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Atomic Theory, Evolution of Atomic Models

Atomic theory, evolution of atomic models

Atomic theory is a scientific theory of the nature of matter. The atomic theory states that matter is made up of units called atoms. Within the atomic theory, several scientists have developed different atomic models to describe the atomic structure.

The atomic theory began as a philosophical concept in ancient Greece. They believed that an atom was a no divisible entity. The word atom originates from the ancient Greek adjective atomic, which means " no divisible." 

From the 19th century on, scientists began to experiment and develop scientific theories.

At the beginning of the 20th century, physicists discovered that the "indivisible atom" is actually a trove of different subatomic particles ( electrons, protons, and neutrons).

Particle physics is the scientific field that studies subatomic particles. In this field, physicists hope to spot the true gut nature of matter.

What Did John Dalton Discover About the Atom?

John Dalton developed the law of multiple proportions and later proposed a first scientific atomic model: Dalton's atomic theory.

Near the end of the 18th century, two laws of chemical reactions emerged, without reference to the notion of atomic theory:

  • Law of Conservation of Mass (Antoine Lavoisier, 1789). This law states that the total mass in a chemical reaction remains constant.

  • Law of Definite Proportions (Joseph Louis Proust, 1799). This law states that if a compound is decomposed into its constituent elements, the constituent compounds' masses will always have the same proportions.

Law of Multiple Proportions

John Dalton studied both laws and developed the law of multiple proportions: 

"If two elements can combine to form a series of compounds, the ratio of the mass of the second element that combines to a fixed mass of the first element will be a ratio of small integers."

Dalton Atomic Model

Dalton proposed that each chemical element was composed of atoms of only one type. Although they could not be chemically modified or destroyed, atoms combine to form more complex structures ( molecules).

Dalton reached these conclusions following a scientific method. Thus, this was the first truly scientific theory of the atom.

The following statements are the base of Dalton's atomic theory:

  • The matter consists of atoms, which are indivisible and indestructible particles.

  • All atoms of the same chemical element are equal in mass and properties and different from any other component's atoms.

  • Compounds are formed by combinations of atoms of different elements.

Plum Pudding Model and the Discovery the Electron

In 1897, JJ Thomson spotted some smaller particles: the electron.

The atom was not the smallest particle of an element.

Soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model tried to explain two properties of atoms then known:

  • Electrons are negatively-charged particles.

  • Atoms have no net electric charge.

Thomson suggested that atoms were divisible. Atoms are electrically neutral, so the presence of negatively charged electrons meant that it had to be other positive charges.

Rutherford's Model: the Discovery of the Atomic Nucleus

In 1909, Ernest Rutherford discovered that most of the atom's mass and positive charge are concentrated in a tiny fraction of its volume in an area that he assumed was in the center.

This spot led Rutherford to propose a planetary atomic model in which a cloud of electrons surrounds a small, compact, positively charged nucleus.

The Quantum Model of the Atom

Quantum theory revolutionized physics in the early 20th century when Max Planck and Albert Einstein postulated that light energy is emitted or absorbed in discrete quantities known as quanta.

The old planetary model raised two significant flaws:

  1. Unlike planets, electrons are charged particles. An electrical charge from the accelerator is known to emit electromagnetic waves. An orbiting particle should steadily lose energy and spiral toward the nucleus, colliding with it in a fraction of a second.

  2. The planetary model could not explain the emission and absorption spectra of the observed atoms.

Bohr's Atomic Theory

In 1913, Niels Bohr developed the atomic model of de Bohr: “electrons can only rotate around the nucleus in a specific circular orbit with angular momentum. The energy fixed distance from the core is proportional to the energy. "

In this model, an electron cannot enter the nucleus because it could not continuously lose energy; Instead, it could only perform quantum jumps in a flash between fixed energy levels.

What Is Wave-particle Duality?

In 1924, Louis de Broglie claimed that all moving particles exhibited some waveform.

Schrödinger's equation (1926) describes an electron as a wave rather than a point particle. Max Born, instead, suggested that the Schrödinger wave function could be used to calculate the probability of finding an electron anywhere around the nucleus.

Both theories introduced the idea of ​​wave-particle duality: "The electron can exhibit both wavelength and particle properties."

Heisenberg Uncertainty Principle 

One consequence of the description of electrons as a wave is that it was not possible to calculate the position and momentum of an electron at the same time. 

This became known as the Heisenberg uncertainty principle (Werner Heisenberg, 1927). This principle invalidated the Bohr model with its clear and clearly defined circular orbits.

What Is the Modern Atomic Model Like?

The modern model of the atom describes the positions of electrons in an atom in terms of probabilities.

An electron can be found at any distance from the nucleus, but, depending on its energy level, it occurs more frequently in some regions around the nucleus than in others; This probability pattern is called the atomic orbital.

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Published: November 13, 2018
Last review: October 11, 2020