Q.What was the basis of classification of elements before the knowledge of electronic structure of atom?
Name the scientists who made the classification of elements in the nineteenth century.
What is Mendeleev's periodic law?
How many elements are discovered uptil now?
How many horizontal rows and vertical columns are present in modern periodic table?

Mendeleev's Periodic Law: the properties of elements are a periodic function of their atomic weights.

Dmitri Mendeleev, in 1869, arranged the then-known elements in a table in order of increasing atomic weight, and observed that elements with similar chemical and physical properties recurred at regular (periodic) intervals in this arrangement. He stated this observation as a law: the properties of elements are a periodic function of their atomic weights. This allowed him to group elements into families (columns) and even predict the properties of then-undiscovered elements (like eka-silicon = germanium) by leaving gaps in the table.

(This law was later refined by Moseley into the Modern Periodic Law — properties are a periodic function of atomic NUMBER, not atomic weight — after anomalies like the Ar/K and Co/Ni pairs were resolved by atomic number.)

Modern periodic law states that the physical and chemical properties of elements repeat periodically when the elements are arranged in order of increasing atomic number.

Mendeleev's original periodic law (1869) was based on atomic mass, which led to a few anomalies (e.g., Ar and K, Co and Ni appearing out of mass order). Henry Moseley (1913), through his X-ray spectra studies, showed that the atomic number (number of protons/nuclear charge) — not atomic mass — is the fundamental property that determines an element's properties. Arranging elements by increasing atomic number removed the earlier anomalies and gave the modern periodic table its correct order.