• Periodic Classification of Elements

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NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements

Question1.
Did Doberiener triads also exist in the columns of Newlands Octaves? Compare and find out?

Answer:Yes, some of the Doberiener triads did exist in the columns of .Newlands Octaves. For example

Question2.
What were the limitations of Doberiener Triads?
Answer: Limitations of Dobereiner Triad
  • All known elements could not be classified into groups of triads on the basis of their properties
  • Not all groups obeyed the Law of Triads
Question3.
What were the limitations of Newlands Law of Octaves?
Answer: Limitations of Newlands Law of Octaves
  • The law of octaves was found to be applicable only up to calcium. It was not applicable to elements of higher atomic masses
  • He thought that only 56 elements existed in nature and no more elements were discovered in future. But the discovered elements were not accommodated in the octaves
  • In order to fit elements into octaves, Newland adjusted two elements in the same slot which were not similar.
Question4.
Use Mendeleev periodic table to predict the formulae for the oxides of the elements: K, C, Al, Si, Ba?

Answer:Oxygen is a member of group VIA in Mendeleev periodic table. Its valency is 2. Similarly, the valencies of all the elements listed can be predicted from their respective groups. This can help in writing the formulae of their oxides

  • Potassium (K) is a member of group IA. Its valency is 1. Therefore, the formula of its oxide is K2O
  • Carbon (C) is a member of group IVA. Its valency is 4. Therefore, the formula of its oxide is C2O4 or CO2
  • Aluminium (Al) belongs to groups IIIA and its valency is 3. The formula of the oxide of the element is Al2O3
  • Silicon (Si) is present in group IVA after carbon. Its valency is also 4. The formula of its oxide is Si2O4 or SiO2
  • Barium (Ba) belongs to group IIA and the valency of the element is 2. The formula of the oxide of the element is Ba2O2 or BaO
Question5.
Besides gallium, which two other elements have since been discovered that fill the gaps left by Mendeleev in his periodic table?

Answer:Two other elements are scandium (Sc) and germanium (Ge). In their gaps, the elements with names Eka-boron and Eka-silicon were placed

Question6.
What were the criteria used by Mendeleev in creating his periodic table?

Answer:Mendeleev used atomic masses of the elements as the criteria for creating his periodic table. In this table, the elements were arranged in order of increasing atomic masses

Question7.
Why do you think the noble gases are placed in a separate group?

Answer:In Mendeleev Periodic Table, the elements have been arranged in different groups on the basis of valency. For example, the elements placed in group I (IA and IB) have valency equal to one. Same is the case with the elements placed in other groups. Since the noble gas elements He, Ne, Ar, Kr, Xe and Rn have zero valency, they could not find a place in Mendeleev Periodic Table. They have been placed in a separate group called zero group in the periodic table. Please note that the noble gas elements were not a part of the Mendeleev Periodic Table. They were added later on

Question8.
How could the Modern Periodic Table remove various anomalies of Mendeleev Periodic Table?

Answer:Modern Periodic Table or Long Form of Periodic Table: In the Modern Periodic Table, the elements are arranged in increasing order of their atomic numbers. The Modern Periodic Table was formulated by Henry Moseley in 1913.
Modern Periodic Law: It states that "the properties of elements are a periodic function of their atomic numbers

Main Features of Modern Periodic Table
  • The elements are arranged according to their electronic configurations
  • There are seven periods, numbering 1 to 7. The period number signifies the number of electron shells of an element. Thus, the period one elements have only one shell. The period two elements have only two shells, and so on. Period 1 is the shortest period with only two elements. Periods 2 and 3 are short periods with eight elements each. Periods 4 and 5 are long periods
Question9.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Answer:Magnesium (Mg) belongs to group 2 known as Alkaline Earth Family. The two other elements belonging to the same group are calcium (Ca) and strontium (Sr). The basis of choice is the electronic distribution in the valence shell of these elements. All of them have two electrons each

