Rutherford scattering

In Rutherford scattering, a beam of positively charged alpha particles is directed at a thin metal foil. Detectors record the directions in which the particles emerge.

Most alpha particles pass through the foil with little or no deflection. Some are deflected through larger angles, while a very small proportion are scattered backwards.

What the scattering reveals

The fact that most alpha particles pass through with little deflection shows that atoms are mostly empty space. The rare large deflections show that an atom’s positive charge is concentrated in a very small region, rather than spread throughout the atom. This region is the nucleus, which also contains almost all the atom’s mass.

Both the alpha particle and the nucleus are positively charged, so they repel each other. An alpha particle passing far from a nucleus experiences little repulsion and is deflected only slightly. One approaching closer experiences stronger repulsion and is deflected through a larger angle. A nearly head-on approach can cause the particle to reverse direction and scatter backwards.

Rutherford scattering and the plum pudding model

The earlier plum pudding model described positive charge as spread throughout the atom. It could not account for the rare large deflections and backscattering observed. These results supported a model in which the positive charge and almost all the mass are concentrated in a tiny nucleus.

Development of the nuclear model

Scientific models of the atom changed as new evidence revealed how its positive charge and mass are arranged. Each model offered an explanation of the evidence available at the time.

From plum pudding to a nuclear model

In the plum pudding model, positive charge was spread throughout the atom, with negatively charged electrons embedded in it. There was no central nucleus.

Rutherford’s nuclear model placed the positive charge and most of the atom’s mass in a tiny central nucleus. This model was supported by experiments in which positively charged alpha particles were directed at thin metal foil. Most passed straight through, but some were deflected and a few bounced back. These results indicated that most of the atom is empty space, while its positive charge and most of its mass are concentrated in a small region.

Protons and neutrons in the nucleus

Later work identified the proton as a positively charged constituent of nuclei. Before the neutron was discovered, nuclei were sometimes modelled as containing protons and electrons.

In 1932, Chadwick discovered the neutron, a particle with no electric charge. Its discovery supported a model in which nuclei contain protons and neutrons. Neutrons add mass to a nucleus without adding positive charge.

This model explains why atoms of the same element can have different masses. They have the same number of protons but can have different numbers of neutrons. Atoms of an element with different numbers of neutrons are called isotopes.