Structure of the Atom (HSC SSCE Physics): Revision Notes

Chadwick's Discovery of Neutrons

What are nucleons?

Following Rutherford's identification of the atomic nucleus, scientists discovered that the nucleus contains two types of particles called nucleons. These are:

• Protons - positively charged particles
• Neutrons - electrically neutral particles

Properties of nucleons

Both protons and neutrons exist inside the nucleus and have very similar masses, but they differ in their electrical charge.

Key properties:

• Location: Both nucleons are found inside the nucleus
• Mass: Protons have a mass of $1.673 \times 10^{-27}$ kg, whilst neutrons have a slightly larger mass of $1.675 \times 10^{-27}$ kg
• Charge: Protons carry a positive charge of $1.602 \times 10^{-19}$ C, whilst neutrons have zero charge

Historical understanding of the nucleus

When Rutherford first described the nucleus, it was understood simply as a concentrated region of positive charge. The internal structure and composition of the nucleus remained unknown. Understanding the nucleus required the work of many scientists over several decades.

Discovery of protons

Protons were the first nucleons to be discovered. Scientists measured the charge-to-mass ratio of protons using a discharge tube containing hydrogen ions. When a hydrogen atom loses its single electron, only a proton remains. This allowed the properties of protons to be measured in a similar way to how electrons were discovered.

Early theories about nuclear composition

After learning about the nucleus and protons, some scientists proposed that the nucleus contained:

• $A$ protons (where $A$ is the mass number)
• $A - Z$ electrons (where $Z$ is the atomic number)

This hypothesis appeared to work well for two reasons:

1. It explained mass numbers: The theory accounted for why atoms had a mass number larger than the number of positive charges. In the sodium example, the 12 electrons in the nucleus would cancel the positive charges of 12 protons, leaving a net charge of +11 rather than +23.
2. It explained beta decay: Scientists observed that electrons could be ejected from the nucleus during radioactive decay (beta particle emission). At the time, they believed this was only possible if electrons existed inside the nucleus.

Bothe's observation (1930)

In 1930, German scientist Walther Bothe made an important observation whilst conducting experiments with alpha particles. He discovered that when beryllium metal was bombarded with alpha particles (which are helium nuclei), it produced a neutral but highly penetrative form of 'radiation'. However, Bothe could not identify the nature of this mysterious 'radiation'.

Chadwick's experiment (1932)

English physicist James Chadwick built upon Bothe's work to solve this mystery. In 1932, Chadwick demonstrated that the unknown 'radiation' from beryllium bombardment was not electromagnetic radiation at all, but rather consisted of particles - specifically, neutrons.

Experimental design

Chadwick's experimental setup consisted of several key components arranged in sequence:

1. Alpha particle source: Provided the bombarding particles
2. Beryllium metal target: When struck by alpha particles, beryllium emitted neutrons
3. Paraffin wax block: Rich in hydrogen atoms and therefore protons
4. Detector: Measured the ejected protons

How the experiment worked

The clever design of Chadwick's experiment allowed him to detect neutrons indirectly:

Why neutrons are difficult to detect

The nuclear equation

The reaction that occurs in Chadwick's experiment can be represented by a nuclear equation:

$^{4}_{2}\text{He} + ^{9}_{4}\text{Be} \rightarrow ^{12}_{6}\text{C} + ^{1}_{0}\text{n}$

In this equation:

• $^{4}_{2}\text{He}$ represents the alpha particle (helium nucleus)
• $^{9}_{4}\text{Be}$ represents beryllium
• $^{12}_{6}\text{C}$ represents carbon (the product nucleus)
• $^{1}_{0}\text{n}$ represents the neutron

Significance of the discovery

Chadwick's work was groundbreaking because he proved the existence of neutrons without directly observing them. Instead, he demonstrated their existence by observing their properties and effects on other particles. This indirect detection method was a brilliant solution to the problem of measuring uncharged particles.

Modelling Chadwick's experiment

To understand the principle of momentum transfer in Chadwick's experiment, you can create a simple model using everyday objects.

The model

The investigation uses:

• A golf ball (representing neutrons - heavier and harder to detect)
• White table tennis balls (representing protons in paraffin wax)
• Orange table tennis balls (representing a second set of protons)

When the golf ball is rolled towards the white table tennis balls, it transfers momentum to them through collision. The white balls then collide with the orange balls, transferring momentum again. This models how neutrons transfer momentum to protons in the paraffin wax, which can then be detected.

Why this model works

1. The golf ball has greater mass than the table tennis balls, similar to how incoming particles have significant momentum
2. The table tennis balls are easy to observe, just as protons are easy to detect
3. The momentum transfer through collisions demonstrates the same principle Chadwick used

Safety considerations

Remember!