Introduction

What this section does

• It sets up the chapter by tracing how the picture of the atom changed: from the indivisible atom of Leucippus and Democritus, to Dalton's atomic theory, to Thomson's discovery of the electron, and then to the models of Rutherford and Bohr that the rest of the chapter derives.
• Dalton's theory held that matter is made of indestructible particles, that all atoms of a given element are identical, and that atoms combine with other atoms to form new substances. Thomson's discovery of the electron showed atoms are not indestructible.
• Two facts are stated here and used throughout: the nucleus has a radius about $10^5$ times smaller than that of the atom, and it holds the entire positive charge together with $99.9\%$ of the atom's mass.

What follows in the chapter

• The three atomic models and the experiments that decided between them, the hydrogen spectrum, the properties of the nucleus, the force that holds it together, its radioactive decays, and the energy that can be got out of it.