Atoms and Electrons

Part 1

Transcriber’s notes:

In this plain-text e-book, italic text is denoted by underscores. Superscripted text is indicated by a preceding caret mark, e.g. 2^{n-1}

ATOMS AND ELECTRONS

J. W. N. Sullivan

DORAN’S MODERN READERS’ BOOKSHELF

ST. FRANCIS OF ASSISI Gilbert K. Chesterton

THE STORY OF THE RENAISSANCE Sidney Dark

VICTORIAN POETRY John Drinkwater

THE POETRY OF ARCHITECTURE Frank Rutter

ATOMS AND ELECTRONS J. W. N. Sullivan

EVERYDAY BIOLOGY J. Arthur Thomson

Other Volumes in Preparation

ATOMS AND ELECTRONS

BY J. W. N. SULLIVAN

NEW YORK GEORGE H. DORAN COMPANY

COPYRIGHT, 1924, BY GEORGE H. DORAN COMPANY

ATOMS AND ELECTRONS —B— PRINTED IN THE UNITED STATES OF AMERICA

GENERAL INTRODUCTION

Of all human ambitions an open mind eagerly expectant of new discoveries and ready to remold convictions in the light of added knowledge and dispelled ignorances and misapprehensions, is the noblest, the rarest and the most difficult to achieve.

JAMES HARVEY ROBINSON, in “The Humanizing of Knowledge.”

It is the purpose of Doran’s Modern Readers’ Bookshelf to bring together in brief, stimulating form a group of books that will be fresh appraisals of many things that interest modern men and women. Much of History, Literature, Biography and Science is of intense fascination for readers today and is lost to them by reason of being surrounded by a forbidding and meticulous scholarship.

These books are designed to be simple, short, authoritative, and such as would arouse the interest of intelligent readers. As nearly as possible they will be intended, in Professor Robinson’s words quoted above, “to remold convictions in the light of added knowledge.”

This “adding of knowledge” and a widespread eagerness for it are two of the chief characteristics of our time. Never before, probably, has there been so great a desire to know, or so many exciting discoveries of truth of one sort or another. Knowledge and the quest for it has now about it the glamour of an adventure. To the quickening of this spirit in our day Doran’s Modern Readers’ Bookshelf hopes to contribute.

In addition to the volumes announced here others are in preparation for early publication. The Editors will welcome suggestions for the Bookshelf and will be glad to consider any manuscripts suitable for inclusion.

The Editors.

CONTENTS

PAGE

Chapter I

Part 2

UNITS AND NOTATION 11

Chapter II

ATOMS AND MOLECULES 21

Chapter III

CONSTITUENTS OF THE ATOM 51

Chapter IV

THE STRUCTURE OF THE ATOM 77

Chapter V

QUANTUM THEORY 111

Chapter VI

THE GROUPING OF ATOMS 137

Chapter VII

THE INNER REGIONS 165

INDEX 187

CHAPTER I: Units and Notation

ATOMS AND ELECTRONS

Chapter I

Units and Notation

Dimensions.—All physical magnitudes are measured in terms of the three fundamental quantities, Length, Mass, Time. When we wish to particularise, we denote these fundamental magnitudes by the letters L, M, and T respectively. Any magnitude which is not simply a length or a mass or a time is derived from them. Thus an area is a length multiplied by a length. If we wish to express this fact we say that the Dimensions of an Area = L × L = L^{2}. Similarly, a velocity is a length divided by a time. Its dimensions are L/T. An acceleration is the rate of increase of a velocity. A stone falling to the earth has an acceleration, since it is moving faster and faster. An acceleration is a velocity divided by a time, and therefore its dimensions are L/T^{2}. A momentum is a mass multiplied by a velocity. Its dimensions are therefore ML/T. These examples suffice to illustrate the general idea.

Metric System.—Throughout all civilised countries scientific men use the metric system of units. The fundamental units are: for length, 1 centimetre; for mass, 1 gramme; for time, 1 second. This is called the centimetre-gramme-second system or, as it is usually written, the C.G.S. system. For those not used to measuring quantities in centimetres and grammes, it may be useful to see how they compare with English units. A centimetre is about 0·39 of an inch; a gramme is about 0·035 of an ounce. The unit of velocity, on this system, is one centimetre per second. The unit of momentum would be one gramme moving with a velocity of one centimetre per second. A very useful notion in science is the notion of force. This term has a perfectly precise meaning. The force acting on a mass is measured by the velocity imparted to that mass in the unit of time. On the C.G.S. system, unit force is that force which gives to a mass of 1 gramme a velocity of 1 centimetre per second in a second. This unit of force is called a dyne.

