NCERT Class X Science Class: Chapter – 5. Periodic Classification of Elements – Part-11 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 49:

An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.

  1. Identify the element X
  2. Write the electronic configuration of X
  3. Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals?
  4. What would be the nature (acidic/basic) of oxides formed?
  5. Locate the position of the element in the Modern Periodic Table.


(a) Element X which is a yellow solid at room temperature, shows catenation and allotropy is sulphur (S) . It forms two oxides SO2 and SO3 which are also formed during thermal decomposition of ferrous sulphate crystals and are the major air pollutants.

(b) Atomic number of S = 16

Its electron configuration is


Both are major air pollutants.

(d) Since, sulphur is a non-metal, therefore, both are acidic oxides. (non-metals form acidic oxides) . They dissolve in water to form the corresponding acids.

(e) Since, sulphur contains six valence electrons; therefore, it lies in group . Further, since atomic number of sulphur (S) is16, electronic configuration is. Therefore, it lies in period.

Question 50:

An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell.

  1. Identify the element X. How many valence electrons does it have?
  2. Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
  3. Draw the electron dot structure for ammonia and what type of bond is formed in it?


  1. Nitrogen (atomic no. 7)

2,5; it has 5 valence electrons

Image the Triple Covalent Bonds
Image the Three Single Covalent Bonds

Question 51:

Which group of elements could be placed in Mendeleev՚s Periodic Table without disturbing the original order? Give reason.


Noble gases

According to Mendeleev՚s classification, the properties of elements are the periodic function of their atomic masses and there is a periodic recurrence of elements with similar physical and chemical properties. Noble gas being inert, could be placed in a separate group without disturbing the original order.

Question 52:

Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at “Periodic Law” ?


1) When Mendeleev started his work, 63 elements were known. He studied the compounds of these elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification.

3) Elements with similar properties were arranged in a group.

4) Mendeleev observed that elements were automatically arranged in the order of increasing atomic masses.

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