For example,
k L M N O
Mg (Z =12) 2 8 2 - -
Ca (Z = 20) 2 8 8 2 2
Sr (Z = 38) 2 8 18 8 2
Question10.
  • three elements that have a single electron in their outermost shells
  • three elements that have two electrons in their outermost shells
  • three elements with filled outermost shells
Answer:
  • Lithium, sodium, potassium (alkali metals)
  • Beryllium, magnesium, calcium (alkaline earth metals)
  • Helium, neon, argon (noble gases)
Question11.
  • Lithium, sodium and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements
  • Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common
Answer:
  • The atoms of all these elements have one electron each in their valence shell. That is why, these elements are placed in the group 1 known as alkali metal group. The electronic configurations of these elements are
k L M N
Li (Z = 3) 2 1 1 - -
Na (Z = 11) 2 8 1 -
K (Z = 19) 2 8 8 1

All three elements evolve hydrogen gas on reacting with water.
2Li + 2H2O → 2LiOH + H2
2Na + 2H2O → 2NaOH + H2
2K+ 2H2O → 2KOH + H2
Apart from this, all the elements happen to be the first elements of their respective periods. For example,

  • Second period starts from lithium (Li)
  • Third period starts from sodium (Na)
  • Fourth period starts from potassium (K).

Both elements have completely filled shells.
Helium (Z = 2) has two electrons in the only shell (K shell). The other element neon (Z = 10) has both K and L shells fully filled (2, 8). Because of the filled shells, the atoms of these elements do not have any desire to take part in chemical combination and they have been placed together in the same group known as group 18 or zero group

Question12.
In the Modern Periodic Table, which are the metals among the first ten elements?

Answer:Metals among the first ten elements are lithium (Li) and beryllium (Be). These are placed towards the left of the table

Question13.
By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristics?
Ga, Ge, As, Se, Be

Answer:Before identifying the metallic character from the list of the elements, we must remember two points

  • The metallic character of an element is related to the electron releasing tendency of its atoms. Greater the tendency, more will be the metallic character
  • In general, metallic character of the elements increases down the group and decreases along a period. With the help of the Modern

Chapter End Questions

Question1.
Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic table?
  • The elements become less metallic in nature
  • The number of valence electrons increases
  • The atoms lose their electrons more easily
  • The oxides become more acidic
Answer:
  • The atoms lose their electrons more easily
Question2.
Element X forms a chloride with the formula XCI, which is a solid with high melting point. X is most likely to be in the same group of the periodic table as?
  • Na
  • Mg
  • Al
  • Si
Answer:
  • Mg
Question3.
Which element has?
  • two shells, both of, which are completely filled with electrons
  • the electronic configuration 2, 8, 2
  • a total of three shells with four electrons in the valence shell
  • a total of two shells with three electrons in the valence shell
  • twice as many electrons in the second shell - as in the first shell
Answer:
  • The elements with completely filled shells are noble gas elements and they belong to group 18. Since the element has two shells, it must be present in second period and it is neon (Ne) with electronic configuration 2, 8. Periodic Table, let us identify the group and period to which these elements belong Since the metallic character increases down a group and decreases along a period, the obvious choice is between two elements. These are Be (beryllium) present in group 2 and Ga (gallium) present in group 13
Period 1 3 4
Group 2 Be - -
Group 13 - - Ga
Group 14 - - Ge
Group 15 - - As
Group 16 - - Se
  • The electronic configuration suggests that the element belongs to third period and second group. It is, therefore, magnesium
  • The element with three shells is present in third period. Since it has four electrons in the valence shell, it must belong to group 14 and is silicon (Si) with electronic configuration 2, 8, 4
  • The element with two shells is expected to be present in the second period. With three electrons in the valence shell, it must belong to group 13 and is boron (B) with electronic configuration
  • The element has only two shells. The first shell can have a maximum of two electrons. The second shell has four electrons which is twice the number of electrons present in the first shell. Therefore, the electronic configuration of element is 2, 4. It is carbon with atomic number (Z) equal to 6
Question4.
  • Which property do all elements in the same column of the periodic table as boron have in common
  • Which property do all elements in the same column of the periodic table as fluorine have in common
Answer:
  • All elements in the same column of the Periodic Table as that boron belong to Group 13 and thus have three valence electrons
  • All these elements belong to Group 17 of the . Periodic Table and thus have seven valence
Question5.
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? (Atomic numbers are given in parentheses). N(7), F(9), P(15), Ar(18)
Answer:
  • The atomic number of the element is 17 (2 + 8 + 7 = 17)
  • It would be chemically similar to fluorine (F) which has also 7 electrons in its valence shell (2, 7)
Question6.
The position of three elements A, B and C in the periodic table are shown below?
Group 16 Group 17
- -
- A
- -
B C
  • State whether A is a metal or a non-metal
  • State whether C is more reactive or less reactive than A
  • Will C be larger or smaller in size than B
  • Which type of ion, cation or anion, will be formed by the element A
Answer:
  • Group 17 represents the halogen family. All the elements included in the family are non-metals. Therefore, element A is a non-metal
  • Reactivity of non-metals is generally due to the electron accepting tendency of their atoms. Down the group, the atomic size increases. Therefore; the attraction of the nucleus for the outside decreases. This means that down the group of non-metals, reactivity decreases. Thus, element C is less reactive than element A
  • Atomic size of the elements decreases along a period. The elements B and C are present in the same period. Since C is placed after B, the size of the element C is less than that of B
  • The element A, as pointed out earlier is a non-metal which belongs to group 17. It has seven valence electrons (2, 8, 7). In order to have the configuration of the nearest noble gas element, it will take up one electron and change to anion, i.e. A-ion
Question7.
Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write their electronic configuration. Which of these will be more electronegative and why?

