NCERT Class X Science Solutions: Chapter 5 – Periodic Classification of Elements Part 2 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Question 6:

The position of three elements A, B and C in the Periodic Table are shown below.

Position of Elements in Periodic Table
Group 16Group 17
  1. State whether A is a metal or non-metal.
  2. State whether C is more reactive or less reactive than A.
  3. Will C be larger or smaller in size than B?
  4. Which type of ion, action or anion, will be formed by element A?


  1. A is a non-metal.
  2. C is less reactive than A, as reactivity decreases down the group in halogens.
  3. C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
  4. A will form an anion as it accepts an electron to complete its octet.

Question 7:

Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?


Position of Elements in Periodic Table (Ans)

Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.

Question 8:

How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?


In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same. Elements across a period show an increase in the number of valence electrons.

Result for Modern Periodic Table

Question 9:

In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12,19, 21, and 38. Which of these have physical and chemical properties resembling calcium?


The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2) .

Question 10:

Compare and contrast the arrangement of elements in Mendeleev՚s periodic Table and the Modern Periodic Table.


Difference between Mendeleev՚s and Modern Perioidic Table
Mendeleev՚s periodic tableModern periodic table
1. Elements are arranged in the increasing order of their atomic masses.1. Elements are arranged in the increasing order of their atomic numbers.
2. There are a total of 7 groups (columns) and 6 periods (rows) .2. There are a total of 18 groups (columns) and 7 periods (rows) .
3. Elements having similar properties were placed directly under one another.3. Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.
4. The position of hydrogen could not be explained.4. Hydrogen is placed above alkali metals.
5. No distinguishing positions for metals and non-metals.5. Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.