FOREWORD

During the last decade there has been a great advance in our knowledge of the structure of the atom and of the relation between the atoms of the chemical elements. In the later stages, science owes much to the remarkable achievements of Professor Niels Bohr and his co-workers in Copenhagen. For the first time, we have been given a consistent theory to explain the arrangement and motion of the electrons in the outer atom. The theory of Bohr is not only able to account in considerable detail for the variation in the properties of the elements exemplified by the periodic law, but also for the main features of the spectra, both X-ray and optical, shown by all elements.

This volume, written by Dr. Kramers and Mr. Holst, gives a simple and interesting account of our knowledge of atomic structure, with special reference to the work of Professor Bohr. Dr. Kramers is in an especially fortunate position to give a first-hand account of this subject, for he has been a valued assistant to Professor Bohr in developing his theories, and has himself made important original contributions to our knowledge in this branch of inquiry.

I can confidently recommend this book to English readers as a clearly written and accurate account of the development of our ideas on atomic structure. It is written in simple language, and the essential ideas are explained without mathematical calculations. This book should prove attractive not only to the general scientific reader, but also to the student who wishes to gain a broad general idea of this subject before entering into the details of the mathematical theory.

E. RUTHERFORD.

Cavendish Laboratory, Cambridge, 8th October 1923.

CONTENTS

chap. PAGE

Preface vii

Foreword xi

I. Atoms and Molecules 1

II. Light Waves and the Spectrum 34

III. Ions and Electrons 61

IV. The Nuclear Atom 83

V. The Bohr Theory of the Hydrogen Spectrum 105

VI. Various Applications of the Bohr Theory 153

VII. Atomic Structure and the

Chemical Properties of the Elements 180

Interpretation of Symbols and Physical Constants 209

COLOURED PLATES

I. Spectrum Plates according to the Original

Drawings of Bunsen and Kirchhoff At end

II. Principal Features of Atomic Structure in Some

of the Elements—Atomic Structure of Radium At end

THE ATOM AND THE BOHR THEORY OF ITS STRUCTURE