The Discovery of Neutrons and Rutherford Gold Foil Experiment

After the discovery of radioactivity, it was observed that radioactive elements, such as uranium, emit spontaneous radiation as a result of their radioactive decay.

There are three types of radiation emitted during radioactive decay:

1. Alpha (α) particles: These particles were found to be deflected by negatively charged plates, indicating that they carry a positive charge.
2. Beta (β) particles: These particles were observed to be deflected by positively charged plates, suggesting that they carry a negative charge.
3. Gamma (γ) rays: These particles were found to be unaffected by any external field and did not exhibit deflection.

In 1910, Ernest Rutherford, a New Zealand physicist and former student of J. J. Thomson, embarked on a study of the atom’s structure using alpha (α) particles. Assisting him in his experiments were two students, Hans Geiger and Ernest Marsden.

During Rutherford’s experiment, he directed a beam of α-particles at an extremely thin gold foil, only one atomic width thick. He observed that most of the rays passed through the gold foil and reached a detecting screen. However, they also noticed that some of the rays were reflected backward or deflected slightly. This phenomenon could only occur if the beam encountered particles carrying the same charge. Rutherford explained these results by proposing his own atomic model.

Rutherford atomic model:

1. Most of the atom must be empty space because the majority of the α-rays passed through the gold foil without any deflection.
2. However, some of the α-rays experienced slight deflection when they encountered the negative electrons. This indicates that electrons are tiny and have smaller mass, resulting in a slight deviation of the α-particles.
3. The atom consists of a positive center called the “nucleus.” When the positive α-particles collide with the nucleus, they bounce back.

 Rutherford Experiment

The Rutherford atomic model differs from the Thomson model:

Thomson proposed the plum-pudding model, suggesting that the atom is a large positively charged sphere with embedded electrons.

Rutherford contradicted this model by stating that the particles that bounced back must have collided with a positively charged entity, causing the scattering of the rays.

The atomic model was modified to include

1. The positively charged nucleus is where the mass of the atom is concentrated.
2. The majority of rays pass through a large empty space, so the atom must contain a large empty space.
3. Negatively charged electrons around the nucleus in the vast space around the nucleus.

 Rutherford atomic model

Notice: At that time, neutrons had not yet been discovered, but the Rutherford experiment prompted scientists to make the discovery of neutrons.

The discovery of neutrons:

Rutherford’s findings puzzled scientists as to why the mass of helium is four times that of hydrogen, even though helium only has two protons while hydrogen has only one proton.

                                                            Helium mass: hydrogen mass

                        Predicted                              2           :                1

                        Actual                                   4           :                 1

Chadwick and the discovery of the neutrons:

James Chadwick was studying the structure of the atom when he directed highly energetic α-particles at a beryllium sheet, resulting in the emission of a radiation similar to gamma rays but with greater penetrating power. This emitted radiation, initially believed to be gamma radiation, was later discovered to be uncharged particles. Furthermore, when this radiation interacted with hydrogen-containing compounds like paraffin wax, it ejected high-energy protons.

Upon further investigation, Chadwick realized that these uncharged particles had a mass similar to that of protons. They were subsequently named neutrons. In a model proposed by Heisenberg and Dmitri Ivanenko, protons and neutrons were described as being located within the nucleus.

The nucleus itself consists of positively charged protons and neutrally charged neutrons. It was through the work of Lise Meitner and Otto Hahn that the highly energetic neutrons were utilized to induce nuclear fission in uranium, eventually leading to the development of atomic weapons.