Class X - Periodic Classification of Elements (PYQs) - Solutions

Periodic Classification of Elements

Periodic Classification of Elements

1. What were the criteria used by Mendeleev in creating the periodic table?

View Answer

Ans. (i) Increasing order of atomic mass

(ii) Formula of oxides and hydrides

2. Why do you think the noble gases are placed in a separate group in the periodic table?

View Answer

Ans. It is because these are least reactive and resemble each other.

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

View Answer

Ans. (i) They are less abundant in nature.

(ii) They are least reactive.

4. What was the basis of classification of elements by Newlands?

Define Newlands’ law of octaves

View Answer

Ans. Elements were arranged in increasing order of atomic mass and every 8th element resembled with the first element.

5. Can the following group of elements be classified as Dobereiner’s triads? Explain by giving reasons.

(a) Na, Si, Cl

(b) Be, Mg, Ca

View Answer

Ans. (a) No, because atomic mass of Si = 58.5/2 = 29.25 which is nearly equal to 28 but these elements do not resemble with each other.

(b) Yes, atomic mass of Mg = (9 + 40)/2 = 24.5 which is nearly equal to 24 and these three elements resemble with each other.

6. Study the data of the following three categories A, B and C.

(a) From the given three categories A, B and C, pick the one which forms Dobereiner’s Triads.

View Answer

Ans. ‘A’ forms Dobereiner’s Triad.

(b) Why did Mendeleev placed elements of category A, B and C in three different groups?

View Answer

Ans. It is because they had different formula of oxides and hydrides.

View Answer

Ans. No, it was not applicable to elements after Ca(20) because after Ca every eighth element did not possess similar properties to first.

7. (a) Why do we classify elements?

View Answer

Ans. It helps to study the properties of elements in a simpler way by studying the properties of 118 elements by studying properties of 18 groups and 7 periods.

(b) What were the two criteria used by Mendeleev in creating periodic table?

View Answer

Ans. (i) Increasing order of atomic mass,

(ii) Formula of oxides and hydrides of elements

View Answer

Ans. These gaps were left for the undiscovered elements.

(d) In Mendeleev’s periodic table, why was there no mention of noble gases like Helium, Neon and Argon?

View Answer

Ans. Noble gases were not discovered by that time.

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

View Answer

Ans. They will be placed in same slot due to same properties.

8. State modern periodic law.

View Answer

Ans. Properties of elements are a periodic function of their atomic number.

9. Write the formula which determines the maximum number of electrons that the shell of an atom can accommodate.

View Answer

Ans. 2n², where ‘n’ represents the number of electronic shell.

10. The electronic configuration of two elements X and Y are 2, 8, 7 and 2, 8, 8, 3 respectively. Write the atomic numbers of X and Y.

View Answer

Ans. 17 and 21 respectively

11. Out of the three elements P, Q and R, having atomic number 11, 17 and 19 respectively, which two elements will show similar properties and why?

View Answer

Ans. P(11): 2, 8, 1, Q(17): 2, 8, 7, R(19): 2, 8, 8, 1

‘P’ and ‘R’, because they have the same number of valence electrons.

12. Write the number of horizontal rows in the modern periodic table. What are these rows called?

View Answer

Ans. There are 7 horizontal rows. These are called periods.

13. Give the number of elements in 2nd and 5th period of modern periodic table.

View Answer

Ans. 2nd period has 8 elements, 5th period has 18 elements.

14. An element ‘A’ has atomic number 17. To which group and period does it belong?

View Answer

Ans. Its electronic configuration is 2, 8, 7. It belongs to group 17 and 3rd period.

15. Find the atomic number of the element whose electronic configuration is 2, 8, 5.

View Answer

Ans. Its atomic number is equal to 2 + 8 + 5 = 15.

16. An element ‘A’ has atomic number 16. To which group and period does it belong?

View Answer

Ans. It belongs to group 16 and third (3rd) period.

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

View Answer

Ans. Henry Moseley

18. How many metals are present in second period of periodic table?

View Answer

Ans. There are two metals Lithium (Li) and Beryllium (Be) in second period of periodic table.

19. Write the name, symbol and electronic configuration of an element X whose atomic number is 11.

View Answer

Ans. Sodium, Na: 2,8,1

20. Predict the maximum number of valence electrons possible for the elements in the first period of periodic table.

View Answer

Ans. 2 valence electrons are present in the last element ‘Helium’ of 1st period.

