The evolution of the atom model

Democritus’ model

The ancient philosopher Democritus (460-370 BC) asserted that matter is made up of very small particles that cannot be broken or divided. He calls these particles atoms (the word atom comes from the Greek word with the same pronunciation and which means indivisible). His representation of matter is called the model of discontinuity, because he believes that these particles are separated by a vacuum.

Aristotle’s model

The philosopher Aristotle (384 – 322 BC) opposes the idea of Democritus. For him, matter must completely fill the space it occupies (the model of continuity). He claims that matter is infinitely divisible and that matter is made up of four elements: earth, fire, air and water. That they are mixed in different proportions to form the different substances around us.

Dalton’s model

The English chemist and physicist John Dalton (1766 – 1844). Dalton builds his model from scientific experiments. He is based on observations made by various scientists and by himself to build his model.

  • Matter is made up of small, invisible and indivisible particles called atoms.
  • Atoms of the same element are the same. They have the same properties and the same mass.
  • Atoms of different elements have different properties and masses.
  • Atoms can combine to form a new substance.
  • The produced molecule has different properties from the atoms that constitute it.

Thomson’s model

The British physicist Joseph John Thomson (1871 – 1937) discovers that the rays emitted by a metallic cathode, whatever the metal, in a cathode ray tube, are made up of negatively charged particles which he calls « electrons ». deduces that it is a constituent element of the atom.

He imagined, in 1902, a model of the atom:
an electrically neutral sphere filled with a positively charged substance in which electrons are frozen.

The description of his model is as follows :

  • The atom is not indivisible, since it is possible to tear electrons from it.
  • The atom is a ball of positive matter dotted with small negative grains called electrons.
  • The negative charges of the electrons are counterbalanced by the positive charge of the ball. This way, the atom is neutral.

Its model is called the plum pudding model, since the crumb of the bread can be compared to the ball of positive matter and the raisins to the electrons that are distributed within the bread.

Rutherford’s model

New Zealand physicist Ernest Rutherford (1871 – 1937) experimentally demonstrates that the structure of the hydrogen atom consists of a very small nucleus surrounded by moving electrons.

To prove the existence of the nucleus, Rutherford bombards a 0.1 μm thick gold leaf with a beam of α positive particles and observes that some bounce back abnormally.

Rutherford therefore concludes that there are positively charged particles in the gold atom which repel α particles. These particles are confined in a small part of the atom called the « nucleus ».

The description of his model is as follows :

  • The atom is mostly made up of vacuum.
  • The atom has a small, dense nucleus made up of positive particles called protons.
  • Electrons revolve around the nucleus.
  • A neutral atom has as many electrons as there are protons.

Niels Bohr’s model

Bohr assumes that electrons travel in orbits, which he calls electronic layers. Each electronic layer corresponds to a specific energy level. The farther away the electron is from the nucleus, the more energy it has.

Electrons can move from one layer to another depending on their energy gain or loss. If an electron is supplied with energy, it moves to a higher layer.

However, the electron does not stay on the top layer. He goes back down to his starting layer. As it descends, it loses energy, which it emits in the form of light.

The description of his model is as follows :

  • The atom is represented by an almost empty space with, in the center, a dense nucleus containing the protons.
  • Electrons circulate around the nucleus of the atom on electronic shells.
  • There can be more than one electron per electron shell.
  • A neutral atom has as many electrons as there are protons.

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