2 Chapter 6 â¢ organising elements Organising elements

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It is said that Mendeleev wrote the names and properties of each element on a small card that he then arranged in order of atomic weight. The cards were then rearranged, maintaining their order, into groups with similar properties. With this organisation complete, Mendeleev proposed the periodic law: ‘Elements have properties that recur or repeat according to their atomic weight.’ More importantly, Mendeleev’s organisation of cards identified ‘holes’ that he attributed to elements that had yet to be discovered. The properties of these undiscovered elements could be predicted from this first periodic table. One of Mendeleev’s predictions was for the element below silicon, which he called ‘ekasilicon’ (‘eka’ is a Greek word meaning ‘first’, ‘beyond’ or ‘after’; see Table 6.2). In all, Mendeleev predicted the properties of 21 unknown or undiscovered elements. His predictions started searches for the missing elements. When these elements were discovered, their Table 6.2 Predicted properties of ekasilicon compared with the actual properties of germanium Ekasilicon (symbol Es) Germanium (symbol Ge) As predicted in 1871: As discovered in 1886: Atomic mass 72 Atomic mass 72.6 Density 5.50 g/mL Colour: grey metal Forms oxide EsO 2 : density 4.70 g/mL, slightly basic Forms chloride EsCl 4 : boiling point 100°C, density 1.90 g/mL Density 5.36 g/ml Colour: grey metal Forms oxide GeO 2 : density 4.70 g/mL, slightly basic Forms chloride GeCl 4 boiling point 86°C, density 1.88 g/mL Fig 6.5 The German chemist Lothar Meyer compiled a periodic table of 56 elements a few months after Mendeleev. Meyer graphed properties of the elements against atomic weight, and noted the repeating properties. properties were very close to the properties that had been predicted by Mendeleev. This convinced many chemists of the accuracy and value of Mendeleev’s periodic table. Mendeleev is given sole credit for the development of the periodic table. This is because of the evidence he provided to support his table and because he assumed that there were missing elements and he accurately predicted the properties of these elements. UNCORRECTED PAGE PROOFS Unit 7.1 • How do we use the products of chemical reactions? 7 CAS_SB10_TXT_06_1pp.indd 7 11/11/11 4:58 PM
8 1894 William Ramsay, a Scottish chemist, used the then new technology of refrigeration to liquefy and separate the components of air. He successfully removed water, carbon dioxide, oxygen and nitrogen, but found he had some unknown gas left behind. This was argon, the first in its group to be discovered. Further experimentation identified helium, neon, krypton and xenon. All these gases form the group of noble gases at the far right of the periodic table. 1940 With the development of nuclear processes, elements heavier than uranium could be created. The US scientist Glen Seaborg, winner of the 1951 Nobel Prize in Chemistry, used a ‘cyclotron’ to slam neutrons into uranium atoms. This created the very first atoms of neptunium and plutonium. Fig 6.7 Atoms heavier than uranium are called the ‘transuranium’ or ‘transuranic’ elements. None of them occurs naturally. The image above shows a sample of neptunium. Today Since the 1940s, similar nuclear processes have been used to synthesise the elements up to and including element 118. These elements are given names based on their atomic number: 118 is called ‘ununoctium’ (1-1-8-ium). In 2010, six atoms of element 117 were created by bombarding berkelium (97) with calcium (20). These six atoms existed for only a fraction of a second, but their creation filled a gap in the periodic table. Chapter 6 • organising elements 1913 By the early 1900s, X-rays could be used to determine the atomic number of each element. Using this technology, a young English physicist by the name of Henry Moseley refined the order of some of the elements in Mendeleev’s periodic table and proposed a minor change to the periodic law: ‘Elements have properties that recur or repeat according to their atomic number.’ Fig 6.6 Henry Moseley’s name is linked to significant advances in X-ray-related chemistry and physics, with many believing him worthy of a Nobel Prize. His work was cut short when he died at just 27 years of age in the Battle of Gallipoli during World War I. What do you know about the birth of the periodic table? 1 Who was the first chemist to lead a team that made elements that did not occur naturally? 2 When Mendeleev proposed the periodic table, he went one step further. What else did he do and why is this significant? 3 Why were the gases that Ramsay discovered not able to be discovered any earlier? 4 Moseley changed the periodic law proposed by Mendeleev by changing one word. What word was changed, and how did this improve the periodic table? UNCORRECTED PAGE PROOFS CAS_SB10_TXT_06_1pp.indd 8 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: 6 Some properties of Dobereiner’s
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 and 20: 20 Emission spectrum and electron s
Page 21 and 22: 22 EXPERIMENT 6.2 Conductivity of i
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