21. Why lithium with atomic number 3 and potassium with atomic number 19 are placed in group one? What will be atomic numbers of first two elements of second group?

View Answer

Ans.

K L M N

Li (3) 2, 1

K (19) 2, 8, 8, 1

Li and K are placed in Group 1 because both have 1 valence electron. Be(4) and Mg(12) are first two elements of Group 2.

22. List two anomalies of Mendeleev’s periodic table which were solved by modern periodic table law.

View Answer

Ans. (i) Position of isotopes were not justified in Mendeleev’s periodic table but it is justified in the modern periodic table.

(ii) Increasing order of atomic masses could not be followed but increasing order of atomic numbers has been followed.

23. (a) Among the following elements identify the one that would form anions:

K, O, Na, F, Ca, Cl, Hg

View Answer

Ans. O, F, Cl will form anions.

(b) Write the electronic configuration of the anions identified above.

View Answer

Ans. O^2– (10) 2, 8

F^– (10) 2, 8

Cl^– (18) 2, 8, 8.

24. An element belongs to third period and second group of the periodic table:

(a) State number of valence electrons in it.

View Answer

Ans. 2, 8, 2 is the electronic configuration. The number of valence electrons = 2

(b) Is it a metal or non-metal?

View Answer

Ans. It is a metal

View Answer

Ans. Magnesium

(d) Write the formula of its oxide.

View Answer

Ans. MgO is the formula of its oxide.

25. State the reasons for the following:

(a) The elements of the same group have similar chemical properties.

(b) The elements of the same period have different properties.

26. (a) State two main characteristics of elements on which modern periodic table is based.

View Answer

Ans. It is because they have the same number of valence electrons.

(b) No fixed position can be assigned to hydrogen in the periodic table. Why?

View Answer

Ans. It is because they differ in the number of valence electrons.

27. (a) State modern periodic law.

View Answer

Ans. Modern Periodic Law: It states that the properties of elements are a periodic function of their atomic number.

(b) 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.

View Answer

Ans. ‘B’ with atomic number 8 is an odd element because it has 6 valence electrons whereas others have 1 valence electron.

28. The electronic configuration of an element is 2,8,4. State its:

(a) Group and period in the Modern Periodic Table.

View Answer

Ans. It belongs to Group 14, third period.

(b) Name and write its one physical property.

View Answer

Ans. Silicon is the element. It is a metalloid, forms covalent bond. It is a semiconductor.

29. An element ‘X’ has atomic number 13.

(a) Write its electronic configuration.

View Answer

Ans. 2, 8, 3,

(b) State the group to which ‘X’ belong.

View Answer

Ans. Group 13

View Answer

Ans. Metal

(d) Write the formula of its bromide.

View Answer

Ans. AlBr₃

30. Choose from the following: ₆C, ₈O, ₁₀Ne, ₁₁Na, ₁₄Si

(a) Elements that should be placed in the same period.

View Answer

Ans. ₆C, ₈O, ₁₀Ne belong to the same period because all these have 2 shells.

₁₁Na, ₁₄Si belong to the same period because both of these have 3 shells.

(b) Elements that should be placed in the same group. State the reason for your selection in each case

View Answer

Ans. ₁₁Na, ₁₄Si belong to the same period because both of these have 3 shells.

31. The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms A and D and the molecular formula of compound formed when A and D will combine.

View Answer

Ans. They belong to third period because these have 3 shells.

A has electronic configuration 2, 8, 1, valence electron 1, valency = 1

D has electronic configuration 2, 8, 7, valence electron 7, valency = 1

Formula: AD or A⁺D^–

32. An element P (atomic number 20) reacts with an element Q (atomic number 17) to form a compound.

Answer the following questions giving reason:

Write the position of P and Q in the Modern Periodic Table and the molecular formula of the compound formed when P reacts with Q

View Answer

Ans. Atomic number of element P = 20

Electronic configuration of element P = 2, 8, 8, 2

Atomic number of element Q = 17

Electronic configuration of element Q = 2, 8, 7

The position of P in the Modern Periodic Table

Period (Number of shells) = 4

Group (Electrons in outer-most shell) = 2

The position of Q in the Modern Periodic Table

Period (Number of shells) = 3

Group (Electrons in outer-most shell) = (10 + 7) = 17

When P reacts with Q, it loses the two valence electrons (valency 2).

