Science Focus ( Issue 21)

At the beginning of the 20th century, scientists still weren’t sure what to make of the periodic table. Older classifications of the chemical elements ran in order of increasing atomic weight. Dmitri Mendeleev’s table aimed to capture periodic trends in their properties, the so-called Periodic Law, forcing him to relegate atomic weights to a secondary consideration [1]. For example, cobalt has a greater atomic weight than nickel, yet the Periodic Law dictates that cobalt comes before nickel based on its chemical properties. These variations in atomic weight also left the possibility of unknown elements that, if discovered, might not fit into the table’s periodic structure [2]. It was clear that atomic weight was not the defining characteristic of the elements, but nobody could confirm what else that might be. Ernest Ruther ford bel ieved the solution might involve a new phenomenon cal led radioactivity, thanks to his experiments with radioactive decay. He assigned Henry Moseley, his new graduate student from Oxford, to study this phenomenon in 1910 [3]. But Moseley had other ideas. He has been following the developing field of X-rays closely ever since their discovery by Wilhelm Roentgen a decade earlier [4]. “Characteristic X-rays” of varying energy would be emitted when an element was struck by a stream of electrons. Moreover, the X-rays would be scattered through sl ightly different angles for each element used, and a technique to determine their wavelengths from this information had recently been developed by William and Lawrence Bragg [2]. This brought the intriguing problem of X-rays back within sight of the periodic table. Moseley decided to extend this line of research and systematically measure the wavelengths of each element’s characteristic X-rays. Returning to Oxford in 1913, he did this with a remarkably simple setup. By running a little train carrying samples of each element through a vacuum tube and then passing the line The oldest periodic table chart discovered at the University of St Andrews in 2014. It is believed to be printed in 1885, constructed according to Mendeleev’s periodic law. Highlighting the periodicity of chemical properties, the heavier cobalt (with an atomic mass of 58.9) appeared before nickel (58.7). 2014 年在聖安德魯斯大學發現最古老的元素週期表掛圖，相信是在 1885 年印刷，根據 Mendeleev 的週期定律而設計的，當中的排列強調元素特 性的週期性，因此較重的鈷（原子量為 58.9）排在鎳（58.7）前。 The photo shows the replica of the St Andrews Periodic Table Chart displayed at HKUST, by permission of the School of Chemistry and Special Collections, the University of St Andrews. 圖為獲聖安德魯斯大學化學系及特藏組授權，展示在香港科技大學的聖安德 魯斯元素週期表掛圖複製品。 15 怎樣「證明」 元素表 的排列是對的？ The Periodic Table Putting Each Element in the Right Spot By Peace Foo 胡適之