The Chemistry of the Metallic Elements

144 Pages·1966·3.479 MB·English

by  David J. Steele and J. E. Spice (Auth.)

The Chemistry
of the Metallic Elements
BY
DAVID STEELE, B.Sc, A.R.C.S.
Chemistry Master, Winchester College
PERGAMON PRESS
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Copyright © 1966 Pergamon Press Ltd.
First edition 1966
Library of Congress Catalog Card No. 66-18401
Printed in Great Britain by Blackie and Son Ltd., Bishopbriggs, Glasgow
This book is sold subject to the condition
that it shall not, by way of trade, be lent,
resold, hired out, or otherwise disposed
of without the publisher's consent
in any form of binding or cover
other than that in which
it is published.
(2824/66)
To Ambrose George King
General Introduction
THE volumes in this division have been planned to provide a comprehensive treatment
of chemistry at the intermediate level, that is, the sixth-form/first-year university
level. Readers are assumed to have a background of O-level chemistry and of O-
or A-level physics and a working knowledge of mathematics.
The books of the division will meet all the requirements of the recently revised
A-level syllabuses of the examining boards and an attempt has been made to
anticipate the nature of future revisions of these syllabuses They will also cover the
ground for university scholarship and for first-year university examinations, such as
those set to intermediate science, medical, and engineering students, etc. They will
provide Ordinary National Certificate students in technical colleges with all they
need and will constitute a useful background and companion to the studies of Higher
National Certificate candidates. In the U.S.A., first and second-year college students
will find them directly relevant to their studies and they will be of value to high-school
students for reference purposes.
The present book, together with that on the Non-Metallic Elements, covers the
usual topics of inorganic chemistry. Both books, while complete in themselves,
presume a knowledge of general chemical theory, particularly of structure and
bonding, and the volumes on Atomic and Nuclear Chemistry and on Chemical Binding
and Structure have been written to provide just this necessary material.
ix
Preface
IN the development of man's material well-being an improved understanding of
metals has played a large part, so large indeed that the stages in this development are
conveniently called Bronze Age, Iron Age and Steel Age. Both the elements and
their compounds are important, but in this book we shall be largely concerned with
the compounds; a detailed account of the properties and uses of the metals belongs
more properly to metallurgy than to chemistry.
When metals enter into chemical combination there are formed many diverse
and useful compounds whose detailed description is a very large undertaking. This
book does not pretend to describe such a large amount of information: it is not a
chemical encyclopedia. Rather is emphasis laid on structural principles from a
knowledge of which the properties of particular substances may be reliably deduced.
Facts concerning the more common metals and their compounds are collected into
tables wherever possible. This is to avoid the fault common in many current text-
books wherein the system underlying chemical facts is obscured by the over-detailed
description of the facts themselves.
Perhaps an outline of the structure of this book will be helpful.
The Text is as concise as possible. The first three chapters are of a general nature;
parts of Chapter 1 may be difficult, but it is felt that some account of the physical
properties of metals in terms of atomic structure is not out of place here. Chapters 2
and 3 are important and in many ways form a summary of what follows. In subse-
quent chapters are described the properties of the elements and their compounds in
relation to their structure, and their position in the Periodic Table and in the Electro-
chemical Series. Emphasis is laid on the comparison of groups of elements.
The Tables. The purely descriptive chemistry of the elements and compounds is
separate from the text and is presented in tables. This is done to facilitate the
memorising of the many chemical facts, so troublesome yet, at present, so necessary
for success in G.C.E. and University examinations. It is also hoped that the gathering
together of the properties of substances in this way will make the book useful for
xi
Xll Preface
reference; not all the facts in the tables are to be memorised. It must be emphasised
that the tables are in no sense appendixes but an integral part of the book and must
be used along with the text.
The Questions. The aim of the questions is threefold: to ensure a proper under-
standing of the text, to provide practice in the style of questions set by the various
examining boards, and, most important, to encourage the pupils to think for them-
selves and to search for information outside this text.
Winchester College,
December 1962
Acknowledgements
THE author wishes to thank the following Examination Boards for permission to
publish questions set in their papers:
The University Press, Cambridge (C.S.),
The Secretary, Natural Science Scholarships, Oxford (O.S.),
Oxford and Cambridge Schools Examination Board (O. & C),
The Senate of London University (Lond.),
Northern Universities Joint Matriculation Board (N.U.J.M.B.).
The symbols in parentheses are those used in the text to denote the sources of the
various questions. Some indication is given whether the questions are Advanced
level (A) or Scholarship level (S).
The information and diagram on the zinc blast furnace were kindly provided by
Imperial Smelting Processes Ltd. of Avonmouth.
The author would like to thank Dr. J. E. Spice for reading the manuscript and
for his helpful advice.
D. M. McC. S.
xiii
CHAPTER 1
The Metals: their Physics
THE atoms of elements called metals have up to three electrons in excess of a stable
electronic arrangement, generally that of an inert gas. In chemical combination such
atoms attain stability by giving up these electrons and forming positive ions iso-
electronic (i.e. having a similar electronic structure) with stable rare gas con-
figurations. In later chapters a comprehensive discussion of the metals in chemical
combination will be our chief concern. Here are considered the metals themselves.
In the metallic state it is to be expected that the atoms attain stability in a way
similar to that mentioned above, and such is the case. Clearly, however, the mech-
