Video Transcript
In the Geiger–Marsden experiments supervised by Ernest Rutherford, known as the Rutherford gold foil experiment, which type of particle was scattered by gold foil, proving that atoms contain dense nuclei?
Geiger and Marsden performed their experiments under Ernest Rutherford between 1908 and 1913. These experiments are sometimes known collectively as the Rutherford gold foil experiment because Rutherford developed his atomic model based on the results. In these experiments, a particle beam was used. The particles were directed at gold foil. While most of the particles passed straight through, some of the particles were deflected a little, and some were reflected back.
These results ran contrary to the popular atomic model of the time, the Thompson model, otherwise known as the plum pudding model. In this model, negative electrons were spread evenly through a field of positive charge. The beam used in the experiment was of positively charged particles.
According to the Thompson model, if a high-energy particle with positive charge would pass through gold foil, it would go straight through. The fact that these positive particles reflected back off the material sometimes suggested that there were dense positively charged nuclei in the atoms, rather than an evenly distributed field.
In this system, a positively charged particle colliding with the gold foil would have a small chance of getting very close to the nucleus of one of the atoms and be repelled. This model would explain why the gold foil reflects some of the particles, but most of them passed straight through. These results led Rutherford to come up with the Rutherford atomic model, with a dense positive nucleus and negative electrons around the outside.
The question asks, which type of particle was scattered by the gold foil? We already know that the particle beam had the same charge as the positive cloud predicted by the Thompson model. In the Geiger–Marsden experiments, an alpha particle source, a radioactive source, was used to provide the positive particles. A radioactive source was the simplest way of producing positive ions.
An alpha particle is a high-energy helium nucleus, having two protons, two neutrons, and an overall charge of two plus. Alpha particles can be produced from unstable radioactive heavy nuclei that decay into alpha particles and a smaller nucleus. You’ll sometimes see alpha particles written like this. So, the answer to our question, which type of particle was scattered by the gold foil in the Rutherford gold foil experiment, is an alpha particle.
The particle beam had to be positive, in order to understand the nature of the positive charge in atoms. This experiment demonstrated that the nucleus must be pretty dense because the charge has to be dense enough in order to repel the high-energy alpha particle, which typically has three to seven megaelectron volts of energy. That’s about one million times the energy of a carbon–carbon bond. So, the answer for the type of particle is an alpha particle.
