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Bohr Models

Bohr Models: Niels Bohr imagined one of the first comprehensive models of the structure of an atom.  He said that the atom had a large center called a nucleus which was composed of protons and neutrons.  He also said that electrons surrounded the nucleus in orbits which had limits to the number of electrons found in them.  These different orbits were believed to have different energy levels associated with them.Bohr said that the electron orbits (or "shells") had up to 2 electrons in the first shell (first ROW in the periodic table), up to 8 electrons in the second shell (second ROW in the periodic table), up to 8 electrons in the third shell (third ROW), up to 18 in the fourth shell, and similar for other shells. 

The Bohr model is a simple model which doesn't actually work well for atoms which are more complex than Helium but the model serves as a reasonable starting point in the discussion of the structure of the atom.

The structure of an atom can be represented by a Bohr model showing the number of protons, the number of neutrons and the shells of electrons shown in orbits of the appropriate numbers (2,8,8,18).

Example: Bohr Model of a Sodium Atom (Na)

***Note: single electron in outer shell...easily given up

Atoms seek a stable state.  This is accomplished by gaining or losing electrons from or to other atoms in order to have a full outer shell of electrons (2,8,8,18, etc.).  According to the Bohr Model, when this is accomplished, the stable state (electron configuration) will be that of the nearest Noble Gas.

Example: Bohr Model of a Sodium Ion (Na +1)

***Note: outer electron gone; configuration like Neon


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