These two valence electrons are accepted by two Q atoms (valency 1).

Hence, the formula of the compound formed between P and Q is PQ₂

33. From the elements ₁₉A³⁹ ,₁₄B²⁸, ₈C¹⁶, ₁₈D⁴⁰ identify:

(a) the most electropositive element.

View Answer

Ans. ₁₉ A³⁹ is most electropositive element

(b) a noble gas.

View Answer

Ans. ₁₈D⁴⁰ is noble gas

View Answer

Ans. ₁₄B²⁸ is metalloid

(d) an element which will gain 2 electrons to attain nearest noble gas configuration.

View Answer

Ans. ₈C¹⁶ will gain 2 electrons to attain nearest noble gas configuration

(e) formula of compound formed between A and C.

View Answer

Ans.

(f) elements belonging to same period.

View Answer

₁₄B²⁸, ₁₈D⁴⁰, belong to same period i.e. 3rd period because they have 3 shells

34. Name the element which has

(a) the electronic configuration 2, 8, 1

View Answer

Ans. Sodium (2, 8, 1)

(b) a total of two shells, with 4 electrons in the valence shell.

View Answer

Ans. Carbon (2, 4)

View Answer

Ans. Aluminium (2, 8, 3)

(d) One shell which is completely filled with electrons.

View Answer

Ans. Helium (2)

(e) twice as many electrons in the second shell as in the first shell.

View Answer

Ans. Carbon (2, 4)

35. Out of Li and K, which one have stronger metallic character and why?

View Answer

Ans. ‘K’, because it can lose electrons easily due to larger size and less effective nuclear charge.

36. List any two properties of the elements belonging to the first group of modern periodic table.

View Answer

Ans. (i) They should have valency equal to 1 and form monovalent positive ions.

(ii) They are highly reactive soft metals.

37. The formula of magnesium oxide is MgO. State the formula of barium nitrate and barium sulphate, if barium belongs to the same group.

View Answer

Ans. Ba(NO₃)₂, BaSO₄

38. The electronic configuration of two elements ‘A’ and ‘B’ are 2, 8, 7 and 2, 8, 8, 2, respectively. Write the atomic number of these elements. What will be the formula of the compound formed and the nature of bond between them, when these elements chemically combine together?

View Answer

Ans. A has atomic number ‘17’, ‘B’ has atomic number ‘20’. BA₂ is the formula of the compound. The bond formed between A and B will be ionic bond.

39. Which has larger atomic radius, K (19) or Ca (20)?

View Answer

Ans. K(19) is larger than Ca(20).

40. What would be nature of oxides formed by the elements on the right hand side of periodic table?

View Answer

Ans. Acidic

41. Arrange the following metals in decreasing order of atomic size:

Ca, Mg, Ba, Be

View Answer

Ans. Ba > Ca > Mg > Be

42. How does valency of an element vary across a period?

View Answer

Ans. The valency of an element first increases and then decreases across a period.

43. Define electropositivity

View Answer

Ans. It is defined as measure of tendency to lose electrons. The greater the tendency to lose electrons, more will be electropositivity.

44. The atomic radii of first group elements are given below:

State the reason behind the observed trend in the above elements

View Answer

Ans. Atomic radii increases down the group because number of shells go on increasing, effective nuclear charge decreases, distance between nucleus and valence shell increases.

45. Write the number of valence electrons present in a nitrogen atom (₇N¹⁴).

View Answer

Ans. It has 5 valence electrons.

46. How does the valency of an element be determined, if its electronic configuration is known? What will be the valency of an element with atomic number 9?

View Answer

Ans. Valency is equal to the number of valence electrons when valence electrons are from 1 to 4 or 8 – no. of valence electrons when valence electrons are from 5 to 8.

F (9): 2, 7; It can gain 1 electron to become stable, so its valency = 1.

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

View Answer

Ans. It decreases due to decrease in atomic size and decrease in tendency to lose electrons.

48. In the periodic table, how does the tendency of an atom to lose electrons changes on moving from (i) left to right across a period? (ii) top to bottom in a group?

View Answer

Ans. (i) It decreases due to increase in effective nuclear charge.

(ii) It increases due to decrease in effective nuclear charge.

49. What is meant by periodicity of properties of elements? Why are the properties of elements placed in the same group of periodic table similar?

View Answer

Ans. The repetition of similar properties of elements after a certain interval of elements is called periodicity of properties.

