Rutherford's alpha particle scattering experiment

Rutherford’s Famous Experiment and the Structure of the Atom

In 1911, scientist Ernest Rutherford conducted a famous experiment that changed our understanding of the atom. Before this, the atom was believed to be a uniform sphere of positive charge with electrons inside it (as proposed in J.J. Thomson’s "plum pudding model"). But Rutherford’s experiment proved this model wrong.

What Was the Experiment?

Rutherford and his team performed the gold foil experiment. In this, they directed a stream of alpha particles (which are positively charged) at a very thin sheet of gold foil. A fluorescent screen surrounded the gold foil to observe the paths of the alpha particles.

Observations

• The majority of alpha particles traveled through the gold foil without changing direction.
• A few particles were slightly deflected from their original path.
• A very few alpha particles bounced back at large angles, even returning toward the source.

Conclusions from the Experiment

• Most of the atom consists of empty space, as most alpha particles moved through without being deflected.
• There is a small, dense, positively charged region in the center of the atom, which caused some alpha particles to deflect or bounce back. This central part was named the nucleus.
• The electrons revolve around the nucleus, like planets around the sun.

Rutherford’s Atomic Model

Based on the experiment, Rutherford proposed a new model of the atom:

• The atom contains a tiny central nucleus that holds the entire positive charge and nearly all of its mass.
• Electrons revolve around the nucleus in circular paths.
• The atom is electrically neutral because the number of electrons equals the number of protons.

Limitations

Rutherford’s model marked significant progress, but it also had an important limitation. According to classical physics, electrons moving in circular paths should lose energy and spiral into the nucleus. But this doesn’t happen. This problem was later solved by Niels Bohr, who modified the atomic model.

Rutherford’s experiment was a turning point in atomic theory. It introduced the concept of the nucleus and helped develop the modern structure of the atom.