Early models of the atom

One of the outstanding problems in the late 1800s was to come up with a model describing properties of the basic constituent of matter - the atom. By bombarding thin metal foils with small particles, Rutherford found an interesting property of materials: since most of the particles went through the metal foil undeflected, only occasionally being scattered, it was concluded that the major mass of the atom was concentrated in a small region, and most of the atom was actually empty space. This and other types of experiments gave rise to the a planetary type of model of the atom, which is pictured below.

 

Figure 12.1: Planetary model of the atom

In this model a central nucleus, consisting of protons and neutrons, lies at the center, with electrons in orbit around the nucleus. Because an electron is about 1800 times less massive than a proton or neutron, most of the mass of the atom is concentrated in the nucleus, and most of the atom is empty space, as was inferred from Rutherford's experiments.

The protons are have positive electric charge, and the electrons are negatively charged. The force of attraction between these two charges keeps the atom together - we will see in the next chapter what keeps the nucleus together. The atom thus behaves much like the earth-sun system, where the gravitational force keeps the earth in orbit around the sun. However, in the atom it is the electrical force which keeps the electron in orbit, as the gravitational force is negligible compared to the electrical force.