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

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(a) Which two criteria did Mendeleev use to classify the elements in his periodic table?

(b) State Mendeleev’s periodic law.

(c) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table?

(d) How and why does the atomic size vary as you go?

i) From left to right along a period?

ii) Down a group?


  • Increasing atomic mass and similarity in chemical properties of elements were the two criteria used by Mendeleev to classify the elements. He took the formulae of the oxides and hydrides formed by the elements as the basis for classification of elements.

  • Mendeleev’s periodic law states that the properties of elements are periodic function of their atomic masses.

  • Hydrogen resembles alkali metals in its electronic configuration and halogens as it also exists as a diatomic molecule and combines with metals and non- metals to form covalent compounds hence it could not be assigned a fixed position in Mendeleev’s periodic table.

  • Atomic size decreases from left to right in the periodic table due to increase in nuclear charge.

    • Atomic size increases down the group because new shells are being added as we go down the group.


Atoms of eight elements A, B, D, E, F, G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compound is added in a small amount to almost all vegetable dishes during cooking. Oxides of elements A and B are basic in nature while those of E and F are acidic. The oxide of D is almost neutral. Based on the above information answer the following questions:

(i) To which group or period of the periodic table, do the listed elements belong?

(ii) What would be the nature of compound formed by a combination of elements B and F?

(iii) Which two of these elements could definitely be metals?

(iv) Which one of the eight elements is most likely to be found in gaseous state at room temperature? (v) The number of electrons in the outermost shell of element C and G be 3 and 7 respectively, write the formula of the compound formed by the combination of C and G.



A 1 group 3rd period

B2 group 3rd period

C 13 group 3rd period

D 14 group 3rd period

E 15 group3 period

F16 group 3rd period

G17 group3d period

H 18 group 3rd period

(ii) Nature of the compound formed by combination of element B and F is ionic.

(iii) Elements A and B are definitely metals.

(iv) Element H belonging to group 18 is most likely to be found in gaseous state at room temperature.

(v) Formula of the compound formed by combination of C and G is CG3,


a) Why do we classify elements?

b) What were the two criteria used by Mendeleev in creating his Periodic Table?

c) Why did Mendeleev leave some gaps in his Periodic Table?

d) In Mendeleev’s Periodic Table, why was there no mention of Noble gases like Helium, Neon and Argon?

e) Would you place the two isotopes of chlorine? CI-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.


(a) We classify elements to systematize the study of elements and make the understanding of properties of elements and compounds simpler.

(b) Criteria used by Mendeleev: i) Atomic mass ii) Properties of hydrides and oxides of elements.

(c) Mendeleev left some gaps in his Periodic Table to leave scope of search for the yet undiscovered elements.

(d) In Mendeleev’s Periodic Table, there was no mention of noble gases since they had not been discovered by that time.

(e) The two isotopes of chlorine, Cl-35 and Cl-37 will be placed in the same slot because their chemical properties are same


a) The Modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.

b) State Modern Periodic Law.

C) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.


a) Dobereiner

Advantage it gave an idea for classification like keeping the elements together which have similar properties.

Limitation: He could identify only three triads from the elements known at that time


Advantage He showed a systematic relationship .When elements were arranged in order of their increasing masses the properties of every eight element were similar to the first one.

Limitation: Law of Octaves was applicable only up to Calcium.


Advantage Noble gases were discovered later and they could be placed in a new group without disturbing the existing order.

Limitation: Anomalous position of hydrogen.

b) Modern Periodic Law: In 1913, Henry Moseley stated “the properties of elements are periodic function of their atomic numbers”.

C) Henry Moseley.


Two elements X and Y have atomic numbers 19 and 17 respectively. Give their electronic configuration. To which period and group of modern periodic table do these two elements belong?

What type of bond will be formed between them?

Also give the chemical formula of the compound formed.


Element ‘X’ has atomic number = 19

Electronic configuration = 2, 8, 8, 1

Therefore, Period=4, Group =1

Element ‘y’ has electronic configuration= 2, 8, 7. It belongs to 31 period and 1 7th group.

Ionic bond is formed between them.

Compound formed is X Y or K Cl


An element placed in Group 14 and 2nd period of the Periodic Table, burns in the presence of oxygen to form an acidic oxide.

a) Identify the element.

b) State the electronic configuration and atomic number of the element.

c) Write the balanced equations to show its burning in the presence of air and the reaction of its oxide with water.(1+2+2)


a) Carbon (1)

b) 2, 4 (1) 6 (1)

c) (1)


Priya took an element X which ¡s a yellow solid at room temperature and shows catenation. X also forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals.

a) Identify the element X.

b) Write the electronic configuration of X.

c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals.

d) What would happen when a pinch of X is added to carbon disulphide liquid?

e) Locate the position of the element in the Modern Periodic table? (5)


a) Sulphur (1)

b) 2, 8, 6 (1)

c) (1)

d) Sulphur is soluble in carbon disulphide (1)

e) 3rd period (1/2) 16th group(1/2)


(a) How does the atomic radius change as we go?

i) From left to right in a period

ii) Down a group in the periodic table

(b) Two elements A and B have atomic numbers 11 and 17 respectively. Answer the questions based on the statement given above.

i) Write the electronic configuration for these elements.

ii) To which period of the modern periodic table do these two elements belong?

Give reason for your answer.

iii) What type of bond would be formed between them and why?



  • It decreases

  • It increases


  • A-2,8,1 B-2,8,7

  • They belong to the third period as they have three shells. Period number of an element is equal to the number of shells in an atom.

  • They will form ionic bond as A is a metal and B is a non-metal. A will lose one electron which will be gained by B.