Answer:The electronic configurations of the two elements are nitrogen (Z = 7) 2, 5; Phosphorus (Z = 15) 2,8, 5.

Since the size of nitrogen is small as compared to phosphorus, it has a greater tendency to take up electrons. It is therefore, more electro-negative than phosphorus

Question8.
How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Answer:The Modern Periodic Table is based on atomic numbers of the elements. Since electronic configurations of the elements depend upon their atomic numbers, this means that the periodic table is based on the electronic configurations of the elements. For example, all the alkali metals have one electron each in their valence shell. These are placed in group 1. Similarly, the alkaline earth metals with two electrons in their valence shell are placed in group 2 and so on

Question9.
In the Modern Periodic Table, calcium (Z = 20) is surrounded by the elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Answer:Only those elements are placed in the same group in which the gaps of atomic numbers are: 8, 8, 18, 18, 32. If we look at the atomic numbers of the elements that are listed, it becomes clear that the elements with atomic numbers 12, 20, 38 fit into this pattern. They are placed in the same group and have similar physical and chemical properties

Question10.
Compare and contrast the arrangement of elements in Mendeleev Period Table and the Modern Periodic Table?

Answer:The main points of distinction between Mendeleev Periodic Table and Modern Periodic Table are

Mendeleev Periodic Table Modern Periodic Table
  • It regards the atomic masses of the elements as the basis of classification
    		•
  • It regards the atomic number of the elements as the basis of classification
    		•
  • No separate positions or slots have been allotted to the isotopes of an element since they have different atomic masses
    		•
  • There is no need for separate slots for the isotopes since they have the same atomic numbers
    		•
  • No justification is made for placing hydrogen in group IA along with alkali metals
    		•  Justification has been made for placing hydrogen along with alkali metals in group 1 since both hydrogen and alkali metals have one valence electron
  • Except for the elements, in group VIII, the remaining groups have been divided with sub-groups A and B
    		•
  • There are no subgroups and all groups are independent in nature
    		•
  • Position of certain elements based on their atomic masses are misfits. For example, the element cobalt (atomic mass = 58.9) has been placed ahead of nickel (atomic mass = 58.7)
    		•
  • Modern Periodic Table is frete from such anomalies. The element cobalt is placed before nickel since its atomic number (27) is more than that of nickel (28)
    		•
  • Electronic configurations and properties of the elements cannot be predicted from their positions in the table
    		•
  • Both electronic configurations and certain properties of the elements can be predicted from their positions in the periodic table
    		•
  • It is not very systematic and is difficult to remember
    		•
  • It is quite systematic and is easy to remember
    		•

    Important Question

    NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements Important Questions with Answers

