Lesson 2 of 13 · 7 min
Mendeleev's periodic table
NCERT §3.2
Mendeleev left empty boxes in his table and described elements nobody had seen. Could a table really forecast the density of an unknown metal?
The lesson in notes
In short
Mendeleev (Russian, 1834-1907) and Lothar Meyer, working independently, both proposed in 1869 that properties recur at regular intervals when elements are listed by atomic weight. Mendeleev is credited with publishing the Periodic Law first.
Mendeleev's Periodic Law: the properties of elements are a periodic function of their atomic weights.
He placed elements in rows and columns so that similar ones fell in the same vertical group, and used a wide range of properties, especially the formulas and behaviour of their compounds.
He broke atomic-weight order when properties demanded it, suspecting faulty weights: iodine, lighter than tellurium, was put in Group VII with fluorine, chlorine and bromine.
He left gaps for undiscovered elements and predicted their properties: eka-aluminium (later gallium) under aluminium and eka-silicon (later germanium) under silicon.
Predicted vs found, eka-aluminium/gallium: atomic weight 68 vs 70, density 5.9 vs 5.94 g/cm³, low melting point vs 302.93 K, oxide E₂O₃ vs Ga₂O₃, chloride ECl₃ vs GaCl₃.
Predicted vs found, eka-silicon/germanium: atomic weight 72 vs 72.6, density 5.5 vs 5.36 g/cm³, high melting point vs 1231 K, oxide EO₂ vs GeO₂, chloride ECl₄ vs GeCl₄. The success of these bold numerical forecasts made his table famous.