Introduction

The Nucleus: A First Look

The previous chapter on atoms left us with a striking picture: almost all the mass and all the positive charge of an atom are crammed into a tiny central core called the nucleus. The rest of the atom — the region where electrons roam — is mostly empty space. How do we know this? The key evidence came from the famous alpha-particle scattering experiments.

When a beam of alpha particles was fired at a thin gold foil, most particles passed straight through, but a tiny fraction bounced back at large angles. This could only happen if the atom’s positive charge was concentrated in a volume far smaller than the atom itself. From these experiments, physicists deduced that the radius of a nucleus is roughly 10,000 times smaller than the radius of the atom.

Because volume scales as the cube of the radius, the nucleus occupies only about $10^{-12}$ of the atom’s volume. In other words, an atom is almost entirely empty space.

This raises a cascade of questions. If the atom has a structure — electrons orbiting a nucleus — does the nucleus itself have a structure? What are its building blocks? What holds those building blocks together, given that they are all positively charged and should repel each other violently? And why are some nuclei stable while others spontaneously break apart?

This chapter begins the journey to answer those questions. We will study the basic properties of nuclei: their size, mass, and the forces that bind them. Then we will explore the dramatic phenomena that arise when nuclei change — radioactivity, nuclear fission, and nuclear fusion.

---

Key Quantitative Result from Scattering Experiments

The alpha-particle experiments gave a direct estimate of the relative sizes:

This means the nuclear radius is about one ten-thousandth of the atomic radius. Consequently, the volume ratio is:

So the nucleus occupies only one trillionth of the atom’s volume. Yet it carries virtually all the mass.

---

What Lies Ahead

The chapter is organised around three broad themes:

1. Nuclear properties — size, mass, density, and the concept of binding energy.
2. Radioactivity — the spontaneous decay of unstable nuclei, the laws that govern it, and the types of radiation emitted.
3. Nuclear reactions — fission (splitting heavy nuclei) and fusion (joining light nuclei), both of which release enormous amounts of energy.

The introduction sets the stage: we know the nucleus exists, we know it is tiny and dense, and we know it has a structure. The rest of the chapter will uncover what that structure is and how it behaves.

---