    Short Answer Type Questions

    Question1.
     The three elements A, B and C with similar properties have atomic masses X, Y and Z, respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called? Give one example of such a set of elements?
    Year of Question :(2014 D)

    Answer: Dobereiner proposed a Triad of elements. He arranged elements in triads. In it, the atomic mass of the middle element is equal to the average atomic mass of the other two elements. Such an arrangement of elements is called a triad.
    For example, the Atomic mass of Lithium, Sodium and Potassium are 6.9, 23.0 and 39.0, respectively. The average mass of Li and K is approximately 23.0, equal to the atomic mass of Na. Thus, Lithium, Sodium and Potassium make a triad

    Question2.
    Elements have been arranged in the following sequence based on their increasing atomic masses.
    F, Na, Mg, AI, Si, P, S, CI, Ar, K?
    Year of Question :(2014 D)
    • (a) Pick two sets of elements with similar properties
    • (b) The given sequence represents which law of classification of elements
    Answer:
    • (a) (i) F and Cl (ii) Na and K have similar properties
    • (b) It represents Newlands law of octaves
    Question3.
     Can the following groups of elements be classified as Dobereiners triad?
    Year of Question :(2014 )
    • (a) Na, Si, CI
    • (b) Be, Mg, Ca Atomic mass of Be 9; Na 23; Mg 24; Si 28; C| 35; Ca 40 Explain by giving a suitable reason
    Answer:
    • (a) Na, Si, and Cl are not a Dobereiners triad. Although the atomic mass of silicon (Si) is the average of atomic masses of sodium (Na) and chlorine (Cl), but these elements do not possess similar properties. Hence, it cant be classified as a Dobereiners triad.
      • 23 (Na) + 35 (Cl) / 2 = 29 (Si)
    • (b) Be, Mg, and Ca is a Dobereiners triad. They have similar properties, and the atomic masses of magnesium (Mg) is approximately the average of the atomic mass of Be and Ca. 9 (Be) + 40 (Ca) / 2 = 24.5 (Si)
    Question4. 
    In Mendeleevs Periodic Table, the elerents were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of 58.93 amu was placed before nickel, having an atomic mass of 58.71 amu. Give a reason for the same?
    Year of Question :(2014)

    Answer: In Mendeleevs Periodic Table, the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of 58.93 amu was placed before nickel, having an atomic mass of 58.71 amu. We did this to ensure that the same group has elements with similar properties. Hence cobalt was placed before nickel despite the higher atomic number of Cobalt than Nickel

    Question5.
      Hydrogen occupies a unique position in Modern Periodic Table". Justify the statement?
    Year of Question :(2014)

    Answer: Hydrogen has an electron configuration of 1s1. It can either accept one electron to get a stable configuration or donate one electron to become stable. Hence the position of hydrogen isnt evident. We can place it either in the metals group as it donates or in the nonmetal group as it accepts an electron. Hence it occupies a unique position in the periodic table

    Question6. 
    Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev?
    Year of Question :(2013)

    Answer: The formulae of chlorides of Eka-silicon and Eka-aluminium are XC4 and XCl3, respectively.

    Question7. 
    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 valencies?
    Year of Question :(2013)

    Answer: A belongs to Group 13, B belongs to Group 14, and C belongs to Group 2. A valency is 3, B valency is 4, and C valency is 2.

    Question8. 
    If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?
    Year of Question :(2012)

    Answer: If an element is placed in group 14, it has 4 electrons in its outermost orbit. Hence, the formula of its chloride is XCl4. It makes compounds by sharing electrons, so its compound will have a covalent bond

    Question9.
     Compare the radii of two species, X and Y. Give reasons for your answer?
    Year of Question :(2013)
    • (a) X has 12 protons and 12 electrons
    • (b) Y has 12 protons and 10 electrons

    Answer: As both X and Y had 12 protons, their atomic number is 12, i.e., magnesium. Y has only 10 electrons, due to which the effect of nuclear charge on the electrons would be more than that of 12 electrons. Hence, the radii of Y would be smaller than that of X

