Write a note on relationship between electronic configuration and valency with reference to period 2 and period 3.

Ans.
1. The valency of an element is determined by the number of electrons present in the outermost shell (valence electrons) of its atoms.

2. From the electronic configuration of an element, one can determine the number of valence electrons.

3. The valency is equal to the number of valence electrons for elements having 1 to 4 valence electrons.

4. For elements having 5 to 8 valence electrons,
The valency = 8 - Number of valence electrons.

Write a note on position of hydrogen in modern periodic table.

Ans.
1. Hydrogen resembles alkali metals. It has one electron in its outermost shell like alkali metals. Its valency is 1 like alkali metals. If roms oxides (H2O) similar to alkali metals (Na2O).

2. Hydrogen also resembles halogens. It is a diatomic molecule (H2) like halogens (F2 , Cl2). Like halogens it requires one electrons to achieve stable electronic configuration similar to halogens.

3. Due to this unique behaviour of hydrogen, it is difficult to assign any definite position to it in the modern periodic table. Generally, it is placed above lithium in the periodic table.

Write a note on demerits of Mendeleev's periodic table.

Ans.
1. Hydrogen resembles alkali metals (group I) as well as halogens (group VII). Therefore, the correct position of hydrogen atom could not be decided.

2. In Mendeleev's periodic table, elements were arranged in an increasing order of atomic masses. But the increase in atomic mass is not uniform. Hence, it was not possible to predict the number of elements that could be discovered between two heavy elements.

3. The whole number of atomic mass of the elements cobalt (Co) and nickel (Ni) is the same. Therefore, it was difficult to interpret the position of these two elements in Mendeleev's periodic table.

4. Isotopes have same chemical properties but different atomic masses. Therefore, it would be difficult to place them in Mendeleev's periodic table.

Write a note on merits of Mendeleev's periodic table.

Ans.
1. Mendeleev was the first scientist to successfully classify all known elements.

2. Mendeleev's periodic table helped to revise the atomic masses of some elements to give them proper places in the periodic table based on their properties.
For Example, the atomic mass of beryllium (Be) was corrected from 14.09 to 9.4 and it was placed before boron (B).

3. Mendeleev's periodic table left some vacant spaces in it for elements which were not discovered that time.
Three of the unknown elements he named as eka - boron, eka - aluminium and eka - silicon. These elements were later discovered and named as scandium (Sc), gallium (Ga) and germanium (Ge), respectively.

4. There was no place reserved for noble gases in Mendeleev's original periodic table. But, when noble gases were discovered later, Mendeleev placed them in a new group called as 'zero group' without disturbing the position of other elements.

Write a note on Position of isotopes in the Mendeleev’s and the Modern Periodic Table.

Ans. 1. Isotopes were discovered long time after Mendeleev developed the periodic table. All isotopes of the same element have different atomic masses but same atomic number. They have similar chemical properties.

2. As isotopes have the same chemical properties but different atomic masses, a challenge was posed in placing them in Mendeleev’s periodic table.

3. In Mendeleev’s periodic table, elements are arranged in increasing order of their atomic masses such that chemically similar elements are placed together in a group.

4. So, it became difficult to place isotopes in Mendeleev’s periodic table. Therefore, isotopes were not given separate place in Mendeleev’s periodic table.

5. In Modern periodic table, elements are arranged in increasing order of their atomic number. Hence, all the isotopes of an elements occupy the same position in the modern periodic table.

Write a note on Mendeleev’s Periodic law.

Answer.
1. The Russian Scientist Dmitri Mendeleev developed the periodic table of elements during the period 1869 to 1872 A.D.

2. Mendeleev organized the periodic table on the basis of the chemical and physical properties of the elements. He considered the fundamental property of elements, namely the atomic mass as standard and arranged 63 elements known at that time in an increasing order of their atomic masses.

3. He found that the elements with similar physical and chemical properties repeat after a definite interval.

4. On the basis of this finding, he stated that ‘properties of elements are a periodic function of their atomic masses’. This is known as Mendeleev’s periodic law.

Write a note on structure of the modern periodic table.

Ans.
1. The modern periodic table has seven horizontal rows called the periods and eighteen vertical columns called the groups. This arrangement results in the formation of boxes in which each boy corresponds to the position of an element.

2. In addition to these seven rows, two series are shown separately at the bottom of the periodic table. The first row is lanthanide series and the second row is actinide series.

3. The entire periodic table is divided into four blocks as follows:

s-block: contains group 1 and 2 elements.
p-block: contains group 13 to 18 elements.
d-block: contains group 3 to 12 elements. (transition elements)
f-block: contains lanthanide and actinide series.

4. A zig - zag line is drawn in the p-block of the periodic table. Metals lie on the left side while nonmetals lie on the right side of this line. Metalloids lie along the border of this line.