Elements of the same group have same number of valence electrons, same valency and therefore possess similar chemical properties.

50. Calcium is an element with atomic number 20

(i) Will it be a metal/non-metal?

View Answer

Ans. It is a metal.

(ii) What will be its valency?

View Answer

Ans. Its valency is equal to 2.

(iii) What would be formula of its chloride?

View Answer

Ans. CaCl₂ is the formula of its chloride.

(iv) Will it be larger/smaller than K?

View Answer

Ans. It will be smaller than K.

51. Given below are four elements with their atomic numbers:

(a) Identify the element which belong to same group of Modern Periodic Table.

View Answer

Ans. ‘B’ and ‘C’ belong to same group.

(b) Arrange the given elements in decreasing order of atomic size.

View Answer

B > D > A > C

View Answer

Ans. B₂O

(d) Which of the above element is a metalloid?

View Answer

Ans. ‘D’ is a metalloid.

52. The following table shows elements represented by the letters A, B, C, D, E, F, G and H

(i) Which of the element has the atomic size (a) biggest and (b) smallest?

View Answer

Ans. (a) ‘A’ is biggest in size

(b) ‘G’ is smallest in size

(ii) Which element has valency (a) 3 and (b) Zero

View Answer

Ans. (a) ‘C’ has valency 3.

(b) H has zero valency

53. What is a metalloid? Name any one of them.

View Answer

Ans. The element which resembles both with metals and non-metals is called a metalloid, e.g. Boron, Silicon.

54. What is place of metalloid in the periodic table?

View Answer

Ans. They are placed between metals and non-metals in a zig-zag manner.

55. An element ‘X’ belongs to 3rd period and group 16 of the modern periodic table.

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

View Answer

Ans. The element is S(16)—2, 8, 6, The number of valence electrons—6, Valency—2.

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

View Answer

Ans.

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

View Answer

Ans. Sulphur, non-metallic.

56. An element ‘M’ with electronic configuration (2, 8, 2) combines separately with NO₃^– , SO₄^2– and PO₄^3– radicals. Write the formulae of three compounds so formed. To which group and period of modern periodic table, ‘M’ belongs to? Will ‘M’ form covalent or ionic compounds? Give reason to justify your answer.

View Answer

Ans. Mg(NO₃)₂, MgSO₄, Mg₃(PO₄)₂

It belongs to Group 2, 3rd period of the periodic table. It will form ionic compounds because it can lose 2 electrons easily to form Mg²⁺ ions

57. Based on the group valency of elements, state the formula of the following, giving justification for each.

(i) Oxides of Group 1 elements.

View Answer

Ans. Group 1 elements can lose one electron to become stable, so its valency is equal to 1, M₂O.

(ii) Halides of the elements of Group 13.

View Answer

Ans. Group 13 elements have valency equal to 3, MCl₃.

(iii) Compounds formed when an element of group 2 combines with an element of Group 16.

View Answer

Ans. Group 2 elements have valency equal to 2, Group 16 elements have 6 valence electrons.

Thus, formula of oxide:

58. In the following table, are given eight elements A, B, C, D, E, F, G and H (here letters are not the usual symbols of the elements) of the Modern Periodic Table with atomic numbers of the elements in parenthesis.

(i) What is the electronic configuration of F?

View Answer

Ans. F(12) 2, 8, 2;

(ii) What is the number of valence electrons in the atom F?

View Answer

Ans. 2

(iii) What is the number of shells in the atom F?

View Answer

Ans. 3

(iv) Write the order of size of the atoms of E, F, G and H in decreasing order.

View Answer

Ans. H > G > F > E;

(v) State whether F is a metal or a non-metal.

View Answer

Ans. F is a metal;

(vi) Out of the three elements B, E and F, which one has the biggest atomic size?

View Answer

Ans. B has biggest atomic size.

59. Table given below shows a part of the periodic table:

Using this table, explain why

(i) Li and Na are considered as active metals?

View Answer

Ans. Li and Na can lose electrons easily due to their large size and hence are more reactive.

(ii) Atomic size of Mg is less than that of Na

Ans. Mg has more effective nuclear charge than Na.

(iii) Fluorine is more reactive than chlorine?

View Answer

Ans. Fluorine can gain electrons more easily than chlorine, due to smaller atomic size.