    Question10.
     Arrange the following elements in increasing order of their atomic radii?
    Year of Question :(2013)
    • (a) Li, Be, F, N
    • (b) CI, At, Br, I
    Answer:
    • (a) Li, Be, N, and F are in the same period, and atomic radii decrease from left to right. Thus, the order will be: F < N < Be < Li
    • (b) Cl, Br, I, and At are in the same group, and the atomic radii increase from top to bottom. Thus, the order will be: CI < Br < I < At
    Question11.
     Identify and name the metals from the following elements whose electronic configurations are given below?
    Year of Question :(2013)
    • (a) 2, 8, 2
    • (b) 2, 8, 1
    • (c) 2, 8,7
    • (d) 2, 1
    Answer:
    • (a) Magnesium; It is a metal
    • (b) Sodium; It is a metal
    • (c) Chlorine; It is a nonmetal
    • (d) Lithium; It is a metal
    Question12.
     Write the formula of the product formed when element A (atomic number 19) combines with element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?
    Year of Question :(2012)

    Answer: The products formula when element A (atomic number 19) combines with element B (atomic number 17) is KCl. The nature of the bond between KCl is ionic. Electron Dot Structure of KCl

    Question13.
     Arrange the following elements in the increasing order of their metallic character: Mg, Ca, K, Ge, Ga?
    Year of Question :(2013)

    Answer: Metallic character increases as we move down the group because there is an increase in atomic size. Thus, the order will be: Ge< Ga < Mg < Ca < K

    Question14.
     Identify the elements with the following property and arrange them in increasing order of their reactivity?
    Year of Question :(2013)
    • (a) An element which is a soft and reactive metal
    • (b) The metal which is an important constituent of limestone
    • (c) The metal which exists in a liquid state at room temperature
    Answer:
    • (a) Sodium is soft and reactive
    • (b) Calcium is an important constituent of limestone
    • (c) Mercury exists in a liquid state at room temperature
      • The increasing order of reactivity will be: Hg < Ca < Na.
    Question15.
     Properties of the elements are given below. Where would you locate the following elements in the periodic table?
    Year of Question :(2013)
    • (a) A soft metal stored under kerosene
    • (b) An element with variable (more than one) valency stored underwater
    • (c) An element which is tetravalent and forms the basis of organic chemistry
    • (d) An element which is an inert gas with atomic number 2.
    • (e) An element whose thin oxide layer is used to make other elements corrosion-resistant by anodising
    Answer:
    • (a) Sodium is soft metal stored under kerosene.
    • (b) Phosphorous shows variable (more than one) valency and is stored underwater
    • (c) Carbon is a tetravalent element and forms the basis of organic chemistry
    • (d) Helium is an element which is an inert gas with atomic number 2
    • (e) Aluminium is an element whose thin oxide layer is used to make other elements corrosion-resistant by anodising

    Long Answer Type Questions

    Question1. 
    An element is placed in the 2nd Group and 3rd Period of the Periodic Table. It burns in the presence of oxygen to form a basic oxide?
    Year of Question :(2010)
    • (a) Identify the element
    • (b) Write the electronic configuration
    • (c) Write the balanced equation when it burns in the presence of air
    • (d) Write a balanced equation when this oxide is dissolved in water
    • (e) Draw the electron dot structure for the formation of this oxide
    Answer:
    • (a) Magnesium
    • (b) The electronic configuration of magnesium is 2, 8, 2
    • (c ) 2 Mg + O2→ 2 MgO
    • (d) MgO + H2O → Mg(OH)2
    • (e) Electron Dot Structure of magnesium oxide
    Question2. 
    An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide?
    Year of Question :(2011)
    • (a) Where in the periodic table are elements X and Y placed
    • (b) Classify X and Y as metal (s), non-metal (s) or metalloid (s)
    • (c) What will be the nature of the oxide of element Y? Identify the nature of bonding in the compound formed
    • (d) Draw the electron dot structure of the divalent halide

    Answer:

    • (a) X belongs to Group 17 and 3rd periods, while Y belongs to Group 2 and 4th
    • (b) X is a Nonmetal, while Y is a metal
    • (c) Y will form a basic oxide, and it will have an ionic bondingv
    • (d) Electron dot structure of the divalent halide
    Question3. 
    The atomic number of a few elements are given below 10, 20, 7, 14?
    Year of Question :(2012)
    • (a) Identify the elements
    • (b) Identify the Group number of these elements in the Periodic Table
    • (c) Identify the Periods of these elements in the Periodic Table
    • (d) What would be the electronic configuration for each of these elements
    • (e) Determine the valency of these elements
    Answer:
    • (a) 10 is the atomic number of neon.
      20 is the atomic number of calcium
      7 is the atomic number of nitrogen.
      14 is the atomic number of silicon.
    • (b) Neon belongs to group 18.
      Calcium belongs to group 2.

