📌 Introduction: Matter exists as elements, compounds, and mixtures. Currently, 118 elements are known, 94 of which are naturally occurring. With increasing discoveries, scientists needed a system to organize elements based on their properties. 📌 Early Attempts of Classification: Döbereiner’s Triads (1817) : Grouped elements in triads (groups of 3). The atomic mass of the middle element was the average of the other two. Limitation: Only a few triads could be identified. Newlands’ Law of Octaves (1866) : Every eighth element had properties similar to the first, like musical notes. Worked only up to calcium. Flawed as he forced placement of dissimilar elements together. 📌 Mendeleev’s Periodic Table (1869): Based on increasing atomic mass and similar properties. Left gaps for undiscovered elements like Eka-Aluminium (later Gallium). Predicted properties of some elements successfully. Limitations: Hydrogen's position was unclear. No place for...
Preparation for Competitive exams SSC RAILWAY