An atomic model is a representation or interpretation of the structure and composition of the atom. It allows us to understand how matter is formed and how its particles interact.
From Democritus to the present day, various atomic models have emerged over the centuries. Thanks to reasoning exercises and scientific experiments, the atom has gone from being a simple small sphere to something much more complex.
What Are Atomic Models?
Today it is known that the atom is made up of several parts, but how did we arrive at the current model? Let’s see below how atomic models have evolved over time.
Evolution Of Atomic Models:
|Description of the atom
|Democritus of Abdera
|5th century BC
|An indivisible, indestructible, incompressible, eternal, invisible and homogeneous particle, which can vary in size and shape.
|Between 1803 and 1807
|A compact, solid, tiny, indestructible, indivisible and eternal sphere.
|Joseph John Thomson
|Between 1897 and 1904
|A compact, indivisible sphere of positive charge, with embedded electrons.
|Jean Baptiste Perrin
|Planetary model with an atomic nucleus, surrounded by electrons as if they were planets.
|Gilbert Newton Lewis
|Developed in 1902, published in 1916
|Cubic structure, with the atomic nucleus in the center and the electrons positioned at the eight vertices.
|Developed in 1903, published in 1904
|Saturnian model, with a positively charged atomic nucleus and electrons arranged like the rings of Saturn.
|Very small and dense atomic nucleus formed by protons, and electrons orbiting around them in different trajectories.
|Electrons orbit the nucleus at different energy levels, depending on the amount of energy they absorb or emit.
|Electrons orbit around the nucleus at different energy levels, describing circular or elliptical paths.
|Electrons behave like waves, positioned around the nucleus in orbitals.
|Nucleus of the atom formed by protons and neutrons, surrounded by electrons positioned in orbitals of different energies.
|Atomic nucleus composed of protons and neutrons, surrounded by electrons positioned in orbitals of different energy and shapes. The atom has wave behavior.
11 Different Atomic Models: Description & History
1. Atomic model of Democritus (5th century BC)
Democritus of Abdera was a Greek philosopher who, under the mentorship of Leucippus, expanded the ideas of the atomic theory of the universe . The philosopher proposed the existence of a very small and indivisible particle , called an atom. He reached this conclusion through exercises of logical reasoning, since philosophy was a popular movement in those centuries.
Democritus attributed a series of characteristics linked to the atom. In addition to being indivisible, they are also indestructible , incompressible and eternal , remaining the same over the years. He also proposed that atoms are invisible and homogeneous , without presenting variations throughout the atom.
Likewise, the philosopher concluded that there is nothing in the space between atoms, but rather that they hang in the void. Therefore, the universe is composed of atoms and vacuum. As a result of this, one substance differs from another depending on the grouping of atoms and the shape or size they have.
More than two millennia would pass before another person proposed a similar model, but with a more critical outlook.
2. Dalton’s atomic model (between 1803 and 1807)
The British scientist John Dalton agreed with Democritus’ proposal when he said that atoms are tiny , indestructible and indivisible particles . Furthermore, these never change over time and are equal to each other when we talk about the same element.
For John Dalton, atoms are compact, solid spheres . Likewise, one element differs from another by having particles of different size, mass and chemical properties.
According to this scientist, compounds are formed by atoms of different elements in fixed proportions. However, he clarified that the smallest possible number of atoms of a single element are combined. As a result, he came to erroneous conclusions, such as that gases are monatomic or that water was formed with a hydrogen and oxygen atom (HO, instead of H 2 O ).
John Dalton also stated that atoms are never created or destroyed, and that chemical reactions only involve a combination, separation or rearrangement of them.
3. Thomson’s atomic model (between 1897 and 1904)
Joseph John Thomson, another British scientist, discovered the electron as part of the atom in 1897 thanks to an experiment with cathode rays. He observed that the rays were bent, leading him to conclude that they were interacting with a charged particle. Within a few years, he developed and published in 1904 an atomic model that was nicknamed the Plum Pudding Model .
What Thomson proposed is that the atom is a compact, indivisible sphere composed of two types of particles. According to him, electrons , negatively charged particles, are embedded inside another larger particle . This second particle contains the majority of the atom’s mass, and has a uniformly distributed positive charge.
Thomson received the Nobel Prize in 1906 for discovering the electron.
4. Perrin’s atomic model (1901)
Jean Baptiste Perrin was a French chemist and physicist who qualified the recently presented atomic model. Starting from Thomson’s premise and experiment, Perrin showed that negative charges are external to the atomic nucleus.
Thanks to the discovery, Perrin concluded that the atom was made up of a positively charged nucleus , without electrons embedded in it. Instead, the electrons are found outside the nucleus , orbiting like planets around the sun.
Later, Jean Baptiste Perrin would receive the Nobel Prize in Physics in 1926 for his study on sedimentation.
5. Lewis atomic model (developed in 1902, published in 1916)
The American chemist Gilbert Newton Lewis proposed a unique model that was not so popular. This is the Cubic Atom Model , which consisted of the following:
- The structure of the atom is cube-shaped.
- The nucleus of the atom is located in the center of the cube.
- The electrons are arranged around the nucleus, positioning themselves in one of the eight vertices of the cube.
He arrived at this proposal using Abegg’s rule, which states that the difference between the maximum and minimum oxidation number is usually eight.
