fluids.

Interestingly, in 1924 Perrin was sworn to Louis de Broglie’s thesis, where he showed the wave properties of electrons.

Nagaoka Atomic Model

The Nagaoka atomic model is known as the Saturnian model.

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 research was 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 positively charged central mass that attracts the other negatively charged particles, with the consequent formation of rings.

This configuration could explain the recently discovered radioactivity phenomena, and the light emission spectra of the elements.

Rutherford’s Atomic Model

For Rutherford, the atom was like the solar system.

It was up to a brilliant student at 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 high-speed, high-energy radioactive alpha particles, bombarded chemical elements, and calculated the angle of deflection (dispersion) of the particles.

If the atom were like the model Thomson proposed, the alpha particles would pass through the element and the deviation would be minimal. Instead, they observed that some particles bounced off. This could only be explained if the atom had a very small and condensed nucleus.

From these results, Rutherford extracted the following postulates:

• There is a small, positively charged dense region called the 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 randomly distributed 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 that orbited around it.

Bohr’s Atomic Model

Bohr’s model looks like the layers of an onion.

The planetary model of the atom had problems: if the electrons freely orbited around the nucleus, they would lose energy and eventually collapse 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 earlier by his teacher.

Bohr used the ideas of Max Planck and Albert Einstein and postulated that electrons could have a certain amount of energy. He 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 smaller one, this would produce a photon, which also solved the phenomenon of absorption spectra of the elements.

Niels Bohr’s postulates are summarized as follows:

• The electrons in an atom move stably at a certain distance from the nucleus with a defined energy. This is what is called the steady state.
• The electrons in each stationary 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 drops in energy level, it emits a photon.
• Stationary levels, or layers, are named with the letters K, L, M, N, and so on.

Bohr’s postulates led to representing the atom as the layers or rings of an onion. However, Bohr’s model did not help explain atoms with more than one electron.

Quantum mechanical model of the atom

Current representation of the atom with electronic clouds surrounding the tiny nucleus.

The quantum mechanical model of the atom is the currently accepted model. 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 it is no longer talking about orbits but electronic clouds. Electronic clouds are spaces around the nucleus where the electron can probably be found.

Here each electron has a specific direction 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 sublayer 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, no two electrons have the same quantum numbers. This is known as the Pauli exclusion principle , thanks to the Austrian physicist Wolfgang Pauli (1900-1958).

What is new in the atom?

The Large Hadron Collider is the most advanced technology for detecting subatomic particles.

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 that of hydrogen. It has no charge and its mass is slightly greater than that of the proton.

In 1970 Albert Victor Crewe (1927-2009) photographed uranium and thorium atoms using a scanning transmission electron microscope.

Today it is known that the atom is not only made up of electrons, protons and neutrons. These in turn are made up of elementary particles known as bosons and fermions.

The standard model is a mathematical model that groups together the elementary particles and explains the forces that govern them. The large hadron collider is the technology that physicists use today to study these particles.