Table Of ContentInorganic
Chemistry
A CONCISE TEXT
j. BASSETT
M.Sc, F.R.I.G.
Senior Lecturer in Inorganic Chemistry
The Woolwich Polytechnic
PERGAMON PRESS
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Copyright © 1965 Pergamon Press Ltd.
First edition 1965
Reprinted (with corrections) 1966
Library of Congress Catalog Card No. 65-21903
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Preface
THE present volume is intended to provide a concise text-book of
inorganic chemistry at a standard intermediate between that
required for Advanced Level in schools and honours degree
courses. The increasing number of students following courses in
chemistry at about this level (e.g. General Degree and Higher
National Certificate) brings with it the need for suitable text-books
and it is with this need in mind that the present book has been
written.
The book is divided into two parts. The aim of the first part is to
provide the reader with a background of basic principles sufficient
to promote a rational understanding of the chemistry of the ele-
ments; this includes simple ionic crystal structures and the shapes of
molecules. Part I of the book is concluded with a chapter des-
cribing the general methods of extraction and purification of
metals. In the second part of the book the chemistry of the elements
is described, mainly in a comparative way, under the appropriate
periodic sub-groups. Transition metal chemistry has, however,
been limited to the first row of transition metals, the presentation
being such as to emphasize the general characteristics of these
elements.
The objects of Part II of the book are twofold. First, to present a
reasonable selection of the more important properties of the ele-
ments and their compounds; every effort has been made to include
up-to-date factual material, for example recent developments in
the chemistry of the noble gases are described in the final chapter
of the book. Secondly, wherever possible, to interpret and explain
viii PREFACE
the descriptive chemistry in the light of modern physical concepts.
In this way, it is hoped, the reader will not only acquire a useful
factual basis of the subject but will also develop an appreciation of
the rational nature of modern inorganic chemistry.
The present book is to some extent based upon a course of lec-
tures given by the author to undergraduate students, but, in addi-
tion, a wide range of the literature of inorganic chemistry, to which
the author is necessarily indebted, has been consulted in the pre-
paration of the text. Direct references to original literature have not,
however, been included since the book is not intended to be a work
of reference.
The author wishes to record his sincere appreciation of the
encouragement given by Dr. Α. I. Vogel, and to thank some of his
colleagues for useful discussions regarding the content of the book.
Particular thanks are due to Dr. C. W. N. Cumper, who read the
preliminary drafts of Chapters II and III and made a number of
helpful suggestions. Finally, it is a pleasure to record my apprecia-
tion of the assistance given by my wife in the preparation of the
manuscript.
Ώ τ
J.b.
Woolwich Polytechnic
June, 1964.
CHAPTER I
The Structure of the Atom
The Electron
Although the name electron had been proposed by G. J. Stoney
in 1891 for the elementary unit of electric charge, such a unit being
suggested by Faraday's Laws of Electrolysis, the idea that electrons
are common constituents of all atoms arose from a study of the con-
duction of electricity through gases.
When a potential difference is applied to a gas, such as air, at
atmospheric pressure, the gas behaves as an insulator practically
no current passing through the gas. At reduced pressures, however,
electrical conduction occurs giving rise to a number of visible
effects. Thus, when a high-tension discharge is passed through a
discharge tube containing a gas at very low pressure (about 0-01
mm of mercury) the glass wall of the discharge tube opposite to the
cathode fluoresces. This fluorescence is caused by rays originating
at the cathode and moving in a direction normal to the surface of
the cathode as shown in Fig. 1.1.
Cathode
Fluorescence
Anode +
FIG. 1.1. Cathode rays.
4 THE STRUCTURE OF THE ATOM
Cathode rays are found to possess the following characteristics :
(a) They travel in straight lines from the cathode. This is shown
by the well defined shadows cast on the wall of the discharge
tube when an opaque object is placed between the cathode
and the end of the tube.
(b) The rays produce strong heating effects when they impinge
on objects in their path.
(c) A hollow metal box placed in the path of the rays acquires a
negative charge; this may be shown by connecting the metal
box to an electrometer.
(d) The rays suffer deflection in applied electrostatic or mag-
netic fields, the direction of their deflection being that
expected for negatively charged particles.
(e) They yield penetrating X-rays when directed against
suitable targets.
(f ) They cause ionization in gases.
Cathode rays thus consist of a stream of negatively charged
particles moving from the cathode with very high velocity. J. J.
Thomson in 1897 carried out experiments to determine the velocity
ν and charge to mass ratio (e/m) of cathode-ray particles. The
apparatus used, shown in Fig. 1.2, allowed rays from the cathode C
of an evacuated discharge tube to pass through a hole in the anode
A. The slit S permitted a narrow beam of the rays to pass between
the plates EE. The latter were connected to a source of high voltage
so that the cathode rays could be subjected to the action of an
electric field, and a magnetic field M was applied by means of a
magnet outside the tube. The position of the beam was shown by
the luminous spot which it produced on striking the wall of the
tube furthest from the cathode.
M 3
IF
FIG. 1.2. Thomson's apparatus for the study of cathode rays.
