6.3 Valence-shell electron-pair repulsion theory The theory of molecular shape that we have been working towards is called valence-shell electron-pair repulsion theory (VSEPR theory). When applied to molecules and ions of the typical elements, its success rate is high. Here is a stepwise procedure that you can follow when applying this theory. It is illustrated with the molecule XeF4 and the ion C1O3−. Xenon tetrafluoride is one of the select band of noble gas compounds that were unknown before 1962
6.1 Introduction Structural formulae of, for example, hexan-1-ol (Structure 6.1) and PF5 (Structure 5.13) merely tell us the immediate neighbours of any particular atom. They are two-dimensional drawings, which ignore the three-dimensional shapes of the molecules. But in studying the structures obtained by X-ray crystallography in Sectio
3.4 Outer electronic configurations and the Periodic Table The essential message of Figure 22 is that the Groups of elements that appear in columns of the Periodic Table usually have atoms with similar outer electronic configurations. Figure 23 incorporates these configurations into our mini-Periodic Table of typical elements; they appear at the top of each Group. They imply that the typi
3.3 Electronic configurations and the Periodic Table
Figure 21 has been designed for use in a particular thought experiment. The purpose of the thought experiment is to see how the electronic configuration of the atoms changes as one moves through the Periodic Table from beginning to end. We start with the hydrogen atom, which has one proton and one electron. Then we
3.2 The electronic configurations of atoms The quantum theory of the atom tells us that we cannot say exactly where an electron in an atom will be at any particular moment; we can speak only of the probability of finding an electron at a particular point. So the precise orbits shown in the Rutherford model of Figure 1 misrepresent the arrangement of electrons about
4.4 Self-assessment questions and problems Find the distance between the numbers 2 − i and 1 + 3i. 1.5 Exercises A vector a has magnitude
|a| = 7 and direction
θ = −70°.
Calculate the component form of a, giving the components
correct to two decimal places. 1.3.5 Journals Journals and articles written by academics or experts are an excellent source of information. Journals are usually published monthly or quarterly, and contain a selection of articles providing details of recent research. Often they will also contain reviews of relevant books. They are usually published more quickly than books, and so are often more up to date. To access content of journals, most publishers require a subscription. There are, however, some journals which you can freely ac 6.3 The role of active citizens and communities Few people agree that individuals should take the main responsibility for tackling environmental issues. For example, in a 2007 poll of over 2000 UK citizens, 70% agreed that the government should take a lead in combating climate change, even if it means using the law to change people's behaviour. However, over 60% disagreed that there was nothing they could do to avert climate change and over half agreed that they would do more if others did more too, although 40% thought that recycling was 1.1.1 Carbon dioxide and other greenhouse gases Carbon dioxide (CO2) is used as the basis for the carbon footprint because it is by far the main contributor to the enhanced greenhouse effect from human activity (mainly burning fossil fuels, clearing forests and making cement). So, often only CO2 is counted in the carbon footprint. However, for a more complete measure of the carbon footprint the other human-generated greenhouse gases are converted into a CO2 equivalent (in kilograms or tonnes CO2e 1.1 What is the carbon footprint and why is it important? The carbon footprint is the annual amount of greenhouse gas emissions, mainly carbon dioxide, that result from the activities of an individual or a group of people, especially their use of energy and transport and consumption of goods and services. It's measured as the mass, in kilograms or tonnes per year, either of carbon dioxide (CO2) emissions alone, or of the carbon dioxide equivalent (CO2e) effect of other greenhouse gas emissions. 2.6.2 The role of modelling studies State-of-the-art models are designed to simulate the workings of the climate system (in so far as this is currently understood), and include the ‘internal’ interactions that generate short-term natural variability in the real world. They provide modellers with a means of carrying out ‘virtual’ experiments on the climate system. In the present context, an important aim of these experiments is to identify the ‘signal’ of a human influence on climate, so studies typically involve ‘ 2.6.1 Weighing up the evidence: the full cast of suspects Figure 36 (again adapted from the TAR) takes your thoughts on Question 11 on a stage. It gives estimates of the cumulative effect since pre-industrial times of the various climate change agents, with the contributions expressed in terms of radiative forcing. Note that the figure also includes yet another device for communicating the IPCC's confidence in a particular finding – an indication of the ‘level of scientific understanding’ that accompanies each estimate. This reflects the autho 2.5.2 Environmental indicators The notion of a link between climatic conditions and the behaviour of plants and animals (e.g. the growth of trees or coral) and the composition of natural communities or ecosystems (the type of vegetation in a given area, say) is fundamental to the use of proxy data to reconstruct past climates. Some examples of biological responses to recent climate change were included in Box 9. Here we should be wary of jumping to conclusions. Such changes involve complex living systems that can respond i 2.5.1 Physical and weather-related indicators The indicators collected in Table 4 have been observed to change over large regions of the Earth during the 20th century. According to the TAR, there is now a good level of confidence that what is being recorded is the result of long-term change rather than short-term natural fluctuations. As we noted earlier (Section 2.2.2), the most recent period of warming has been almost global in extent, but particularly marked at high latitudes. So are the changes in Table 4 consistent with rising tempe References 1.2.8 In praise of cheap offshore labour? continued There are two points which are central to this line of thinking. One, according to Wolf (2004), is that the whole process, as odd as it may sound, is about mutual exploitation. Outside firms do indeed exploit the poor by taking advantage of the profitable opportunities that a pool of cheap labour represents. But Indonesian or Chinese workers, for instance, could be said to exploit the incoming firms by extracting higher pay from them and taking advantage of opportunities that previousl 1.1.1
Introduction Many of the smaller branded goods on sale to consumers in Europe and North America – the latest in clothing and footwear or the smart toys and electronic gadgets on offer – are made in factory ‘sweatshops’. Found in the backstreets of modern, Western cities, but more often than not a feature of the poorer parts of the world, factory sweatshops are an integral part of today's global economy. Increasingly, as you can see from Author(s): 6.6.2 The World Energy Council scenarios What are the possibilities for radical changes in our energy systems when viewed from a world perspective? There have been numerous studies of the various future options for the world's energy systems. One of the most recent and most comprehensive was produced in 1998 by the International Institute for Applied Systems Analysis (IIASA) and the World Energy Council (WEC), a version of which was published in 2000 as part of the United Nations’ World Energy Assessment (United Nations Dev 6.6.1 The Royal Commission on environmental pollution scenarios The UK's Royal Commission on Environmental Pollution produced (RCEP) its 22nd report Energy: the Changing Climate in June 2000. The Commission examined what changes would be needed in Britain's energy systems if, as suggested by the various reports of the Intergovernmental Panel on Climate Change (IPCC, 2001), it should prove necessary to reduce the country's emissions of greenhouse gases by about 60 per cent by 2050. The Commission investigated the various possibilities very tho
SAQ 25
Answer
Exercise 1
