13.2.1. Discovery of Neutron

Since the nuclei of deuterium and tritium are isotopes of hydrogen, they must contain only one proton each. But the masses of the nuclei of hydrogen, deuterium and tritium are in the ratio of 1:2:3. Therefore, the nuclei of deuterium and tritium must contain, in addition to a proton, some neutral matter. The amount of neutral matter present in the nuclei of these isotopes, expressed in units of mass of a proton, is approximately equal to one and two, respectively.

Scientists realized that if deuterium and tritium are both isotopes of hydrogen (which means they each contain one proton), then the differences in their masses imply that something else must be present. Specifically, deuterium's mass is about twice that of hydrogen, and tritium's is about three times. This suggested that these isotopes have additional particles that do not carry a charge, which came to be known as neutrons.

Think of a car (representing the hydrogen nucleus, or a proton) that goes through upgrades to make it more powerful (representing the addition of neutrons). The more powerful car (deuterium) has a heavier frame, while an even more powerful version (tritium) is heavier still. However, the foundational piece remains the same; they all started with the basic car (the proton).

This fact indicates that the nuclei of atoms contain, in addition to protons, neutral matter in multiples of a basic unit. This hypothesis was verified in 1932 by James Chadwick who observed emission of neutral radiation when beryllium nuclei were bombarded with alpha-particles.

James Chadwick, through his experiments in 1932, bombarded beryllium with alpha particles. He noticed the emission of a type of radiation that was not charged—they were neither positive nor negative. This neutral radiation suggested the existence of an unknown particle, which was later confirmed as the neutron. Chadwick's work validated the earlier hypothesis that neutral particles were part of atomic nuclei.

Imagine trying to figure out what ingredient is missing from a recipe. You mix various spices (alpha particles) with a dish of beryllium, and suddenly something new appears that you weren't expecting (the neutrons). Just as you analyze this new flavor to complete your dish, Chadwick pieced together that this new radiation indicated the presence of neutrons in the atomic nucleus.

The mass of a neutron is now known to a high degree of accuracy. It is m = 1.00866 u = 1.6749×10–27 kg. Chadwick was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron.

The neutron, which has a mass very similar to that of a proton, plays a crucial role in the nucleus of atoms. It helps to stabilize the nucleus by balancing the repulsive forces that protons have for each other due to their positive charges. This stability allows atoms to exist as they do and enables the complexity of matter as we know it.

Think of the neutrons as the glue that binds a book (the nucleus) together. While the pages (protons) may push against each other, the glue (neutrons) holds everything in place, preventing the book from falling apart. Just as a book can be thin or thick, depending on how many pages it has, different atoms have different numbers of neutrons that influence their stability.

A free neutron, unlike a free proton, is unstable. It decays into a proton, an electron and a antineutrino (another elementary particle), and has a mean life of about 1000s. It is, however, stable inside the nucleus.

While neutrons are stable when they are part of a nucleus, isolated neutrons undergo decay, meaning they will eventually turn into a proton and an electron after about 1000 seconds. This is an important aspect of understanding nuclear reactions and stability, as it shows how neutrons contribute to the behavior of nuclei over time.

Picture a decorated cake (the nucleus) where the icing (neutrons) holds the layers of cake (protons) together. If you remove the icing, the layers may slide apart and collapse—much like how free neutrons decay and are unable to provide stability when they are not within a nucleus.