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The History Of Periodic Table And Variation Of Physical Properties And Classification Of Elements

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1 © Copyright reserved Kizza Emmanuel PERIODICITY I Competence: The learner analyses the trends and periodic properties of elements , to explain and predict the reactivity and properties of elements in the Periodic Table. THE PERIODIC TABLE Historical Development: The development of the periodic table involved key contributions from Dmitri Mendeleev and Henry Moseley . However, the there are some earlier versions whic h did not impact much on the development of the Periodic Table . 1. Early Attempts Before Dmitri Mendeleev . (a) Dobereiner’s Triads (1829): (i) Grouped elements in threes based on similar properties. (ii) Middle element’s atomic mass ≈ average of the other two. Example: Ca, Sr, Ba. (b) Newlands’ Law of Octaves (1864): (i) Arranged elements by increasing atomic mass. (ii) Every 8th element had similar properties. (iii) Failed for elements beyond calcium. 2. Dmitri Mendeleev’s Periodic Table (1869) . (a) Arrangement Principle: Mendeleev arranged elements in order of increasing atomic mass, noting that elements with similar chemical properties appeared at regular intervals into vertical columns. (b) Key Achievements: He grouped elements with similar pro perties into vertical columns (groups). 2 © Copyright reserved Kizza Emmanuel (i) Prediction of Missing Elements . He left gaps in the table for elements not yet discovered (e.g., gallium, scandium, germanium), and accurately predicted their properties based on the patterns he observed. (ii) Co rrection of Atomic Masses . He prioritized chemical behavior over strict atomic mass order, sometimes placing elements slightly out of order based on their chemical properties. For example, he adjusted by swapping the positions of iodine and tellurium based on (to fit the pattern of) their chemical similarities to other group VII element s (halogens ) and group VI elements, respectively even if mass order seemed “wrong” . (iii) Periodic Law (Mendeleev’s version): "The properties of …
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