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Physics A Level | Chapter 15: Atomic structure 15.1 Looking inside the atom

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LEARNING INTENTIONS

In this chapter you will learn how to:
- describe the nuclear model of the atom and the evidence for it
- show an understanding of the nature and properties of $\alpha - $, $\beta - $ and $\lambda - $radiations
- understand that in $\alpha $ and $\beta $ decay a nuclide changes into a different nuclide
- recognise that there are two classes of sub-atomic particles – leptons and hadrons
- recognise that leptons are fundamental particles
- appreciate that electrons and neutrinos are leptons
- recognise that hadrons are not fundamental particles
- understand that hadrons are made up of particles called quarks.

BEFORE YOU START

- Try drawing the structure of the atom.
- Suggest why, in the late $19th$ century, physicists felt that atoms were not the basic building blocks of matter and that the atoms themselves had an internal structure. Discuss your ideas with your fellow students.

RADIOACTIVITY AT WORK

Radioactive substances have many uses, for example, in engineering and medicine.
In the $1950s$, many shoe shops had an X-ray machine where you put your feet into an opening and you could view the bones in your feet on a fluorescent screen – quite exciting for a young child! These have long since disappeared. Why do you think they are not used anymore?
Radioactive substances must be handled with great care to ensure that no-one becomes contaminated and so exposed to the radiation that comes from these substances (Figure 15.1).
Do you know how modern-day workers who are likely to be exposed to radiation (such as radiographers in a hospital) are protected from radiation? Are the short-term and long-term protections different?
In this chapter, we will look at the structure of the atom, and then the nature of radioactive substances and the different types of radiation they produce.

**Figure 15.1:** A worker at a nuclear power station is checked for any radioactive material on his body.

The idea that matter is composed of very small particles called atoms was first suggested by the Ancient Greeks about 2000 years ago. However, it was not until the middle of the 19th century that any ideas about the inside of the atom were proposed.
It was the English scientist J.J. Thomson who suggested that the atom is a neutral particle made of a positive charge with lumps of negative charge (electrons) in it. He could not determine the charge and the mass of the negative particles separately, but it was clear that a new particle, probably much smaller than the hydrogen atom, had been discovered. Since atoms are neutral and physicists had discovered a negatively charged part of an atom, it meant that there were both positive and negative charges in an atom. We now call this the plum pudding model of the atom (positive pudding with negative plums!).
Other experiments show that the electron has a mass of approximately $9.11 \times {10^{31}}\,kg\,({m_e})$ and a charge of $ - 1.60 \times {10^{ - 19}}\,C\,( - e)$. Today, we use the idea of the electron to explain all sorts of phenomena, including electrostatics, current electricity and electronics.

Physics A Level | Chapter 15: Atomic structure 15.1 Looking inside the atom

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