NCERT Class 10 Chapter 4 Classification of Elements CBSE Board Sample Problems Short Answer

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Two elements ‘A’ and’ B’ have atomic numbers 11 and 19 respectively.

State their position in the Modern Periodic Table.

Which element has bigger atomic radius.

What is the nature of their oxides?


Element ‘A’ and’ B’ belong to same group that is group 1 and ‘A’ belongs to 3rd period and’ B’ belongs to 4th period.

B has bigger radius as it has more number of shells.

Their oxides are basic in nature.


An element Q belongs to 3rd period and group 17 of the Modern Periodic table. Answer the following questions:

What is the valency of Q.?

What is the name of the element?

Write the formula of the compound formed in the reaction of Q with sodium.





The atomic number of an element is 16. Predict

i. the number of valence electrons in its atom

ii. its valency

iii. Its group number

iv. Whether it is a metal or a non-metal y. the nature of oxide formed by it

vi. The formula of its chloride


Atomic number = 16 Electronic configuration = 2, 8, 6

  • Number of valence electrons in its atom = 6

  • Valency = 2

  • Group number =16

  • It is a non-metal

  • Acidic oxide


F, Cl and Br are elements each having seven valence electrons. Which of these:

(i) Has the largest atomic radius

(ii) Is most reactive?

Justify your answer stating reason for each.


  • F, Cl and Br all are in the same group and thus have the same effective nuclear charge. Br has the largest atomic radius among all because it uses the largest number of electron energy levels since the valence electrons are placed in larger orbitaIs i.e. the principal quantum number increases on going down the group.

  • Fluorine is the most reactive since it has the greatest tendency to gain electrons because it has a higher effective nuclear charge and uses fewer energy levels than Br and Cl.


In the modern periodic table, the element calcium (atomic number = 20) is surrounded by elements with atomic numbers 12. 19.21 and 38. Which of these elements has physical and chemical properties resembling those of calcium and why?


Ca: Electronic configuration is: 2,8,8,2 the physical and chemical properties of elements with atomic number 12 and 38 will resemble those of calcium. This is because they all belong to the second group and all of them have two electrons in the valence shell.


How does the metallic character of elements change along a period of the periodic table from the left to the right and why?


Metallic character decreases from left to right along a period of the periodic table because on moving from left to right, size of the atoms decreases and nuclear charge increases. Hence, the tendency to release electrons decreases. Thus, the electropositive character decreases


Can the following groups of elements be classified as Dobereiners triad?

Na, Si, Cl b) Be, Mg, Ca

Atomic mass of be - 9; Na - 23; Mg - 24; Si - 28; CI - 35; Ca-40

Justify your answer in each case.


a) Na, Si, Cl - The properties of these three elements are not similar to each other, so no Dobereiner triad.

Be, Mg, Ca - The properties are similar to each other so it is Dobereiner triad.

Atomic mass of Mg =

= 24.5


An atom has electronic configuration 2, 8 7

What is the atomic number of this element?

To which of the following elements would it be chemically similar?

N, F, P, Ar


a) The atomic number of this element is 17.

b) It will be chemically similar to Fluorine as they both have same number of valence electrons.


The electronic configuration of an element is 2, 8, 4. State its Group and period in the Modern Periodic Table Name and write its one physical property.


Group = 14, Period = 3


It is a metalloid; poor conductor of electricity


How can it be proved that the basic structure of the Modern Periodic Table is based on the electronic configuration of atoms of different elements?


Modern periodic table consists of groups and periods, where number of valence electrons determines the group and number of shells determines the period.


Name the element which has

One shell only which is completely filled with electrons.

Twice as many electrons in second shell as in first shell.

A total of three shells, with 3 electrons in valence shell.


a) Helium

b) Carbon

c) Aluminium


Nitrogen (atomic number 7) and phosphorous (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 electro negative? Why?


  • Electronic configuration of Nitrogen=2, 5

  • Phosphorus=2, 8, 5

  • Nitrogen is more electronegative element because it ¡s smaller in size hence its nucleus exerts greater force on the electrons.


Give reasons

K atom is bigger than Na atom even though both these elements belong to same group.

The metallic character of elements increases as we move down a group.

Mg atom is smaller than Na even though these elements belong to same period.


Atomic number of K is 19 and atomic number of Na is 11. K has an additional energy shell in comparison to Na so its size is bigger than K.

The effective nuclear charge on valence electrons decrease which makes it easier for the atom to lose its valence shell electron.

As we move from left to right in a period, the nuclear pull on electron increase due to increase in nuclear charge this results in the decrease of atomic size.


An element X belongs to group 17 and third period of the periodic table

Write electronic configuration of the element. What is its valency?

Predict its nature, whether it is metal or a non-metal. Give reason.

Give the formula of the compound formed when it combines with an element Y having valency three?


(a) 2, 8, 7, Valency is 1

(b) It is a non-metal. It needs one electron to complete its octet to attain stability. It accepts one electron and hence is a nonmetal.



Arrange the following elements in the increasing order of their:

j) Atomic radii

ii) Non-metallic character. Give reason for your answer.

Li, Be, F, N


  • Atomic radii- F, N, Be, Li

  • All the elements lie in the second period of the periodic table. Li is in first group, be in second group, N in the 15 group and F in 17 group. As we move from left to right in same period, atomic no increases, both no of protons and no of electrons increase but the electrons are added in same shell, so effective nuclear pull increases and hence atomic radii decreases.

  • Nonmetallic character -Li Be, N, F,

  • As effective nuclear pull increases from left to right in the period, so ability to lose electrons decreases, ability to gain electrons increases and hence Nonmetallic character increases.

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