2 Chapter 6 â¢ organising elements Organising elements

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Hydrogen 1 H He Forming ions Nature likes balance. We see this in food webs and in the action of forces, and the behaviour of electrons is no exception. Electron shells are most stable when they are full—eight valence electrons. The behaviour of valence electrons can be explained by the atom seeking a stable state: electrons can be gained and lost from neighbouring atoms in an attempt to achieve balance, which, in the case of atoms, means a full outer (valence) shell of electrons. In certain cases, electrons are shared between atoms to achieve this balance. The easiest way to achieve stability for atoms with only a few valence electrons is to lose these electrons. In contrast, if the valence shell is almost full, there is a greater likelihood of that atom gaining electrons to fill the gaps in the shell. Because electrons carry a negative charge, the movement of the electrons results in an overall charge for the atom. The atom is then referred to as an ion—a charged atom. Electron Fig 6.27 Hydrogen has unique properties—no other element is like it! Hydrogen was originally placed in group 1, even though all the other elements in group 1 are metals, simply because its uncharged atoms have one electron in their valence shell, like all the other elements in this group. However, because of its unique properties, it is placed alone and is not part of any group. Fig 6.28 Helium is placed in group 18. All members of group 18, except helium, have eight electrons in the valence shell of their uncharged atoms; helium atoms only have two. However, this means that, in all group 18 elements including helium, the valence shell of their uncharged atoms contains as many electrons as possible. Because helium also has very similar properties to the other elements in group 18, it remains placed within this group. Electron shell e – Nucleus Proton Neutron Fig 6.28 Oxygen tends to gain two electrons to fill its valence shell. Overall, there will be two extra negative charges compared with positive charges (from the protons in the nucleus), so the ion is written as O 2 –. Electron Na Cl Na + + Nucleus Electron shell Cl – Proton Neutron Fig 6.29 Calcium has two electrons in its valence shell, so it tends to lose them to achieve stability. The calcium ion formed is then written as Ca 2 + to show that there are two more protons compared with the number of electrons. The ions formed by the various elements impact on the elements they are likely to interact with. Again, it’s all about balance. An ion with a 2+ charge is likely to combine (bond) with an ion of 2– charge or with two ions each with a charge of 1– (oppositely charged ions attract each other). The positive charge is balanced out by an equal negative charge. The bonds that are formed after ions interact are referred to as ionic bonds. Fig 6.30 Sodium chloride is formed when sodium donates an electron to chlorine. UNCORRECTED PAGE PROOFS Unit 7.3 • What are the risks of using chemicals? 21 CAS_SB10_TXT_06_1pp.indd 21 11/11/11 4:58 PM
22 EXPERIMENT 6.2 Conductivity of ionic compounds Aim To investigate the electrical conductivity of a number of ionic compounds as a solid and in aqueous solution. Equipment Large sodium chloride crystals Large potassium bromide crystals Coarse sea salt crystals One small Petri dish 3-V battery or other 3-V DC power source Ammeter Wires Alligator clips Two graphite electrodes Three 100-mL beakers Large spatula Glass stirring rod Paper towel Method 1 Set up the electrical circuit as shown in Figure 6.31. Have your teacher check that it is correct before proceeding. Ensure that you know how to use the ammeter and its scales correctly. 2 Using the spatula, place the largest sodium chloride crystal onto a Petri dish, then touch each end with an electrode, making sure that the two electrodes do not touch each other. Does the crystal conduct electricity? If it doesn’t appear to, connect the wire to the more sensitive scale on the ammeter. Does a reading register now? Record your result. 3 Now place several sodium chloride crystals into a beaker, half-fill the beaker with water and then stir. 4 Place the electrodes into this solution, again ensuring they do not touch each other. Does the solution conduct electricity? If it doesn’t appear to, connect the wire to the more sensitive scale on the ammeter. Does a reading register now? Record your result. Chapter 6 • organising elements 5 Rinse the electrodes with fresh tap water, then dry them with a paper towel. 6 Repeat steps 1–5 for the potassium bromide crystals. 7 Repeat steps 1-5 for the coarse sea salt crystals. Results Devise a simple table or spreadsheet in which to record your results. Discussion 1 Sea salt is a mixture of different ionic compounds, including sodium chloride. What can you conclude about the ability of solid ionic compounds to conduct electricity, whether they are pure or mixed up together? 2 What effect does dissolving an ionic compound in water have on its ability to conduct electricity? 3 To conduct electricity, a substance must have charged particles that can move about. Suggest an explanation for your findings. 4 The melting point of sodium chloride is 801°C, so it is not practical to melt it in the school laboratory. Predict whether molten sodium chloride would conduct electricity and justify your answer. Fig 6.31 Experiment set up. UNCORRECTED PAGE PROOFS CAS_SB10_TXT_06_1pp.indd 22 11/11/11 4:58 PM
Page 1 and 2: 2 6 Chapter 6 • organising eleme
Page 3 and 4: 6.1 4 Why do we organise elements?
Page 5 and 6: 6 Some properties of Dobereiner’s
Page 7 and 8: 8 1894 William Ramsay, a Scottish c
Page 9 and 10: 10 Properties and structure 6.1 F
Page 11 and 12: 12 Grouping elements The two main t
Page 13 and 14: 14 • Many of the transition metal
Page 15 and 16: 16 What do you know about non-metal
Page 17 and 18: 6.3 18 How are properties linked to
Page 19: 20 Emission spectrum and electron s
Page 23 and 24: SKILLS LAB 24 Ionic compounds Ionic
Page 25 and 26: 26 Table 6.6 gives the electrical c
Page 27 and 28: 28 Properties and structure 6.3 Ho
Page 29 and 30: 30 >>ZOOMING OUT
Page 31 and 32: 6 Properties and structure Element
Page 33 and 34: 34 Given the fact that so many coun

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2 Chapter 6 â¢ organising elements Organising elements