      Nitrogen belongs to group 15.
      Silicon belongs to group 14
    • (c ) Neon belongs to period 2.

      Calcium belongs to period 4.
      Nitrogen belongs to period 2.
      Silicon belongs to period 3
    • (d) The electronic configuration of neon is 2, 8.

      The electronic configuration of calcium is 2, 8, 8, 2.
      The electronic configuration of nitrogen is 2, 5.
      The electronic configuration of silicon is 2, 8, 4
    • (e) The valency of neon is equivalent to 8 - no of valence electrons, i.e. 8 - 8 = 0.

      The valency of calcium is equivalent to the no of valence electrons, i.e. equal to 2.
      The valency of nitrogen is equivalent to 8 - no of valence electrons, i.e. 8 - 5 = 3.
      The valency of silicon is equivalent to 8 - no of valence electrons, i.e. 8 - 4 = 4
    Question4. 
    Complete the following crossword puzzle (Figure 5.1)?
    Year of Question :(2013)
    • (1) An element with atomic number 12
    • (3) Metal used in making cans and member of Group 14
    • (4) A lustrous non-metal with 7 electrons in its outermost shell. Down:
    • (2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene.
    • (5) The first element of the second Period
    • (6) An element which is used in making fluorescent bulbs and is the second member of Group 18 in the Modem Periodic Table
    • (7) A radioactive element which is the last member of the halogen family
    • (8) Metal is an important constituent of steel and forms rust when exposed to moist air
    • (9) The first metalloid in Modem Periodic Table whose fibres are used to make bullet-proof vests

    Answer:

    • (1) Magnesium has 12 atomic numbers
    • (2) Sodium is a highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene
    • (3) Tin is used in making cans and is a member of Group 14
    • (4) Iodine is a lustrous non-metal with 7 electrons in its outermost shell
    • (5) Lithium is the first element of the second Period
    • (6) Neon is used in making fluorescent bulbs and is the second member of Group 18 in the Modem Periodic Table
    • (7) Astatine is a radioactive element which is the last member of the halogen family
    • (8) Iron is an important constituent of steel and forms rust when exposed to moist air
    • (9) Boron is the first metalloid in Modem Periodic Table whose fibres are used to make bullet-proof vests
    Question5. 
    • (a) In this ladder (Figure 5.2), symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table
    • (b) Arrange them in the order of their group also

    Answer:

    • (a) The arrangement of elements in the increasing order of their atomic number in the Periodic Table. H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, CI, Ar, K, Ca
    • (b) The arrangement of elements in groups.
      Group 1: H, Li, Na, K
      Group 2: Be, Mg, Ca
      Group 13: B, Al
      Group 14: C, Si
      Group 15: N, P
      Group 16: O, S

      Group 17: F. Cl
      Group 18: He, Ne, Ar
    Question6. 
    Mendeleev predicted the existence of certain elements not known at that time and named two of them Eka-silicon and Eka-aluminium?
    Year of Question :(2014)
    • (a) Name the elements which have taken the place of these elements
    • (b) Mention the group and the period of these elements in the Modern Periodic Table
    • (c) Classify these elements as metals, non-metals or metalloids
    • (d) How many valence electrons are present in each of them
    Answer:
    • (a) Eka-silicon was replaced by Germanium, while Gallium replaced Eka-aluminium
    • (b) Gallium belongs to Group 13 and Period 5 of the periodic table, while Germanium belongs to Group 14 and Period 4
    • (c) Gallium is metal, while Germanium is metalloid
    • (d) Gallium has three valence electrons, while Germanium has 4
    Question7. ?
    • a) Electropositive nature of the element(s) increases down the group and decreases across the period
    • (b) Electronegativity of the element decreases down the group and increases across the period
    • (c) Atomic size increases down the group and decreases across a period (left to right)
    • (d) Metallic character increases down the group and decreases across a period. Based on the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.
    • (a) Name the most electropositive element among them
    • (b) Name the most electronegative element among them
    • (c) Name the element with the smallest atomic size
    • (d) Name the element which is a metalloid
    • (e) Name the element that shows maximum valency