60. What is the position i.e. group number, period number of element, iodine (atomic number 53)? What is the physical state and nature of this element (metal or non-metal)?

View Answer

Ans. I (53): 2, 8, 18, 18, 7

It belongs to group 17, 5th period.

It is a solid. It is a non-metal.

61. Explain the basic character of oxides of elements down the group and across the period.

View Answer

Ans. Basic character of oxides increases from top to bottom in a group because metallic character increases down the group due to increase in tendency to lose electrons. Basic character of oxide decreases along a period from left to right because the atomic size decreases, tendency to lose electrons decreases, metallic character decreases.

62. Two elements ‘P’ and ‘Q’ belong to the same period of the modern periodic table and are in Group 1 and 2 respectively. Compare the following characteristics in tabular form:

(a) The number of valence electrons in their atom.

(b) Their metallic character

(d) The formulae of their oxides

(e) Their tendency to lose electrons

(f) The formula of their chloride

View Answer

Ans.

63. Given below are some elements of modern periodic table. Atomic number of elements are given in parenthesis:

A (4), B (9), C (14), D (19), E (20)

(a) Select the element that has one electron in outermost shell. Also write the electronic configuration of this element.

View Answer

Ans. D (19) has one valence electron. Its electronic configuration is 2, 8, 8, 1.

(b) Which two elements amongst there belong to the same group? Give reason for your answer.

View Answer

Ans. A (4), E (20) belong to same group because they have same number of valence electrons.

View Answer

Ans. A and B belong to same period; A is bigger than ‘B’

D and E also belong to same period, ‘D’ is bigger than E.

64. The elements Be, Mg and Ca are having two electrons in their outermost shells are in periods 2, 3 and 4, respectively of the modern periodic table. Answer the following questions, giving justification in each case.

(i) Write the group to which these elements belong.

View Answer

Ans. They belong to Group 2 because they have 2 valence electrons.

(ii) Name the least reactive element.

View Answer

Ans. Be is the least reactive element due to smallest size and least tendency to lose electrons.

(iii) Name the element having largest radius.

View Answer

Ans. Ca has largest radius because it has the most, four shells (2, 8, 8, 2).

65. Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (i) from left to right in a period, and (ii) down a group? Give reason to justify your answer.

View Answer

Ans. In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.

Metallic character: It is defined as the tendency of an atom to lose electrons.

Across the period i.e., from left to right: Metallic character decreases.

Down the group i.e., from top to bottom: Metallic character increases.

Reason: Across the period, the effective nuclear charge increases, thus decreasing its atomic radius. This favours the increase of electronegativity and therefore the tendency to lose electrons is less. This accounts for the decrease in the metallic character along the period.

But as we move down the group the number of shells keep on increasing and therefore the atomic size increases. This means that the electronegativity decreases. This enhances the ability to lose electrons and therefore the metallic character increases down the group.

66. Na, Mg and Al are the elements of the 3rd period of the Modern Periodic Table having group number 1, 2 and 13 respectively. Which one of these elements has the (a) highest valency, (b) largest atomic radius, and (c) maximum chemical reactivity? Justify your answer stating the reason for each.

View Answer

Ans. Given are the three elements Na, Mg and Al belonging to group 1 , 2 and 13 respectively. The electronic configurations of the three elements are as follows:

(a) The element having the highest valency signifies the maximum number of electrons present in the valence shell of an atom. Hence, as per the given electronic configurations, the element having highest valency is aluminium (Al).

(b) As we move across the period, i.e., from left to right the atomic radius decreases. Therefore, the element having the largest atomic radius will be sodium (Na).

(c) The given three elements are metals. So, the chemical reactivity of a metal is determined by its metallic character which is the tendency of an atom to lose its electrons. We know that the metallic character of element decreases across the period, i.e., from left to right. So, the element having highest chemical reactivity is sodium (Na).

67. Write the names given to the vertical columns and horizontal rows in the Modern Periodic Table. How does the metallic character of elements vary on moving down a vertical column? How does the size of atomic radius vary on moving left to right in a horizontal row? Give reason in support of your answer in the above two cases.

View Answer

Ans. In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.

As we move down the group, the electrostatic attraction between the nucleus and the outer-most electron decreases due to increase in the distance between them. This happens because, on moving down the group, a new shell is added. So the valence electrons can be easily lost by the element. As we know, metallic character is characterised by the ease of loss of electrons, thereby, metallic character increases on moving down the group in the Modern Periodic Table.