anism must be different from that in chemical bonds, for there are no "acceptors" to
take up the liberated electrons, which must exist therefore in a free state. These
electrons are not completely free, but move in the space between, and in the field
created by, the three-dimensional lattice of positive ions. Their movement is subject
to special laws, different from those governing electrons in ordinary chemical bonds.
A brief discussion of these laws will be given later; a complete description lies without
the scope of this book. Such a system of electrons is called the electron gas, and this
theory of positive ions and an electron gas, shown diagrammatically in Fig. 1.1,
satisfactorily accounts for most of the characteristic properties of the metallic state:
opacity, lustre, malleability, ductility, and high thermal and electrical conductivity.
Mobile electrons in the surface of a metal tend to annul the electric field associated
with electromagnetic waves falling on the surface: they constitute a node. If the
electrons are sufficiently mobile, almost all the energy will be re-emitted as reflected
waves and very little will be transmitted through the metal; hence the high reflecting
power (lustre) of metallic surfaces and their opacity, even in thin layers. It is inter-
esting to note that if the incident radiation is of high frequency, the electrons will be
insufficiently mobile to reflect the incident waves and they will be transmitted; hence
the transparency of the metals to radiation of short wavelength such as X-rays.
Layers of independent, positively charged, spherical ions, at once held together
and "lubricated" by a "fluid" of mobile electrons, will easily move one over the
1
2 The Chemistry of the Metallic Elements
other. This explains the malleability and ductility of the metals. It will be appreciated
that as the layers slide over each other, no bonds are broken since the ions are
bonded to the electron gas and not to each other. Slip occurs easily because the ions
are in what is called a "close-packed" formation; that is, they are arranged in layers
mi Electron gas
FIG. 1.1 The electron gas structure in metals
giving smooth planes along which slip can occur. Several actual arrangements, all
close-packed, are possible—body-centred cubic, face-centred cubic and hexagonal
are the three most common. For definitions of these terms reference should be made
to the volume in this series on Chemical Binding and Structure* It should be men-
tioned that if there was perfect close-packing and the layers of ions were completely
free from faults, all metals would be very malleable; indeed, it is thought that when a
piece of metal is completely free from faults, the elastic limit is zero. This effect is
never observed in practice for dislocations occur.
When the layers slip over one another, not all the ions move at once, but the
movement begins in a small area and spreads throughout the metal. The effect is
shown in Fig. 1.2. The ions to the right of A have slipped; those to the left of D
Direction of 'slip'
6§mssb
FIG. 1.2. A dislocation diagram.
have not. C and B are slipping, causing a temporary distortion of the lattice as they
move over the lower layer. The distortion, which in the bulk of the material consti-
tutes a dislocation, in effect moves to the left in the diagram. In a piece of metal
"waves" of dislocations spread throughout the material from the points where slip
begins. In the simple dislocation diagram it will be seen that there is a force acting
* J. E. SPICE, Chemical Binding and Structure, Pergamon Press, 1964.
The Metals: their Physics 3
on the ion C tending to keep it in the space between W and X and prevent distortion
of the lattice. This means that there is a resistance to the slipping of the upper layer
and the material as a result will be less malleable. There is also a similar force in the
opposite direction tending to draw B into the space between Y and Z. It is possible
for C to go to W-X and B to Y-Z and a permanent gap or fault develops. This leads
to increased brittleness of the metal. The formation of dislocations is affected to a
certain extent by alloying and heat treatment, and a proper understanding of this
aspect of metallic structure has given the metallurgist a certain control over his
materials.
A system of mobile electrons accounts for the conduction of both heat and
electricity. Thermal energy absorbed in one part of the metal increases the kinetic
energy of the electrons and, since they are free to move through the metal, this
energy is conducted through the metal. If an electrical potential is applied across a
metal then the electrons, having negative charge, move under the potential: this
constitutes a current flowing through the metal.
So far this theory of the electron gas has provided an adequate explanation of the
behaviour of metals, but the discussion has been only qualitative. When the pro-
perties of metals, particularly their conductivity, are studied quantitatively, then this
simple theory must be modified. A quantitative account of the structure of metals is
without the scope of this book, so also is a rigorous mathematical treatment of the
explanatory theory; what follows is merely an outline and those who wish for a more
substantial treatment are referred to the appropriate chapters in works on Quantum
and Wave Mechanics.
An important law of the metallic state is the Wiedemann-Franz law which shows
the relation between electrical conductivity (c), thermal conductivity (t), and absolute
temperature (T). It is:
c.T
where k is a constant for all metals. The electron gas theory leads to an equation of
this form but the value of k is 30 per cent too small, and it is only from a more
sophisticated quantum mechanical approach that a value for k agreeing with the
experimental value is obtained. Another discrepancy is in the value for the atomic
heat of a metal. From the simple electron gas theory and assuming that the electrons
behave as a monatomic gas, the calculated value for the atomic heat is 9 cal. The
atomic heat for most metals at ordinary temperatures is about 6 cal (Dulong and
Petit's law). Calculation of this quantity using classical thermodynamics and
ignoring the electron gas gives a value of 5-96 cal.
From these two important examples it is apparent that the original simple electron
gas theory is inadequate, and a more precise idea of the state of these electrons is desir-
able. In 1926 Fermi suggested that the Pauli Exclusion Principle applies as well to this
2 CME