CLASSIFICATION OF ELEMENTS – 2020 Important Questions


Q1. Upto which element, the  Law of Octaves was found to be applicable?

Q2. Why are noble gases missing from Newlands’ Octaves?

Q3. Atomic number is considered to be a more appropriate parameter than atomic mass for classification of elements in a periodic table. Why?

Q4. Out of Na, Al, Si, P which element exhibits maximum number of valence electrons?

Q5. Elements  A,   B,  C  and   D  have atomic numbers  1,  8,   11  and    19  respectively. Choose the odd element and give reason for your answer. 

Q6. Write the correct increasing order of the atomic radii of O, F and N.

Q7. Which group of elements will form an acidic oxide?

Q8. An element X is in  second period of group 16 of the periodic table. Is it metal or non-metal? Give reason for your answer.

Q9. Write two reasons responsible for late discovery of noble gases. 

Q10. Why hydrogen should be placed in Group I?

Q11. State one reason for placing  Mg and   Ca in the same group of the periodic table.

Q12. The atomic numbers of three elements A, B and C are 12, 18 and 20 respectively. State giving reason, which two elements will show similar properties.

Q13. The atomic number of three-element X, Y and  Z are 3, 11 and 17 respectively. State giving reason which two elements will show similar chemical properties.

Q14. How does the electronegativity changes as we move from left to right across a period?

Q15.  What is meant by periodicity in properties of elements with reference to periodic table?

Q16. Why is lithium atom smaller than potassium atom?

Q17. What is the basis for the classification of modern periodic table?

Q18. Name the two elements whose valencies are equal to their group number.

Q19. Why does silicon has valency  4 and chlorine  1?

Directions (Q. Nos. 10 – 14)  In the following questions a statement of Assertion is followed by a statement of Reason. Mark the correct choice as:

(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.            ·

(c) If Assertion is true but Reason is false.

(d) If Reason is true but Assertion is false.

(e) If both Assertion and Reason are false.

Q10. Assertion: Atomic size of potassium is greater than that of sodium.

           Reason: As we go down the group, atomic radius increases.

Q11. Assertion: Alkali metals have smallest size in a period.

            Reason: They are at the extreme left of the periodic table.

Q12. Assertion: Nobel Elements were not included by Mendeleev’s Periodic Table.

            Reason: Mendeleev did not know where to place in his Periodic Table.

Q13. Assertion: Electronegativity decreases from left to right along a period in the Modern Periodic Table.

            Reason: As electron is added to the same shell when moving from left to right along a period, the force of attraction increases between the nucleus and the electron on the last shell.

Q14. Assertion: Atomic Mass of certain elements are expressed in fraction taking in account the availability of its various isotopes.

            Reason: Isotopes of an element have the same atomic mass but different atomic number.

Q15. When you add a few drops of acetic acid to a test-tube containing sodium bicarbonate powder, which one of the following is your observation?

(a ) No reaction takes place.

(b) A  colourless gas with pungent smell is released with brisk effervescence.

(c) A  brown coloured gas is released with brisk effervescence.


Q1.  Compare the radii of two species X and Y. Give reasons for your answer.  (i) X has 12 protons and 12 electrons  (ii) Y has 12 protons and 10 electrons. 

Q2. The atomic number of elements A, B, C, D and E are given below.







Atomic Number






From the above table, answer the following questions.

(i)  Which two elements are chemically similar?

(ii)  Which element is an inert gas?

(iii)  Which element belongs to 3rd period of periodic table?

(iv) Which element among these is a non-metal?

Q3.  ‘Hydrogen occupies a  unique position in modern periodic table’, Justify the statement.           

Q4.  An element Y has a total of three shells, with six electrons in its valence shell.

(i) What will be the atomic number of this element?

(ii) In which period will you find this element?

(iii) Name another element which belongs to the same group as this element.

Q5.  Atomic radii of the elements of the second period are given below:

Period 2 elements







Atomic Radii (pm)







(i) Arrange the elements in the decreasing order of their atomic radii.

(ii) Identify the element that restively has :

            (a) largest radius          (b) smallest radius

(iii) Give reasons for the variation in atomic radii.

Q6.  From the part of a periodic table, answer the following questions:























(i) Atomic number of oxygen is 8.  What would be the atomic number of fluorine?

(ii) Out of X and Q which element has larger atomic size? Give reason for your answer.

(iii) Out of Y and Z which element has smaller atomic size? Give reason for your answer.

Q7.  An element A reacts with oxygen to form A2O.

(i) State the number of electrons in the outermost orbit of A.

(ii) To which group of periodic table does A belong?

(iii) State whether A is a metal or non-metal.

Q8.  Two elements P and Q belong to the 3rd period of the modern periodic table and are in group 1 and group 2 respectively. Compare their following characteristics  in tabular form:

  • The number of electrons in their atoms
  • The sizes of their atoms
  • Their metallic character
  • Their tendencies to lose electrons
  • The formula of their oxides
  • The formula of their chlorides.

Q9. State the main aim of classifying elements. Which is the more fundamental property of elements that is used in the development of  Modern Periodic Table? Name and state the law based on this fundamental property.  On which side of the periodic table one can find metals, non-metals and metalloids?

Q10. An element ‘X’ with electronic configuration (2,8,2) combines separately with two radicals, (NO3 ) and (SO4)2−.

(i) Is ‘X’ a metal or a non-metal? Write the nature of its oxide.

(ii) Write the formula of the compounds of ‘X’ formed by the combination of these radicals. Are these compounds covalent or electrovalent?

Q11.  How   does   the  atomic   radius of  the  elements change on going

(i) from left to right in a period, and

(ii) down  a group

in the  Modern Periodic  Table  ?  Give reason in support of your answer.

