The development of the atomic models

 Dalton model (Billiard Ball Model)

John Dalton proposed a modern atomic model based on experimentation. The Law of Multiple Proportions states that when elements combine, they do so in the ratio of small whole numbers. Carbon and Oxygen can form either CO ( a 1:1 ratio) or CO2 (a 1:2 ratio). In his theory, there are six basic ideas:

–          All matter is composed of atoms

–          Atoms cannot be made or destroyed

–          All atoms of the same element are identical

–          Different elements have different types of atoms

–          Chemical reactions occur when atoms are rearranged

–          Compounds are formed from atoms of the constituent elements.

THE KELVIN’S-THOMSON’S ATOMIC MODEL -THE PLUM CAKE: Physics and radioactivity after the discovery of polonium and radium.

 When the phenomena of the radioactivity was discovered, the Dalton’s theory of the indivisible atom fell down. Basing on the researches on polonium and radium ( discovered by Marie e Pierre Curie), lord Kelvin formulated a model, assuming that every particle is more or less but radioactive.

The “plum pudding model” was J.J. Thomson’s theory of atomic structure that he developed in 1904. In this model the electrons and protons are uniformly mixed throughout the atom.

In the years 1909-1911 Ernest Rutheford and his students – Hans Geiger (1882-1945) and Ernest Marsden conducted some experiments, testing Thomson’s hypothesis by devising his “gold foil” experiment.

The experiments caused the creation of the new model of atom – the “planetary” model. An alpha particle ( a heavy, positively charged particle) is a helium nucleus released by radioactive substances. Polonium was put into a lead box that sent out alpha particles to a thin sheet of gold foil. The foil was then surrounded by a luminescent zinc sulphide screen that served as a backdrop for the alpha particles to appear on.  A microscope was placed above the screen so they could observe any contact made between the alpha particles and the screen. In order to prove Thomson’s “plum pudding model”, the alpha particles were supposed to go straight through the foil. Shockingly, although most alpha particles passed straight through the gold foil, some did not.


Most alpha particles indeed went through the foil with only small deflections. But a few were scattered by large angles or bounced directly back at source. Thomson’s “plum pudding model” was proven incorrect. Thomson’s theory said that an atom was made up of empty space, but that couldn’t be correct if the particles had bounced back because they had to have hit something. The way and angles in which the alpha particles bounced off the foil indicated that the majority of the mass of an atom was concentrated in one small region, that Rutherford later called the nucleus. He reasoned that the nucleus held all the positive charge, while electrons occupied most of the atom’s space.

The Bohr’s Model

In 1913 Niels Bohr published a theory about the structure of the atom based on an earlier theory of Rutherford’s. Rutherford had shown that the atom consisted of a positively charged nucleus, with negatively charged electrons in orbit around it.

Bohr expanded upon this theory by proposing that electrons travel only in certain successively larger orbits. Bohr also described the way atoms emit radiation by suggesting that when an electron jumps from an outer orbit to an inner one, that it emits light. The Bohr’s Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun. The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons.

There were problems with the Bohr Model:

  • It violated the Heisenberg Uncertainty Principle because it considered electrons to have both   a known radius and orbit.
  • The Bohr Model provided an incorrect value for the ground state orbital angular momentum.
  • It made poor predictions regarding the spectra of larger atoms.
  • It did not predict the relative intensities of spectral lines.
  • The Bohr Model did not explain fine structure and hyperfine structure in spectral lines.
  • It did not explain the Zeeman Effect.

The Zeeman effect describes the splitting of spectral lines in the presence of a magnetic field. In the absence of a magnetic field, emission is observed as a single spectral line and is dependent only on the principal quantum numbers of the initial and final states. In the presence of an external magnetic field, the principal quantum number of each state is split into different substates.


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