32. Read the following texts about the development of the periodic table.
The Periodic Table, a vital tool to predict the way elements react, is a method of categorizing elements according to their properties. Scientists started to look for a way to categorize the known elements around 200 years ago. In 1817, Johann Döbereiner noticed that the atomic mass of Sr fell midway between the weights of Ca and Ba. These were elements which possessed similar chemical properties. They formed a triad of elements. Other triads were also discovered, composed of:
Cl, Br, I, and Li, Na, K.
He called this the ‘Law of Triads’.
In 1865, John Newlands, an English chemist, arranged the 56 known elements in order of increasing atomic mass. He found every eighth element in the series was similar: H Li Be B C N O F Na Mg Al Si P S Cl K He likened this to music and called it the ‘Law of Octaves’.
In 1869, the Russian Dmitri Mendeleev arranged all the 63 known elements in order of increasing atomic mass but in such a way that elements with similar properties were in the same vertical column. If necessary, he left gaps in the table. Mendeleev’s periodic table was tested by making predictions about elements that were unknown at that time but could possibly fill the gaps. As new elements were discovered, they were found to fit easily into the classification. For example, Mendeleev predicted the properties of the missing element ‘eka-silicon’ with the color, density and melting point as well as its atomic mass. In 1886 the element germanium was discovered in Germany by Clemens Winkler; its properties were almost exactly those Mendeleev had predicted. In all, Mendeleev predicted the atomic mass of ten new elements, of which seven were eventually discovered. His periodic table was quickly accepted by scientists as an important summary of the properties of the elements.
32a. Use the verbs (with correct form) in the box below to complete the following summary of the texts. [7 marks]
Summary
The Periodic Table (1) elements based on their properties and is crucial in predicting how they (2). Scientists (3) looking for a way to organize elements two centuries ago. Johann Döbereiner (4) triads of elements with similar properties, while John Newlands found that every eighth element was similar. Dmitri Mendeleev (5) all known elements in order of (6) atomic mass and left gaps where necessary, (7) properties of unknown elements. His predictions were validated when new elements were discovered. Mendeleev's periodic table was quickly accepted as a summary of element properties.
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