Atom models

Atom models

1. What is an atom?

   • An atom is the smallest particle that maintains the properties of a chemical element and the basic unit of matter composed of of a nucleus with protos (positive) and neutrons (neutral), and electrons (negative) orbiting the nucleus. The number of protons in an atom defines which chemical element it is.

2. The beginning

   • In the 5th century BC, the Greek philosopher Democritus imagined matter as consisting of small, invisible particles- atoms ( derived from the Greek word fo indivisible). This idea was not accepted and was only taken up much later by Dalton.

3. Dalton's atomic model

   • Dalton in 19th century, revived the ideaof atoms as the basic constituents of matter. For him Atoms were small, indivisible and indestuctible particles.
   • Rutherford's atomic model
     • Later, Rutherford demostrated that most of the atom was empty space, with the positive charge located in the nucleus (the atom's central point), which had most of the atom's mass. The electrons would be orbiting the nucleus.
     • Rutherford also discovered the existence of protons, positively charged particles found in the nucleus.
     • This model does not explain why electrons do not fall into the nucleus, due to the attraction they have for the positive charges there.

4. Thomson's atomic model (Plum pudding model)

   • In 1897, Thomson discovered electrons, negative particles much smallerthan atoms, thus proving that atoms were not indivisible.
   • He formulated this theory, that atoms would be made up of a spherical central part with a positive eletric charge where electrons would be spread out, in sufficient numbers so that the total charge of the atom would be zero.

5. Bohr atomic model

   • In 1911, Bohr introduced changes to Rutherford's model: electrons revolve aroud the nucleus but can only occupy well-defined energy levels, in which each energy level corresponds to an orbit around the nucleus.
   • The innermost orbits represent lower energy levels and as they are further away from the nucleus the energy value of that level is higher.
   • Although this model is widely used for simple explanations of how the atom works (the levels represented by orbits are easy to draw and/or represent) this is not the current model of the atom.

6. Electron cloud model (or quantum model). The current model

   • Unlike the Bohr model, electrons do not occupy well-defined orbits corresponding to energy levels. In the current model (electron cloud model or quantum model), we only know the probality of finding each electron in an atom in a region of space _ this region is called an orbital.
   • The electron cloud of an atom. This cloud represents the probability of finding electrons in a given location in space, representing the sumof the different orbitals occupied by electrons.

7. Beatriz Pereira, Nº5, 7ºC

8. Webgraphy - fq.pt/atomo/historia-do-atomo

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