In the metric system, a very convenient system of prefixes is used which play the part of multipliers or dividers. Thus the prefix “mega” in front of some unit, such as a dyne or a gramme, means a million dynes or grammes. The prefix “milli,” on the other hand, divides the unit by a thousand. Thus a milligramme is a thousandth of a gramme. We append a table of these prefixes.

mega is equivalent to multiplying the unit by 1,000,000 myria " " " " " " 10,000 kilo " " " " " " 1,000 hecto " " " " " " 100 deka " " " " " " 10 deci " " dividing " " " 10 centi " " " " " " 100 milli " " " " " " 1,000 micro " " " " " " 1,000,000

A centimetre, therefore, as its name denotes, is the hundredth part of a metre. A kilogramme is a thousand grammes. And so on.

Electrostatic and Electromagnetic Units.—The reader of this book will notice that quantities of electricity are sometimes expressed in what are called electrostatic units and sometimes in electromagnetic units. Both systems of units are constantly employed in physics, and they exist because there are two radically different ways of measuring electric magnitudes. The reader probably knows that there are two kinds of electricity, positive and negative. Two electric charges of the same kind repel one another; if of unlike minds, they attract one another. It is on this property of attraction or repulsion that the electrostatic system is based. The electrostatic definition of a unit quantity of electricity is as follows: The unit quantity of electricity is that which, when concentrated at a point at unit distance in air from an equal and similar quantity, is repelled with unit force. On the C.G.S. system the unit distance is one centimetre and the unit force one dyne. The unit magnetic charge, or magnetic pole, as it is called, is defined in a similar way.

The electromagnetic system, on the other hand, starts with the dynamic, not the static, properties of electricity. A wire conveying an electric current produces a magnetic field. The lines of magnetic force exist as circles round the wire. If we imagine the wire itself to form a circle, we see that there will be a certain magnetic force at the centre of this circle. This leads us to the electromagnetic definition of the unit quantity of electricity. We proceed in two steps. We first define the unit current as such that, if it is flowing in a circular arc 1 centimetre in length where the circular arc forms part of a circle having a radius of 1 centimetre, then it will exert a force of 1 dyne on a unit magnet pole placed at the centre of the circle. This defines the unit current. We get to the unit quantity of electricity by saying that it is the quantity conveyed by the unit current in 1 second. The electromagnetic unit of quantity is enormously greater than the electrostatic unit. It is thirty thousand million times bigger. The ratio of the electromagnetic to the electrostatic unit is, in fact, equal to the velocity of light. This is no mere meaningless coincidence. Maxwell showed that this ratio gave the velocity of propagation of electromagnetic waves, and this velocity is precisely the velocity of light. This was one of the chief points confirming the theory that light itself is an electromagnetic phenomenon.

Large and Small Numbers.—Physicists often have to express very large and very small quantities, and to that end they have adopted a useful and simple convention. A large number like ten million is not written 10,000,000 but as 10^{7}. The figure 7 shows how many 0’s are to be written after the 1. If the number had been thirty million it would have been written 3 × 10^{7}. Thus 100 = 10^{2}; 1,000 = 10^{3}; 10,000 = 10^{4}; and so on. Such numbers as 10^{24} or 3·5 × 10^{24} would be tedious to write out, and they are of frequent occurrence.

Similarly, very small numbers are expressed much more conveniently in this way. But the minus sign is prefixed to the figure above the 10. Thus one millionth is 10^{-6}. One million millionth is 10^{-12}. Seven one hundred-thousandths is 7 × 10^{-5}. Thus 1/10 = 10^{-1}; 1/100 = 10^{-2}; 1/1000 = 10^{-3}; 1/10000 = 10^{-4}; and so on. So that when we say that the weight of a hydrogen atom is 1·65 × 10^{-24} gramme, the quantity we express in this way is

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