NCERT Class X Science Class: Chapter –5. Periodic Classification of Elements – Part-7

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Question 33:

Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valences.


Table of Element and Valency
Three elements A, B and C have 3, 4 and 2


Group No.











Question 34:

If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?


As the element X is placed in group 14 then the valency of the element is 4.

Now we know that the valency of chlorine is 1 as it’s atomic number is 17.

Therefore, as the element X has the valency 4 it can neither gain nor lose electrons to attain octet.

Therefore, it makes a covalent bonding with four atoms of chlorine to form a compound .

Question 35:

Compare the radii of two species X and Y. Give reasons for your answer.

  1. X has 12 protons and 12 electrons

  2. Y has 12 protons and 10 electrons


From electron arrangement

Since Y has fewer energy levels its atomic radius will be smaller than that of X.


X and Y have the same no. of protons

Y is actually an ion of X obtained when X loses the two electrons in its outermost energy level.

Question 36:

Arrange the following elements in increasing order of their atomic radii.


As the atomic radii decreases along a period

Thus, the given elements will be arranged in the following order

As the atomic radii increases down a group

Thus, the given elements will be arranged in the following order

Question 37:

Identify and name the metals out of the following elements whose electronic configurations are given below.


  1. Magnesium

  2. Sodium

  3. Chlorine

  4. Lithium

Metals are (a) Magnesium, (b) Sodium and (d) Lithium as they have 1 to 3 valence electrons. Whereas, (c) chlorine is a non-metal.

Question 38:

Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?


The electron configuration of elements A with atomic number 19 is 2,8,8,1

Therefore valance electron is q and valency will be 1

The electronic configuration of elements B with atomic number 17 is 2,8,7

Therefore, valance is electron is 7 and valency will be 1 hey need one electron to complete the octet

Chemical formula between the A and B is AB

Imagee electronic dot structure of AB

Imagee Electronic Dot Structure of AB

Imagee electronic dot structure of AB

Ionic bond.

A = K (Potassium) B = Cl (Chlorine)

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