    Answer:

    • (a) The most electropositive element among them is Lithium
    • (b) The most electronegative element among them is Fluorine
    • (c) The element with the smallest atomic size is Fluorine
    • (d) Boron (5) is a metalloid
    • (e) Carbon shows maximum valency
    Question8. 
    An element X, a yellow solid at room temperature, shows catenation and allotropy. X forms two oxides formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants?
    Year of Question :(2016)
    • (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 be the nature (acidic/ basic) of oxides formed
    • (e) Locate the position of the element in the Modem Periodic Table
    Answer:
    • (a) The element X is Sulphur
    • (b) The electronic configuration of X is 2, 8, 6
    • (c) Thermal decomposition of ferrous sulphate: 2 FeSO4 → Fe2O3 + SO2 + SO3
    • (d) Oxides of Sulphur are acidic
    • (e) Sulphur belongs to Group 16 and the third period of the periodic table
    Question9. 
    An element X of group 15 exists as a diatomic molecule and combines with hydrogen at 773 K in the presence of the catalyst to form a compound, ammonia, which has a characteristic pungent smell?
    Year of Question :(2017)
    • (a) Identify the element X. How many valence electrons does it have
    • (b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it
    • (c) Draw the electron dot structure for ammonia, and what type of bond is formed in it
    Answer:
    • (a) The element X is Nitrogen. It has five valence electrons
    • (b) The electron dot structure of the diatomic molecule of X.
        Dinitrogen forms a covalent bond
    • (c ) The electron dot structure of the ammonia.
      Ammonia forms a covalent bond
    Question10.
     Which group of elements could be placed in Mendeleevs Table without disturbing the original order? Give reason?
    Year of Question :(2013)

    Answer: Noble gases could be placed in Mendeleev.s periodic table without disturbing the original order.
    Noble gases are inert due to a wholly filled valence electron shell. They are present in low concentrations in our atmosphere and do not form any compound with other elements. During the evolution of the periodic table by Mendeleev, these gases were not known. After their discovery, they were placed in a separate group as their properties did not resemble that of any other group in the periodic table. So, their existence did not affect the existing order of Mendeleevs periodic table

    Question11.
     Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at Periodic Law?
    Year of Question :(2012)

    Answer: Mendeleev tried to classify the elements based on their chemical properties to ease the study of elements. He wrote the properties of each element on a separate card and arranged them in diverse ways. He put all 63 elements in the order of their increasing atomic masses in horizontal rows called periods. Elements with similar properties were placed below the other in the same vertical column called groups. There was a total of seven periods and eight groups. The classification was based on resemblances in physical and chemical properties and the compounds formed by elements with hydrogen and oxygen. He noticed that elements of similar properties would repeat at regular intervals (8th, 18th, or 32nd position). Then, he stated it as the periodic law, i.e. The properties of elements are a periodic function of their atomic masses

    Important Questions and Answers from Chapter:Periodic Classification of Elements

    Question 1:
    What is Döbereiner’s Law of Triads?

    Answer:
    Johann Wolfgang Döbereiner (German scientist) introduced the "Law of Triads" in the early 19th century.
    It states that when three elements with similar properties are arranged in increasing order of their "atomic weights", the atomic weight of the middle element is the average of the other two.
    Example:
    The triad of Lithium (Li), Sodium (Na), and Potassium (K):
    Atomic weight of Sodium ≈ (Atomic weight of Lithium + Atomic weight of Potassium) ÷ 2.

    Question 2:
    What were the limitations of Döbereiner’s Triads?

    Answer:
    Only a few elements could be grouped into triads.
    Several known elements didn’t fit into any triad.
    Failed to classify all elements known at that time.

