Atomic Models

Dalton’s Atomic Model

In 1808 Dalton formulated the atomic theory, a theory that broke with all traditional ideas derived from the ancient Greek philosophers (Democritus, Leucippus). It introduces the idea of the discontinuity of matter, i.e., it is the first scientific theory that considers that matter is divided into atoms. The basic postulates of this theory are:

  • Matter is divided into indivisible and unalterable particles called atoms.
  • Atoms are very small particles and cannot be seen with the naked eye.
  • All atoms of the same element are equal to each other, equal mass and equal properties.
  • Atoms of different elements have different mass and different properties.
  • Compounds are formed when atoms are bonded together in a constant and simple relationship.
  • In chemical reactions atoms separate or unite; but no atom is created or destroyed, and no atom of one element becomes an atom of another element. This conception was maintained for almost a century.

Thomson’s Atomic Model

Later, in 1897, the electron, one of the subatomic particles that make up the atom, was discovered. In 1898 Thomson proposed an atomic model that took into account the existence of this subatomic particle. His model was static, since it assumed that the electrons were at rest inside the atom, and that the whole was electrically neutral.

Thomson also explained the formation of ions, both positive and negative. When the atom loses any electron, the structure remains positive and positive ions are formed; but if the atom gains any electron, the structure remains negative and negative ions are formed.

Rutherford’s Atomic Model

After the discovery of the proton, Rutherford formulated his atomic model. In 1911, Rutherford used alpha particles to determine the internal structure of matter.

Ernest Rutherford, who studied with Thomson at Cambridge University, used alpha particles to demonstrate the structure of atoms. Together with his colleague Hans Geiger and an undergraduate student named Ernest Marsden. Rutherford performed a series of experiments using very thin films of gold and other metals as targets for particles from a radioactive source. They observed that most of the particles passed through the foil without deflection, or with a slight deflection. Occasionally, some alpha particles were scattered (or deflected) from their trajectory at a large angle. In some cases, the alpha particles returned along the same path back to the radioactive source.

From that experiment he deduced that:

  • Most particles pass through the sheet without deflection (99.9%).
  • Some particles are deflected (0.1%).
  • Seeing that the model proposed by Thomson was not fulfilled, Rutherford formulated the nuclear model of the atom. According to this model, the atom consists of a nucleus and a crust:
  • In the nucleus is concentrated almost the entire mass of the atom, and it has a positive charge.
  • The crust is formed by the electrons, which revolve around the nucleus describing circular orbits (miniature solar system).
  • Likewise, he also said that matter is neutral, since the positive charge of the nucleus and the negative charge of the crust neutralize each other.

Rutherford deduced that:

Matter is almost empty; the nucleus is 100,000 times smaller than the radius of the atom. Most alpha particles are not deflected because they pass through the crust, not the nucleus. Those that pass close to the nucleus are deflected because they are repelled. When the atom releases electrons, the atom remains negatively charged, becoming a negative ion; but if, on the other hand, the atom gains electrons, the structure will be positive and the atom will become a negative ion. The atom is stable.

Böhr Atomic Model

After the discovery of the neutron, in 1913 Böhr tried to improve Rutherford’s atomic model by applying Planck’s quantum ideas to his model. To realize his atomic model he used the hydrogen atom; he described the hydrogen atom with a proton as nucleus and an electron revolving around it. The new ideas on the quantization of energy are the following:

  • The atom is quantized, since it can possess only a few certain energies.
  • The electron revolves in circular orbits around the nucleus, and each orbit is a stationary state that is associated with a natural number, “n” (principal quantum number), and takes values from 1 to 7. Likewise, each level “n” is formed by different sublevels, “l”. And in turn, these are split into others (Zeeman effect), “m”. And finally, there is a fourth quantum number that refers to the sense, “s”. The allowed energy levels are multiples of Planck’s constant.
  • When an electron passes from one energy level to another, energy is either absorbed or emitted.
  • When the electron is at n=1 it is said to be in the fundamental level (minimum energy level); when changing levels the electron absorbs energy and is called an excited electron.
  • Böhr placed the electrons in exact places in space. It is the planetary model of Böhr.

Mechano-Quantum Model

It is the current model; it was exposed in 1925 by Heisenberg and Schrödinger.

Characteristic aspects:

Wave-particle duality: Broglie proposed that material particles have wave properties, and that every particle in motion carries an associated wave.
Indeterminacy principle: Heisenberg said that it was impossible to place an electron at an exact point in space. The equations of the quantum-mechanical model describe the behavior of electrons within the atom, and reflect their wave character and the impossibility of predicting their exact trajectories. Thus they established the concept of orbital, a region of the atom’s space where the probability of finding an electron is very large.

Characteristics of orbitals:

  • The energy is quantized. What makes the difference with the Böhr model is that this model does not determine the exact position of the electron, but the greater or lesser probability. Within the atom, the electron is interpreted as a cloud of negative charge, and within this cloud, where the density is higher, the probability of finding an electron will also be higher.
  • The behavior of the electrons within the atom is described by the quantum numbers. The quantum numbers are responsible for the behavior of the electrons, and the electronic configuration for their distribution. And finally, given the number of elements, a classification was needed. Today the Periodic Table is used, although it was preceded by many other proposals. In the Periodic Table the elements are classified according to atomic number.

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