The molecular world

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	Time: 20 hours Level: Intermediate

	Introduction Introduction Resource This unit will provide you with a detailed understanding of some of the important problems and topics that are being studied by the chemists of today, and of the ways in which associated problems might...

	1 Everything that you can see is made of atoms 1.1 Introduction Resource The idea that everything that we can see is an assembly of tiny particles called atoms is chemistry's greatest contribution to science. There are about 120 known kinds of atom, and each one is distinguished... 1.2 Chemical elements Resource Atoms of the same atomic number behave virtually identically in chemical reactions. They are therefore given the same chemical name and chemical symbol. For example, the atom of atomic number 6, which... 1.3 Chemical compounds Resource Chemical elements contain atoms of the same atomic number. But most materials consist of chemical compounds. These are a combination of the atoms of two or more chemical elements. Such combinations often... 1.4 Molecular substances Resource Chlorine, bromine and iodine belong to a family of elements called the halogens. At room temperature, chlorine (Figure 2e) is a gas, bromine (Figure 2d) is a liquid and iodine is a dark-purple solid. All... 1.5 Non-molecular substances Resource Non-molecular substances defy attempts to pick out discrete molecules from their structures. One example is common salt, NaCl, which is built up from the tiny cubes shown in Figure 10a. Look first at the... 1.6 Binding forces in molecular and non-molecular substances – a first look Resource As we shall see in Section 4, elementary bonding theories imply that materials as different as salt, iodine and aluminium are held together by different types of chemical bond. However, all binding forces... 1.7 Summary of Section 1 Resource All materials are made of atoms of about 120 different chemical elements, each element being characterised by an atomic number which lies in the range 1–120.

	2 Chemical patterns are to be found in the periodic table 2.1 Chemical periodicity Resource The chemistry of the elements is immensely varied. But amidst that variety there are patterns, and the best known and most useful is chemical periodicity: if the elements are laid out in order of atomic... 2.2 The Group number of the noble gases Resource In Figure 18, the Period numbers increase steadily from 1 to 7 down the columns. It obviously seems appropriate that the Group numbers should show a similar steady increase from I to VIII across the rows.... 2.3 Elements on parade: an audiovisual interlude Resource Here you have the opportunity of viewing seven video sequences which show both reactions and properties of some chemical elements. The seven sequences provide examples of the way in which Periodic Tables... 2.4 Summary of Section 2 Resource The typical elements can be displayed in a mini-Periodic Table of eight Groups and seven Periods (Figure 18). The Periods are numbered from 1 to 7 and the Groups are labelled I-VIII.

	3 Chemistry can often be explained by electronic structure 3.1 Introduction Resource Section 2 used some simple examples to illustrate chemical periodicity. But how can we explain such periodicity? The answer lies in the way that the electrons in atoms are arranged about the positively... 3.2 The electronic configurations of atoms Resource 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... 3.3 Electronic configurations and the Periodic Table Resource 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... 3.4 Outer electronic configurations and the Periodic Table Resource 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... 3.5 Electron states and box diagrams Resource So far, we have represented the electronic state of an atom as a collection of sub-shells. Now we turn to the states of the electrons within those sub-shells. Just as shells can be broken down into sub-shells,... 3.6 Summary of Section 3 Resource The electronic configuration of an atom can be obtained by allocating its electrons to s, p, d and f sub-shells in the order given by Figure 21. This procedure generates a periodicity in electronic configuration...

	4 Chemical bonds consist of shared pairs of electrons 4.1 Introduction Resource Simple theories of chemical bonding are based on the idea of the electron-pair bond, and the extent to which the electron pair is shared between the bound atoms. There is also an assumption that the electronic... 4.2 Ionic and covalent bonding Resource We begin by applying simple bonding theories to molecular chlorine gas (Cl2) and non-molecular sodium chloride (NaCl), whose structures were discussed in Section 1. Figure 28 shows the result. 4.4 Metallic bonding Resource Two familiar properties of metals point to a simple model of metallic bonding. Firstly, metals have a strong tendency to form positive ions. Thus, when sodium reacts with water, and when magnesium and... 4.4 A classification of chemical substances Resource We now have a provisional but useful classification of chemical substances. First they are divided into molecular and non-molecular types, largely on the basis of their structures. Then a further division... 4.5 More about covalent bonding Resource So far, the valencies in Table 1 have just been numbers that we use to predict the formulae of compounds. But in the case of covalent substances they can tell us more. In particular, they can tell us how... 4.6 Summary of Section 4 Resource The chemical formulae of many substances can be understood by arguing that their atoms attain noble gas structures by chemical combination.

	5 Molecular reactivity 5.1 Molecular reactivity is concentrated at key sites Resource Reactivity is not spread evenly over a molecule; it tends to be concentrated at particular sites. The consequences of this idea are apparent in the chemistry of many elements. However, in organic chemistry,... 5.2 Summary of Section 5 Resource The structural formulae of organic molecules can be divided into the carbon-hydrogen framework or skeleton, and the functional group(s). In the first approximation, the functional groups are the sites...

	6 Molecular shape affects molecular reactivity 6.1 Introduction Resource 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... 6.2 The shapes of some molecules Resource Here we shall look at the shapes of some simple molecules of the typical elements. In doing so, we shall meet the problem of representing three-dimensional shapes on two-dimensional paper. Let's use methane,... 6.3 Valence-shell electron-pair repulsion theory Resource 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... 6.4 Summary of Section 6 Resource Molecules have a three-dimensional shape. Bulky irregularities in the shape of a molecule around a reactive site can exclude a potential reactant. Such effects are described as steric.

	7 Reactivity needs a favourable rate and equilibrium constant 7.1 Introduction Resource So far, we have concentrated on the electronic and spatial structures of chemical substances, but we have not said much about chemical reactions. Now we turn to the question of why chemical reactions happen.... 7.2 Is the equilibrium position unfavourable? Resource The first possibility is that the reaction system has been able to reach chemical equilibrium, but the equilibrium position is not favourable. How does this come about? If equilibrium has been reached,... 7.3 Is the rate of reaction very slow? Resource If the equilibrium position is very favourable, then the reason why Reaction 8.1 fails to occur at 525 °C must be that its rate is very slow. Usually, a reasonable response would be to increase the temperature... 7.4 Equilibrium positions and rates of reaction in this unit Resource Section 7 showed that if a reaction is to occur at a particular temperature, two conditions must be fulfilled: its equilibrium constant must be sufficiently large, and its rate sufficiently great. We finish... 7.5 Summary of Section 7 Resource The equilibrium constant of a reaction is fixed at any particular temperature. It depends only on the natures of the initial reactants and the final products; what happens as reactants change into products...

	8 Reviewing and reflecting 8 Reviewing and reflecting Resource Figure 55 is a conceptual diagram that summarises this unit. Molecules are made of atoms, so it was with atoms, to the left of Figure 55, that we began. Early in Section 1 they acquired a structure...

	References and Acknowledgements Acknowledgements Resource

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Topic outline

Introduction

1 Everything that you can see is made of atoms

2 Chemical patterns are to be found in the periodic table

3 Chemistry can often be explained by electronic structure

4 Chemical bonds consist of shared pairs of electrons

5 Molecular reactivity

6 Molecular shape affects molecular reactivity

7 Reactivity needs a favourable rate and equilibrium constant

8 Reviewing and reflecting

References and Acknowledgements

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