    Question 3:
    Explain Newlands’ Law of Octaves.

    Answer:
    John Newlands, a British scientist, proposed the "Law of Octaves" in 1864.
    Main Points:
    When elements are arranged in the order of "increasing atomic weights", the properties of every eighth element are similar to the first, like musical notes.
    He compared it to musical octaves where every eighth note is similar.
    Limitations:
    The law worked only up to Calcium.
    Newlands placed elements with different properties in the same group (e.g., Iron with Oxygen and Sulfur).

    Question 4:
    What is Mendeleev’s Periodic Law? What were its achievements?

    Answer:
    Mendeleev’s Periodic Law:
    Dmitri Ivanovich Mendeleev, a Russian chemist, proposed this law in 1869.
    The "properties of elements are a periodic function of their atomic masses".
    Achievements:
    Prediction of new elements: Mendeleev left gaps in his table for undiscovered elements and predicted their properties (e.g., Eka-aluminium was later discovered as Gallium).
    Correction of atomic weights: Mendeleev corrected the "atomic weights" of some elements like Beryllium.
    Grouping of similar elements: Elements with similar properties were grouped together, making their study easier.

    Question 5:
    What were the limitations of Mendeleev’s Periodic Table?

    Answer:
    Position of hydrogen: It could not be clearly assigned to a specific group because hydrogen behaves like both alkali metals and halogens.
    No space for isotopes: Isotopes (same element with different atomic masses) were not given separate positions.
    Irregular atomic mass: Some heavier elements were placed before lighter ones (e.g., Cobalt before Nickel).
    Uncertainty for elements: In some cases, the increase in atomic weights between elements was not regular, making it hard to predict the number of elements between them.

    Question 6:
    Explain Moseley’s Modern Periodic Law.

    Answer:
    Henry Moseley, a British physicist, proposed the "Modern Periodic Law" in 1913.
    It states that "the physical and chemical properties of elements are periodic functions of their atomic numbers".
    Key Points:
    Moseley used X-rays to study elements and found that "atomic number", not atomic weight, is the fundamental property.
    This law led to the arrangement of elements by increasing "atomic number" in the "Modern Periodic Table".

    Question 7:
    What is the Modern Periodic Table? How is it structured?

    Answer:
    Structure:
    Developed by Bohr and based on Moseley’s work.
    Elements are arranged in increasing order of "atomic numbers".
    It has 18 vertical columns called "groups" and 7 horizontal rows called "periods".
    Key Features: Elements in the same "group" have similar properties (e.g., all elements in Group 1 are alkali metals).
    The table includes both naturally occurring elements (1 to 92) and synthetic elements (93 to 118).

    Question 8:
    What are the periodic properties of elements?

    Answer:
    Valency:
    Valency refers to the number of electrons in the outermost shell.
    Increases from 1 to 4 across a "period", then decreases to 0.
    Atomic Size:
    Atomic radius increases down a "group" (due to added shells).
    Decreases across a "period" (due to increased nuclear charge pulling electrons closer).
    Metallic and Non-metallic Properties:
    Metals are on the left side of the periodic table and lose electrons to form positive ions.
    Non-metals are on the right side and gain electrons to form negative ions.
    Ionization Energy:
    Energy required to remove an electron from an atom.
    Decreases down a "group" and increases across a "period".
    Electron Affinity:
    Energy released when an atom gains an electron.
    Increases across a "period", decreases down a "group".
    Electronegativity:
    Ability of an atom to attract shared electrons in a bond.
    Increases across a "period", decreases down a "group".

    Question 9:
    How does atomic size change across a period and down a group?

    Answer:
    Across a period:
    Atomic size decreases as the nuclear charge increases, pulling electrons closer to the nucleus.
    Down a group:
    Atomic size increases because new electron shells are added, increasing the distance between the nucleus and outermost electrons.

    Question 10:
    Why were inert gases placed in a separate group in the periodic table?

    Answer:
    Inert gases (e.g., Helium, Neon) have fully filled outermost electron shells.
    They are "chemically unreactive" and do not readily form compounds.
    A separate group (Group 18) was created for these gases in both Mendeleev’s and the modern periodic table.

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