The most important properties of atomic and molecular structure may be exemplified using a simplified picture of an atom that is called the Bohr Model.

This model was proposed by Niels Bohr in 1915; it is not completely correct, but it has many features that are approximately correct and it is sufficient for the explanation of many observed phenomena.

The Bohr Model is probably familar as the "planetary model" of the atom illustrated in the adjacent figure.

 In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). The adjacent image is not to scale since in the realistic case the radius of the nucleus is about 100,000 times smaller than the radius of the entire atom, and as far as we can tell electrons are point particles without a physical extent.