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400 BCE
Democritus's atomic model
He considered that matter was made up of small particles that could not be divided, which he called atoms.
The atoms are eternal, immutable, indivisible.
Experiment: There was no experiment because he was empirical.
Limitations of the model: His ideas were lacking of an experimental basis. -
Dalton's atomic model/ theory
The chemical compounds would be formed by combining the atoms of two or more elements.
As it happens when the water molecule is formed from two hydrogen atoms and one oxygen atom.
Limitations of the model: Dalton's atom served to explain the properties of atoms, but not to explain the functioning of electrical and light phenomena.
Developed an atomic theory that proposed that atoms were responsible for the combinations of elements found in compounds. -
Thomson's atomic model "Plum-pudding model"
Atoms are a positive sphere with embedded electrons.
Electrons have a negative charge.
Has a region with a positive electric charge.
Discovered the isotopes.
Experiment: He performed his experiment using cathode rays.
Proved that the rays were formed by a stream of negatively charged particles "electrons".
Limitations of the model: It was insufficient to explain various types of radiation. -
Rutherford's dispersion (Gold foil experiment)
The atom has a small central nucleus, with a positive electric charge, which contains almost all its mass.
The electrons rotate at great distance around the nucleus in circular orbits.
The atom is neutral.
Experiment: It consisted in bombarding a sheet of gold with positive particles and observe the deviation they suffered.
Limitations of the model:
The inability to explain the atomic spectra and to represent the atom as a terribly unstable entity. -
Bohr's atomic model "Rutherford-Bohr atomic model"
Consists of a nucleus formed by a proton on which an electron rotates following a circular orbit.
It also predicted that as electrons spiralled inward, their emission would rapidly increase in frequency as the orbit got smaller and faster.
Experiment: When an electron receives energy from outside, it can absorb a certain amount that forces it to change to higher orbits.
Limitations of the model: He could not justify the Zeeman effect (unfolding of levels and sublevels). -
Chadwick's atomic model
The atoms are composed by a nucleus and electrons.
In the nucleus most of the mass is concentrated and inside are the neutrons and protons. Around them electrons move in their respective orbits.
Experiment: If the nucleus were a ball of 1cm in diameter, the complete atom would be 1km in diameter.
Revealing that most of the atomic volume is given by the volume of the region occupied by the electrons and that it is mostly empty space.
Limitations: Data related to the mass did not agree. -
Schrödinger's atomic model "Cloud model"
Conceived the electrons as waves of matter.
Thus the equation was interpreted as the wave equation that described the evolution in time and space of this material wave.
Experiment: It recognizes an atomic nucleus and electrons distributed around a "cloud" according to the energy they possess by the force of attraction exerted by the nucleus on them,
using quantum mechanics.
Limitations: It doesn't take into account the spin of electrons, ignores the relativistic effects of fast electrons.
