###### Write short notes on:
a. Mendeleev’s periodic law:
→ Mendeleev found that, when the elements are arranged in the order of their increasing atomic masses, the properties of the elements repeat after a certain interval. Based on his observations, Mendeleev stated the periodic law as follows:
The physical and chemical properties of elements are a periodic function of their atomic masses.
He prepared a Periodic Table of elements based on his peridic law.
2 Marks
b. Structure of the modern periodic table:
→ In the modern periodic table, the elements are arranged in the increasing order of their atomic numbers. It contains 118 elements. They are classified into 18 groups and 7 periods. Two rows are placed separately below the main body of the periodic table. They are called Lanthanides and Actinides. On the basis of electronic configuration, the elements in the modern periodic table are divided into 4 blocks viz. s-block, p-block, d-block and f-block. Elements from groups 1 and 2 are known as s-block elements. Elements from groups 13 to 18 are known as p-block elements. Elements from groups 3 to 12 are known as d-block elements. The d-block elements are called transition elements. Elements in the lanthanide and actinide series are known as f-block elements. They are known as inner transition elements. In the p-block, a zig zag line separates metals from non-metals. The metals are on the left while the non-metals are on the right of the zig zag line. The elements along the zig zag line are metalloids.
2 Marks
c. Position of isotopes in the Mendeleev’s and the modern periodic table:
→ Isotopes have same chemical properties. Isotopes of an element have same atomic number but different atomic masses. Mendeleev’s periodic table is based on atomic masses. Thus, isotopes cannot be placed in Mendeleev’s periodic table. Modern periodic table, on the hand, is based on atomic numbers. Thus, the isotopes of an element can be given the same place in the modern periodic table. e.g. $Cl1735$ and $Cl1737$ are the isotopes of chlorine. They can’t be placed in the Mendeleev’s periodic table, but they are given the same place in the modern periodic table.
2 Marks