Science Focus ( Issue 21)

of fire of an electron beam, he was able to capture the positions of the emitted X-rays on photographic plates [1]. Knowing the angle by which they had been scattered, he could then calculate the wavelengths of the character i st ic X- rays for each element. Moseley found that as the elements progressed up the periodic table, the scattered X-rays decreased in wavelength – and by taking the inverse square root of the wavelength, this relationship became a straight line [5]. Thi s became known as Moseley’s law, and Moseley could also explain how it came about. Two years ear l ier, Ruther ford devised a model of the atomic structure: negatively charged electrons orbit a dense positively charged nucleus within the atom, with these charges cancelling each other out [5]. Moseley argued that the increasing size of this positive charge as atoms progressed up the periodic table would halt his electron beam more effectively, triggering a greater release of energy in the form of higher-frequency and lower-wavelength X-rays [2]. Because these positive charges could not be altered by chemical means, and were clearly a basic property of the atom, he suggested referring to them as “atomic numbers”. Thanks to this breakthrough, Moseley could now “cal l the rol l” of the elements. I f the X- ray wavelengths of two elements differed by a k n own 特性的週 期性，這又稱為週 期定律，迫使他把原子量擱 在一旁，成為次要的考慮因素 [1]。舉 例說，鈷（cobalt）的原子量比鎳（nickel）大， 但根據它們的化學特性，週期定律表明鈷應該排在鎳 前面。這些原子量上的差別也對怎樣將新發現的元素 納入既定的元素表造成困難 [2]。原子量顯然不是定 義元素的恰當特性，但當時並沒有人能確定什麼才是。 Ernest Rutherford 從他的放射衰變實驗 中取得靈感，相信答案可能涉及放射現象。在 1910 年，他把研究放射現象的工作分配了給 從牛津大學來的新研究生 Henry Moseley [3]，但 Moseley 卻有其他想法。從十年 前 Wilhelm Roentgen 發 現 X 光 起， minimum, there could be no other elements between them. From hydrogen to uranium there were exactly 92 elements, and it soon became obvious where the missing elements had to go. As Mendeleev had done, Moseley and others identified gaps at atomic numbers 43, 61, 72, 75, 85, 87 and 91, all of which were filled in the following 30 years [4]. The reversal of cobalt and nickel was also completely justified by cobalt’s lower nuclear charge, the proper basis for its order in the table [4]. Henry Moseley had brought meaning to the order of the elements and set the periodic table on a firm foundation in the process, grounding it in a reality far deeper than the chemical and physical properties Mendeleev saw [4]. If anyone can be said to have “proved” the periodic table, it can only have been him. 在 20 世紀初，科學家還未知道應該如何歸納元素週 期表。較舊的分類方法以遞增的原子量（atomic weight）把 化 學 元 素 排 列，而 Dmitri Mendeleev 的元 素表則希望突 顯 元 素