(a) Alkali metals are soft and reactive. For example Na, K etc.,
(b) Limestone is calcium carbonate, therefore, the important constituent of limestone is calcium.
(c) Metals which exist in the liquid state at room temperature are mercury (Hg) and francium (Fr)

Arranging the given elements into different groups and periods in order of their increasing atomic numbers we have:

Since the metallic character decrease along a period from left to right, therefore, metallic character increases in the order: Ge < Ga < Ca < K. .
Since metallic character increases down the group from top to bottom, therefore Ca is more metallic than Mg.
Combining above two results, the overall metallic character increases in the order: Ge < Ga < Mg < Ca < K.

(i) Element X has 3 valence electrons, therefore its valency is 3 and thus belongs to group 12(3+10). As such it could be any one of the following element: B, AI, Ga, In or TI.
(ii) Element Y has 4 valence electrons, therefore its valency is 4 and it belongs to groups 14(4+10). The element Y could be any one of the following C, Si, Ge, Sn or Pb.
(iii) Element Z has two valence electrons, therefore, its valency is 2 and it belongs to group 2. The element Z could be any one of the following: Be, Mg, Ca, Sr, Ba or Ra.

Eka-Silicon is germanium (Ge). It lies in group IV of the Mendeleev's periodic table and thus has a valency of 4. Therefore, the formula of its chloride is GeCl. Eka ₄. Eka-aluminium on the other hand is gallium (Ga). It lies in group III of the Mendleev's periodic table and thus has a valency 3. Therefore, the formula of its chloride is GaCl₃.

The electronic configuration of the element A is 2, 8, 6. Therefore, it belongs to group 6A.

Be has the maximum metallic characteristics because all other elements are situated at the right hand side in periodic table than Be. Due to the position their metallic characteristics decreases as we go from left to right.

Lithium, Beryllium, Boron are the metals in Modern Periodic Table among the first ten elements.

Since the noble gas elements such as Helium, Neon, Argon, Krypton, Xenon and Radon have zero valency, they could not find a place in Mendeleev's periodic table. Thus, these have been placed in a separate group called zero group in the periodic table.

The criteria for Mendeleev periodic table was the use of atomic masses. Mendeleev arranged the elements in increasing order of their atomic masses and according to their chemical properties.

When the various elements were arranged in order of their increasing atomic masses, elements having similar properties fell under on another in the same vertical column called the group. He further observed that repetition of elements with similar physical and chemical properties occurred after certain regular intervals. On the basis of these observations, Mendeleev proposed his periodic law which states that the physical and chemical properties of elements are a periodic function of their atomic masses.

At the time when Mendeleev's gave his periodic table, noble gases such as helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) were not known. When these gases were discovered much later, these were placed in a new group called the zero group without disturbing the original order.

(a) The element A which is a yellow solid at room temperature and shows catenation and allotropy is sulphur (S).
(b) The atomic number of sulphur is 16. Therefore, its electronic configuration is 2, 8, 6.
(c) \(\underset { Ferroussulphate }{ { 2FeSO }_{ 4 } } \underrightarrow { Heat } \underset { Ferricoxide }{ { Fe }_{ 2 }{ O }_{ 3 } } +\underset { Sulphurdioxide }{ { SO }_{ 2 } } +\underset { Sulphurtrioxide }{ { SO }_{ 3 } } \)
    Both SO₂ and SO₃ are major air pollutions.
(d) Since sulphur is a non-metal, therefore both S02 and S03 are acidic oxide since they dissolve in water to form the corresponding acids.
        \(\underset { Sulphurdioxide }{ { SO }_{ 2 } } +\underset { Water }{ { H }_{ 2 }O } \rightarrow \underset { Sulphurousacid }{ { H }_{ 2 }{ SO }_{ 3 } } \\ \underset { Ferroustrioxide }{ { SO }_{ 3 } } +\underset { Water }{ { H }_{ 2 }O } \rightarrow \underset { Sulphuricacid }{ { H }_{ 2 }{ SO }_{ 4 } } \)
(e) Since sulphur contains six valence electrons, therefore it lies in group 6 + 10 = 16.Further, since atomic number of sulphur (S) is 16. It lies in the 3rd period.

(a) Eka-silicon for germanium (Ge) and eka-aluminium for gallium(Ga).
(b) Group number of Ga is 13 and its period is 4th and group number of Ge is 14 and its period is also 4th.

(c) Both Ga and Ge are metalloids.
(d) Ga lies in group 13, therefore, it has 13 - 10 = 3 valence electrons. Similarly, Ge lies in group 14 and hence it has 14 - 10 = 4 valence electrons.

(a) The properties of all elements in the boron family
      (i) All the elements show valency of 3.
(b) The properties of  all the elements in halogen family
    (i) All the elements possess seven electrons in the valence shell.