When we move across a period, the number of electrons in the same shell increases. This leads to greater electrostatic attraction between the nucleus and the outer-most electron. This increased attraction pulls the outer-most electron closer to the nucleus, thereby decreasing the atomic size.

68. Based on the group valency of elements write the molecular formula of the following compounds giving justification for each

(i) Oxides of first group elements

View Answer

Ans. Group 1 elements have valency equal to 1.

(ii) Halides of group 13 and

View Answer

Ans. Group 13 elements have 3 valence electrons, valency equal to 3

(iii) Compound formed when an element A of group 2 combines with element B of group 17

View Answer

Ans.

A has valency equal to 2 and B has valency equal to 1.

69. Atoms of eight elements A, B, C, 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 the 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 belongs to?

View Answer

Ans. They belong to the same period but different groups. ‘G’ belongs to Group 17. A and B belong to Group 1 and 2, . . . their oxides are basic. ‘C’ belongs to Group 13 due to 3 valence electrons. ‘D’ belongs to Group 14. E and F belong to group 15 and 16, respectively. ‘H’ belongs to group 18.

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

View Answer

Ans. B and F will form an ionic compound.

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

View Answer

Ans. A and B are metals.

(iv) Which one of the eight elements is most likely to be found in gaseous state, at room temperature?

View Answer

Ans. G, H are gases at room temperature.

(v) If the number of electrons in the outermost shell of elements C and G are 3 and 7 respectively, then write the formula of the compound formed by the combination of C and G.

View Answer

Ans. CG₃

70. (a) How is valency of an element determined if its electronic configuration is known? Determine the valency of an element of atomic no. 9

View Answer

Ans. Valency is equal to number of valence electrons or 8— number of valence electrons.

F (9): 2,7

Its valency is equal 1 because it will become stable on gaining one electron.

(b) Given below are some elements of modern periodic table. Atomic number of elements are given in parentheses.

A (4), B (9), C (14), D (19), E (20)

View Answer

(i) With the help of electronic configuration, find out which ore of the above elements will have one electron in its outermost shell.

Ans. A (4): 2,2

B (9): 2,7

C (14): 2,8,4

D (19): 2,8,8,1

E (20): 2,8,8,2

‘D’ has one valence electron.

(ii) Which two elements belong to the same group? Give reasons for your answer.

View Answer

Ans. ‘A’ and ‘E’ belong to same group because they have same number of valence electrons.

(iii) Which one of the above element belonging to the fourth period has bigger atomic radius and why?

View Answer

Ans. ‘D’ has larger atomic radius than ‘E’ because it has 19 protons which attract 19 electrons which is there less strongly than 20 protons can attract 20 electrons as in E.

71. 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

View Answer

Ans. Valency increases from left to right till middle, then decreases because valence electrons go on increasing.

(b) Change of atomic radius

View Answer

Ans. Atomic radius decreases due to increase in effective nuclear change.

View Answer

Ans. Metallic character decreases, non-metallic character increases due to increase in tendency to gain electrons as atomic size decreases, effective nuclear change increases.

(d) Electronegative character

View Answer

Ans. Electronegative character increases due to increase in effective nuclear change.

(e) Nature of oxides

View Answer

Ans. Acidic character of oxides increases, basic character of oxides decreases because metallic character decreases and non-metallic character increases.

72. The position of certain elements in the Modern Periodic Table are shown below

Using the above table answer the following questions giving reasons in each case:

(i) Which element will form only covalent compounds?

View Answer

Ans. E will form only covalent compounds.

(ii) Which element is a non-metal with valency 2?

View Answer

Ans. B is non-metal with valency 2.

(iii) Which element is a metal with valency 2?

View Answer

Ans. D is metal with valency 2.

(iv) Out of H, C and F which has largest atomic size?

View Answer

Ans. ‘F’ has largest atomic radius.

(v) To which family does H, C and F belong?

View Answer

Ans. It belongs to noble gases

73. Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why is it so?

View Answer

Ans. Atomic size is the distance between centre of nucleus and valence shell.

Its unit of measurement is picometre (10–12 m) denoted by pm.

Atomic radius increases down the group due to increase in number of shells and decrease in effective nuclear charge.

Atomic radius decreases along a period from left to right because effective nuclear charge increases but number of shells remain the same.

Class X – Periodic Classification of Elements (PYQs) – Solutions