Although the model was discarded soon after, it was useful in introducing the concept of valence and its importance in chemical bonds. The valence consists of electrons located in the last energy level, capable of reacting or bonding with other elements.
Following this, Lewis introduced the Lewis structure, the concept of the electron pair in covalent bonds, and the octet rule.
6. Nagaoka atomic model (developed in 1903, published in 1904)
Hantaro Nagaoka was a Japanese physicist who rejected the model proposed by Thomson. The reason he gave is that one charge cannot contain another, stating that electric charges are impenetrable. Therefore, electrons could not be embedded in the positively charged nucleus of the atom.
Instead, Nagaoka proposed an atom with a distribution similar to Saturn and its rings. Saturn represents the positive charge of the atom , while the rings are the electrons that orbit around it , separated from the nucleus. This model was nicknamed the Saturnino Model.
Nagaoka explained that the electrons rotated around, bound by electrostatic forces. However, he did not take into account the fact that electrons repel each other when they are close to each other, so this model was discarded.
7. Rutherford’s atomic model (1911)
Ernest Rutherford was a New Zealand physicist who contributed much to the study of radiation. In one of his experiments, he sent alpha particles at high speed against a gold foil. The physicist noticed that some particles deviated and even bounced, describing a trajectory of 90º or more.
For this phenomenon to occur, Rutherford deduced that the atomic nucleus had to be very small and dense . Likewise, it had to contain a type of positively charged particle, the positive electron or proton .
Electrons orbit around the atomic nucleus. Unlike Nagaoka, Rutherford said that electrons describe random trajectories . Between them and the protons there is empty space.
With this model, Rutherford affirmed two things. On the one hand, the mass of the atom is approximately equal to the mass of the electrons and protons. On the other hand, stable atoms have the same number of electrons and protons.
Ernest Rutherford received the Nobel Prize in Chemistry in 1908 for his studies and discoveries on radioactive particles.
8. Bohr’s atomic model (1913)
Niels Bohr was a Danish physicist who developed an atomic model based on Rutherford’s theories. During an experiment, Bohr saw that the radiation emitted by the excited atoms of a gas described discontinuous radiation spectra.
By observing the different spectra, Bohr understood that electrons move in orbits with different energy levels. It is from this idea that the layers or energy levels arise , with the electrons furthest from the nucleus being those with the greatest energy.
He furthered this idea by explaining that an electron can rise to a higher energy level by absorbing photons. Likewise, if an electron emits a photon, it loses energy and drops to a lower energy level. With this, Bohr explained the emission and absorption spectra of radiation.
This model proposed by Niels Bohr was a great scientific advance, laying part of the foundations of current quantum mechanics. The problem with this model is that it did not serve to explain the structure of atoms other than hydrogen, which contains a single electron.
Niels Bohr received the Nobel Prize in Physics in 1922 for his work on radiation and atomic structure.
9. Sommerfeld’s atomic model (1916)
Arnold Sommerfeld, a German physicist, refined Bohr’s model to represent atoms with more than one electron. Relying on Albert Einstein’s theory of relativity, Sommerfeld proposed a relativistic atomic model.
According to him, the orbits of electrons could be circular or elliptical . Furthermore, he expanded on the idea of energy levels, stating that sublevels also exist . That is, two electrons could be in the same energy level, but in a different sublevel. With this, Sommerfeld approached the concept of electronic clouds, without going into it.
His proposal on energy sublevels and elliptical or circular trajectories was suitable for atoms with several electrons. However, Sommerfeld ignored the interaction between electrons.
10. Schrödinger’s atomic model (1926)
Erwin Schrödinger was an Austrian physicist and philosopher who developed an atomic model based on wave-particle behavior, proposed by Louis de Broglie.
According to Schrödinger, electrons are undulations or waves of matter . For practical purposes, what he suggested is that we cannot define the exact position of an electron, but rather the possible regions in which it can reside. Therefore, this model proposed an atom with a central atomic nucleus, surrounded by electrons arranged around it in different regions, called orbitals .
Erwin Schrödinger received the Nobel Prize in Physics in 1933 for developing the Schrödinger equation.
11. Chadwick’s atomic model (1932)
James Chadwick, an English physicist, carried out an experiment in which he bombarded a thin sheet of beryllium with alpha particles. In this experiment, Chadwick realized that a type of particle without an electrical charge was emitted , which he would later name neutron .
Thanks to this discovery, James Chadwick proposed an atomic model that consisted of the following:
- The atom is made up of a central nucleus made up of protons and neutrons.
- Most of the mass of the atom is found in the nucleus itself.
- Electrons, with a much lower mass, move around the nucleus in orbitals.
He received the Nobel Prize in Physics in 1935 for the discovery of the neutron.
The current model is the quantum atomic model . This model establishes that the atom and its parts have wave behavior. Broadly speaking, electrons reside around the nucleus in orbitals or electronic clouds of different shapes, without an exact defined position and with different energy levels.
- Rutherford’s Contributions
- What are the defects of Rutherford Atomic model?
- Postulates of Atomic Model of Bohr
- Atomic Model of Thomson
- Atomic Model of Dalton
- Atomic Model of Sommerfeld
- Atomic Model of Democritus
- Atomic Model of Democritus
- Biography of Joseph John Thomson
- Difference between Atom and Molecule
- Classification Of Atoms According Their Properties With Examples