Atoms and atomic structure

The atom

For many years it was speculated that the ultimate state of subdivision of matter was discrete indivisible particles termed atoms. In the early nineteenth century John Dalton gave quantitative support to these ideas by assigning a self-consistent set of masses to atoms of elements. Elements are substances which cannot be decomposed by ordinary types of chemical change, or made by chemical union. However, subsequent work by Faraday, Crookes and Goldstein pointed out the electrical nature of atoms and that they are divisible. The simplest picture of an atom now shows a central nucleus surrounded by a cloud of electrons.

Electrons

When a neutral atom is supplied with sufficient energy it will ionise yielding positively and negatively charged fragments. The former, the positive ion, is characteristic of the particular parent atom, whilst the negative fragment, the electron, is the same irrespective of the parent atom. The charge and mass of electrons can be determined by studying their behaviour in electrical and magnetic fields. Early experiments revealed the ratio of charge to mass, e/m, which for all electrons is 1.7588 × 108 coulombs per gram (Cg−1).

To evaluate the mass of an electron, experiments were carried out to determine the applied potential required to prevent oil droplets carrying negative charges from settling due to gravity.

Natural abundance of the elements

Our knowledge about the elemental composition of the earth is restricted to the composition of the crust, atmosphere and oceans. Although the mantle and core represent over 99% of the earth's mass, their elemental compositions are not accurately known. Hence natural abundances of elements quoted in Table 3.1 refer only to the crust. Since the crust (35 km average thickness under continents and 10km under oceans) is not homogeneous and different parts of the earth contain different minerals, these values are averages of a large number of estimations. The relative homogeneity of ocean waters (Tables 6.3 and 6.4) and the atmosphere (Table 3.2) renders their compositions far less variable for major components.

Oxygen is the major element in the crust, with silicon coming second in abundance. Formation processes may have enriched the crust with certain elements in comparison with the mantle and core. The water and air masses are very different in composition from the crust, and, although elemental abundances may vary slightly with water depth or atmospheric height, the overall relative proportions of the major elements remain almost constant in the oceans and in the lower and middle atmosphere.

Although many problems in environmental chemistry are related to the abundance of elements in the environment, the chemical behaviour and properties of elements are largely independent of their abundances.

All four authors of this book are involved in teaching and/or research in environmental science and ecology. In the course of this work, we have found no shortage of advanced books in specialised aspects of our subject, and the availability of teaching texts on specific aspects of environmental chemistry has improved markedly in recent years. There does, however, in our view continue to be a need for a basic book covering the chemistry necessary to comprehend more specialised books on chemical aspects of environmental science and ecology. This book is intended to fulfil that need.

In preparing the book, we have been struck by the enormous range of aspects of chemistry which are involved in studying the environment; all major branches of the subject are involved to some degree. This has made our task more difficult and explains the involvement of such a large band of authors, needed to give expert coverage to all the topics included. Although specific individuals have prepared first drafts of whole or part chapters, these have been reviewed by all other authors, and the finished work is a group effort.

The book is not aimed at specialist chemists, although we hope that they may find some of the more applied material useful. Rather, it is aimed at students specialising in environmental science or ecology, and requiring a grounding in basic chemical concepts to make chemical aspects of their studies accessible.

General principles of analytical chemistry

Introduction

Analytical chemistry encompasses the science of characterising a sample. Two broad categories can be considered. Qualitative analysis deals with the question of what elements or compounds are present. In the simplest case, this may involve only sample identification. For instance, is an unknown water sample fresh or saline? Several screening tests may rely on determining whether or not a particular substance is present or absent. This has applications in quality control during food processing where the analysis might serve to distinguish foreign bodies. Urine samples from athletes and racehorses are routinely examined for the presence of prohibited drugs. Qualitative analyses may also be used to trace the source of a sample. Pollen analysis of honey indicates the country of origin. Comparing the ‘fingerprint’ of substances in an oil slick with that in the bilge tanks of ships may enable the responsible vessel to be identified. Quantitative analysis involves determining how much of an element or molecule is present. Several types of quantitative analyses can be contemplated. These are exemplified below using the analysis of petrol (gasoline). A complete analysis would quantify the amount of each constituent in the sample. This would involve the determination of hexane, octane, toluene, tetraethyllead, etc. Such analyses are generally impossible and for environmental samples usually unnecessary. For an ultimate analysis the amount of each element is quantified.

Environmental issues continue to be raised in the public arena. Sometimes these are not presented accurately, but only in a way designed to arouse the public and stimulate reaction. Chemistry and chemicals often comprise the target for public concern and disquiet. The only sure way to lessen the ability of pressure groups to manipulate public opinion is by educating a greater proportion of the population in environmental science. Our aims with the first edition of this book were to ensure that the fundamental principles of chemistry could be presented in an honest and straight-forward way to a wide, non-specialist audience. Hence, the text was aimed originally at undergraduate science students who were not majors in chemistry. The success of the book and feedback from users indicate that it has found a wider market than anticipated, which has included chemists requiring an introduction to the environment.

We are pleased to be able to present the second edition. We have taken the opportunity to broaden the scope of the book to satisfy this extended audience. A new section introduces the concepts of biogeochemical cycling, considering both the speciation of elements in environmental reservoirs and the exchange of material between these pools. Also included is a brief description of the role of environmental chemistry with respect to climate, dealing notably with global warming and ozone depletion.

Introduction to biogeochemical cycles

Much of the discussion of environmental processes in Chapter 6 will deal with single environmental reservoirs, such as the atmosphere, or the oceans. Many important processes involving chemicals, some of which have given rise to significant environmental problems, have involved transfer of chemicals from one reservoir or compartment to another. The transfer of chemical elements and compounds between compartments, taking into account the important influences of the biota, is termed biogeochemical cycling. Much work has been carried out to elucidate the biogeochemical cycles of the elements, but much remains to be done to describe the cycles in adequate detail.

The major transfers taking place in a biogeochemical cycle appear in Figure 5.1. An element is washed from rock and soil and enters fresh water streams and lakes in runoff, and to a lesser extent, in water flowing from ground waters into surface waters. The major part of that entering the fresh waters is carried by streamflow into the oceans. Direct coastal discharges of pollutants can also be important, especially in fully and partially enclosed seas like the North Sea. Volatile elements (e.g. mercury) and compounds are degassed from land surfaces and the sea. Less volatile materials enter the atmosphere from the land surface in wind-blown dusts (and man-made emissions) and from the sea surface in sea spray, the smaller particles of which form marine aerosol.