What NEET Asks
- Direct questions on Mendeleev's Periodic Law statement are common (1-2 marks).
- Questions often test the basis of his classification or its limitations (e.g., anomalous pairs, isotopes).
- Understanding predicted elements and their modern equivalents can fetch 4 marks.
Key Points
- Mendeleev's Periodic Law (1869): "The properties of the elements are a periodic function of their atomic masses."
- Elements were arranged in increasing order of atomic masses.
- He also considered the similarities in chemical properties (especially formation of hydrides and oxides).
- Merits: Left gaps for undiscovered elements (Eka-boron, Eka-aluminium, Eka-silicon), predicted their properties accurately, and accommodated noble gases without disturbing the existing table.
- Demerits: Position of isotopes (same atomic number, different mass), anomalous pairs (e.g., Ar-K, Co-Ni, Te-I), position of hydrogen, and lack of clarity on cause of periodicity.
- Eka-Aluminium corresponds to Gallium (Ga); Eka-Silicon to Germanium (Ge); Eka-Boron to Scandium (Sc).
Must-Know Concept
Mendeleev's Periodic Law: The physical and chemical properties of elements are periodic functions of their atomic masses.
- Atomic Mass: The fundamental property used for arrangement.
- Periodic Function: Properties repeat after regular intervals.
Common Mistakes
- Students often confuse Mendeleev's basis (atomic mass) with the Modern Periodic Law's basis (atomic number).
- Don't forget the specific anomalous pairs like Argon (Ar, mass 39.9) placed before Potassium (K, mass 39.1).
- Students sometimes struggle to recall which modern elements correspond to Mendeleev's 'Eka' predictions.
Rapid Revision
Mendeleev classified elements by atomic mass and chemical properties, leaving gaps for predictions. Remember his Law, key merits like predicted elements (Eka-Al, Eka-Si), and crucial demerits, especially the anomalous pairs and isotope placement issues. This forms the foundation for the modern periodic table.