THE STRUCTURE OF THE ATOM 5
The force acting on a particle of charge e in an electric field of
strength Ε is Ee. A similar particle moving with velocity υ in a
magnetic field of strength Η applied at right angles to the particle
direction, is subject to a force Hev. Moreover, under the influence of
the magnetic field only, the particle moves in a circular path of
radius r in which the magnetic field just balances the centrifugal
force
2
Hev = mvlr (1)
Hence e/m = vjHr (2)
If the electric and magnetic fields are arranged so that they
deflect the particle in opposing directions, and the field strengths
are then adjusted to give no resultant deflection of the particle
from its path, it follows that
Hev = Ee (3)
and hence ν = EjH (4)
The velocity ν of cathode-ray particles was thus determined from
the relative strengths of the exactly compensating electric and
magnetic fields. The radius of curvature r of the circular path of
the particles under the influence of the magnetic field only, could be
calculated from the observed deflection on the scale F. This
enabled e/m to be found by equation (2).
Thomson found that cathode-ray particles moved with the
extremely high velocity of 3 χ 109 cm/sec (about one-tenth of
the velocity of light). It was also found that the ratio e/m for these
particles, which according to more recent determinations is
1·758χ 108 coulombs/g, was the same regardless of the nature of
the cathode or of the residual gas in the discharge tube. It thus
became clear that the cathode-ray particles or electrons are
ultimate constituents of all matter. Assuming that the electron and
the hydrogen ion, H+, carry a charge of the same magnitude,
their relative masses may be calculated as follows :
e Ne 96500
coulombs/g
m Nm 1-008
H n
6 THE STRUCTURE OF THE ATOM
where e is the charge and m the mass of the hydrogen ion, Ν being
H
the Avogadro number (the number of atoms in a gram-atom of an
element). Using the value already given for the ejm ratio of the
electron (1·758χ 108 coulombs/g), we have:
e _ 96500 (1-758 xlO8)
• ' m ~ 1-008
Hence m/m = 1/1836
H
The mass of the electron is thus 1 /1836 part of the mass of a hydro-
gen atom.
To sum up, the electron is a common constituent of all atoms,
carries unit negative charge and has a negligible mass compared
with the mass of the lightest atom, the hydrogen atom.
Alpha Particle Scattering
The modern concept of the nuclear atom arose directly from a
study of the behaviour of alpha particles emitted by a radioactive
source. Rutherford (1906) observed that when a narrow beam of
such particles fell on a photographic plate after passing through a
very thin metal foil, the resulting photographic trace faded off at
the edges, instead of being sharp. This diffuseness suggested that
the alpha particles were deflected from their path by some inter-
action with the atoms present in the thin metal foil. Further
studies showed that the great majority of the incident alpha
particles were undeflected by passage through the metal foil, but
A.
Θ
Beam
7
of
alpha
particles
Θ
Θ
— Electron
FIG. 1.3. Scattering of alpha particles by an atomic nucleus.
THE STRUCTURE OF THE ATOM 7
that a small fraction, about one particle in 8000, were deflected
through an angle of 90°, or even more (Fig. 1.3).
In order to explain these observations Rutherford suggested
that the atom consisted of a very small, positively charged, central
nucleus surrounded at a relatively large distance by electrons.
The fast-moving alpha particles will not be deflected to any
appreciable extent by an electron, which has a very small mass,
and the majority of the particles thus pass through the atom with-
out being deflected to any significant extent. The scattering through
large angles is seen to be due to single encounters between the posi-
tively charged alpha particle and the positively charged atomic
nucleus. The minute size of the latter compared with the atom as a
whole results in such encounters being comparatively rare.
The relatively "empty" nature of the atom is indicated by the
fact that if nuclei could be closely packed together, the resulting
material would have the enormous density of about 10 12 g/cm3.
Such densities are only realized under the conditions of extremely
high pressure and temperature prevailing at the centres of stars.
Characteristic X-Rays
X-Rays, discovered by Röntgen in 1895, are emitted when fast-
moving cathode rays, produced in a highly evacuated discharge
tube, are allowed to impinge upon a metal target called the anti-
cathode (Fig. 1.4). X-Rays have been found to be electromagnetic
radiations of short wavelength; they have high penetrating power
and are not deflected by magnetic or electric fields.
FIG. 1.4. X-Ray tube.
8 THE STRUCTURE OF THE ATOM
If the anti-cathode of a discharge tube is made of, or contains, a
particular element, then X-rays with wavelengths characteristic
of that element are emitted. These characteristic X-rays fall into
several series, known as the K, L, M, JV, etc., series, in order of
decreasing penetrating power. In 1913 H. G.J. Moseley studied the
characteristic X-ray spectra of a number of consecutive elements in
the periodic system. The spectrum of each element was recorded
photographically, so that the positions of the lines produced on the
photographic plate gave the wavelengths of the particular X-rays.
A comparison of the position of a specific line in the X-ray spectra
of the elements showed that this line was displaced regularly
towards shorter wavelengths as the atomic weight of the element
increased. Moseley also found that the atomic number (Z) °f an
element was related to the frequency ν of a specific line in the X-ray
spectrum by the equation
Vv = a{Z-b) (5)
where a and b are constants (the atomic number was initially
taken as the number which gives the order of the element in the
periodic system, i.e. H=l, He = 2, Li = 3, etc.). Thus a plot of
the square root of the frequency of a given line in the X-ray spec-
trum against the atomic numbers of the corresponding elements
gives a straight line (Fig. 1.5).
d
io 20 3(5 40 50 6(5 70 80" 90
Atomic number
FIG. 1.5. Relation of frequency of characteristic X-rays and atomic
number.