Q12. What is meant by ‘group’ in the modern periodic table? How do the following change on moving from top to bottom in a group?

(i) Number of valence electrons

(ii) Number of occupied shells

(iii) Size of atoms

(iv) Metallic character of element

(v) Effective nuclear charge experienced by valence electrons.

Q13.  An element ‘X’ belongs to third period and  second group of the Modern Periodic Table.

(i) Write its electronic configuration.

(ii) Is it a metal  or non-metal? Why?

(iii) Write the formula of the compound formed when ‘X’ reacts with an element.

                   (a) Y of electronic configuration 2, 6 and 

                   (b) z with electronic configuration 2, 8, 7.

Q14. An element ‘X’ belongs to 3rd period and group 16 of the Modern Periodic Table.

(i) Determine the number of valence electrons and the valency of ‘X’.

(ii) Molecular formula of the compound when ‘X’ reacts with hydrogen and write its electron dot structure.

(iii) Name the element ‘X’  and state whether it is metallic or non-metallic.

Q15. Two elements X  and  Y belong to group  1 and  2 respectively in the same period of the periodic table. Compare  these elements with respect to :

(i) number of electrons in their outermost orbit

(ii) their valencies

(iii) metallic character

(iv) their atomic size

(v) formula  of their chlorides

(vii) formula  of their sulphates.

Q16. Given  below  are  some  elements of the modern periodic  table :

4Be, 9F, 14Si, 19K, 20Ca.

(i) Select the element that has one electron in the outermost shell and write its electronic configuration.

(ii) Select two elements that belong to the same group. give reason for your answer.

(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?

Q17.  State the difference between modern periodic table and Mendeleev’s periodic table.

Q18. (a) Can the following groups of elements be classified as Dobereiner’s triad?

                        (i). Na, Si, Cl        (ii) Be, Mg, Ca

Explain by giving reason.

(Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40)

         (b) Upto which element, the law of octaves was found to be applicable?

Q19.       Four elements A, B, C and D have atomic numbers 12, 13, 14 and  15 respectively. Answer the following questions giving reasons.

(i)  What is the valency of B?

(ii)  Classify these elements as metals and non-metals.

(iii)  Which· of these elements will form the most basic oxide?

Q20.       X is an element of modern periodic table. It is surrounded by elements having atomic number  17, 34, 36 and  53.

(i)   What is the atomic number of X?

(ii)   Predict its position in the periodic table.

(iii) Which among these do not belong to the same Group?


Q1. Mendeleev’s predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.

(i)  Name the elements which have taken the place of these elements.

(ii)   Mention the group and the period of these elements in the modern periodic table.

(iii) Classify these elements as metals, non-metals or metalloids.

(iv) How many valence electrons are present in each one of them?

Q2. What is meant by group in the modern periodic table?   How do the following changes occur on moving from top to bottom in a group?

  • Number of valence electrons
  • Number of occupied shells
  • Size of atoms
  • Metallic character of element
  • Effective nuclear charge experienced by valence electrons

Q3. Consider two elements A (atomic number = 17) and B (atomic number= 19)

  • Write the positions of these elements in the modern periodic table giving justification.
  • Write the formula of the compound formed when A combines with B.
  • Draw the electron dot structure of the compound and state the nature of the bond formed between the two elements.      

Q4. An element is placed in 2nd   group and 3rd  period of the periodic Table,  burns in presence of oxygen to form a basic oxide. 

  • Write the electronic configuration.
  • Write a balanced equation when it burns in the presence of air.
  • Write a  balanced equation when this oxide is dissolved in water.
  • Draw the electron dot structure for the formation of this oxide. \
  • Identify the element.

Q5. (i) Why do we classify elements?

        (ii) What are the two criteria used in the development of Modern Periodic Table?

         (iii) State the position of

                (a) metals,

               (b) non-metals and

               (c) metalloids in the periodic table.

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

Q6. (a) Why did Mendeleev have gaps in his periodic table?

         (b) State any three limitations of Mendeleev’s classification?

          (c )How do electronic configurations of atoms change in a period with an increase in atomic number?

Q7. (a) What was the basis of Mendeleev’s classification of elements?

         (b) List two achievements of Mendeleev’s  Periodic table.

          (c) List any two observations which posed a challenge to Mendeleev’s Periodic  Law.

Q8. (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) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

Q9. Explain   giving    justification  the   trends   in the following properties of elements, on moving from left  to right  in  a period,  in  the  Modern Periodic Table:

(a) Variation of valency.

(b) Change of atomic radius.

(c) Metallic to non-metallic character.

(d) Electronegative character.

(e) Nature of oxides.

Q10. (a) An element which is an essential constituent of all organic compounds belongs to which group?

  • Account for the following:
  • Elements of group 18 are called zero-valent.
  • If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?
  • An element X forms a chloride with formula XCI3. The element X would most likely be in the same group of the periodic table as for which of the following
  • Na, Mg, AI or Ca.
  • Elements of which groups will form acidic oxides?

Q11. (a) Consider the following elements:

            Na (11), AI (13), Cl (17), K (19)

For the given elements, find out

          (i) valency            (ii) period number              (iii) group number

         (b) An element X belongs to group 17 and element Y belongs to group 1. What type of bond will they form?

Q11.       The   positions   of   five   elements  in   the  periodic table are shown below:


Group 1

Group 2

Group 15

Group 16

Period 1





Period 3





What are the number of valence electrons in  B? Identify the element(s) which

(i)  will have the. smallest atomic radius?

(ii)  will be the most metallic?

(iii)  will form basic oxide?

(iv)  will show valency three?

(v) will show greatest tendency to gain electron?


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