13.1. INTRODUCTION

In the previous chapter, we have learnt that in every atom, the positive charge and mass are densely concentrated at the centre of the atom forming its nucleus. The overall dimensions of a nucleus are much smaller than those of an atom.

An atom consists of a nucleus at its center, which holds the positive charge and most of the atom's mass. This nucleus is significantly smaller than the entire atom, which means that atoms are mostly empty space. The nucleus can be thought of as a tiny ball compared to the spacious area around it, which holds the electrons.

Imagine a large stadium where the seats (representing empty space) are filled with people (representing electrons), but the actual playing field is only as large as a soda can (representing the nucleus). Most of the stadium is empty, symbolizing the vast distances between subatomic particles.

Experiments on scattering of α-particles demonstrated that the radius of a nucleus was smaller than the radius of an atom by a factor of about 10^4. This means the volume of a nucleus is about 10–12 times the volume of the atom.

The nucleus has a radius that is approximately 10,000 times smaller than that of the atom itself. Consequently, while the atom occupies a larger volume, the nucleus makes up an extremely small fraction of that volume. This illustrates how atoms, despite containing significant mass, have a massively larger empty space around their nuclei.

Think of the sun as the nucleus and the solar system as the atom. The sun is a massive body, but in comparison to the vast distances between the planets and the sun, its size is minuscule. The empty space in the solar system allows planets to orbit around the sun, just like electrons orbit around a nucleus.

Nevertheless, the nucleus contains most (more than 99.9%) of the mass of an atom. Does the nucleus have a structure, just as the atom does? If so, what are the constituents of the nucleus? How are these held together?

Most of an atom’s mass resides within the nucleus, which is composed of protons and neutrons. It's important to understand not only what makes up the nucleus but also how these particles are held together tightly within the small space of the nucleus. This exploration of structure and binding is fundamental to understanding nuclear physics.

Consider a tightly packed suitcase filled with clothes (representing the protons and neutrons); while the suitcase may be small, it can hold a significant amount of material, emphasizing how a nucleus, though small, contains most of an atom's mass.

In this chapter, we shall look for answers to such questions. We shall discuss various properties of nuclei such as their size, mass and stability, and also associated nuclear phenomena such as radioactivity, fission and fusion.

This chapter will delve into the characteristics of nuclei. It aims to explain important concepts, such as why nuclei vary in size and mass and how they behave under different conditions. Additionally, the chapter will introduce concepts like radioactivity, and processes such as fission (splitting) and fusion (combining) which are crucial for energy production in nuclear reactions.

Think of the chapter as a journey through a nuclear landscape where we explore different environments (properties and behaviors of nuclei) to understand how nuclear energy can be harnessed similar to exploring different terrains to gather resources for a community.

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