Short Summary

Chapter 5, Periodic Classification of Elements, details the historical evolution and scientific principles behind organizing chemical elements based on their physical and chemical properties. Early attempts to classify elements included Döbereiner’s triads and Newlands’ Law of Octaves, which grouped elements according to recurring similarities in their properties. Dmitri Mendeléev arranged 63 known elements in order of increasing atomic mass and considered their chemical reactivity with oxygen and hydrogen, proposing that the properties of elements are a periodic function of their atomic masses. He also deliberately left gaps in his periodic table to predict the existence and properties of elements that had not yet been discovered. Later, Henry Moseley established that atomic number is a more fundamental property than atomic mass, resolving anomalies in the earlier arrangement and providing the basis for the modern periodic table. The modern periodic table arranges elements in increasing order of atomic number and consists of 18 vertical groups and 7 horizontal periods. Elements belonging to the same group generally have the same number of valence electrons, which gives them similar chemical properties. The periodic table also demonstrates predictable trends across periods and down groups, including changes in valency, atomic size, and metallic and non-metallic character.