Class 10 Science Chapter 5 Periodic Classification of Elements

NCERT Solutions For Class 10 Science Chapter 5 Periodic Classification of Elements. Question Answer.

Question 1.

Did Doberiener’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Yes, some of the Doberiener’s triads did exist in the columns of .Newlands’ Octaves. For example,
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 1

Question 2.
What were the limitations of Doberiener’s Triads?
•  Limitations of Dobereiner’s Triad

  1. All known elements could not be classified into groups of triads on the basis of their properties.
  2. Not all groups obeyed the Law of Triads.

Question 3.
What were the limitations of Newlands’ Law of Octaves?
•  Limitations of Newlands’ Law of Octaves

  1. The law of octaves was found to be applicable only up to calcium. It was not applicable to elements of higher atomic masses.
  2. 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.
  3. In order to fit elements into octaves, Newland adjusted two elements in the same slot which were not similar.

Question 4.
Use Mendeleev’s periodic table to predict the formulae for the oxides of the elements: K, C, Al, Si, Ba.
Oxygen is a member of group VIA in Mendeleev’s 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 K 2O.
  •  Carbon (C) is a member of group IVA. Its valency is 4. Therefore, the formula of its oxide is C 2O 4 or CO 2.
  •  Aluminium (Al) belongs to groups IIIA and its valency is 3. The formula of the oxide of the element is Al 2O 3.
  •  Silicon (Si) is present in group IVA after carbon. Its valency is also 4. The formula of its oxide is Si 2O 4 or SiO 2.
  •  Barium (Ba) belongs to group IIA and the valency of the element is 2. The formula of the oxide of the element is Ba 2O 2 or BaO.

Question 5.
Besides gallium, which two other elements have since been discovered that fill the gaps left by Mendeleev in his periodic table?
Two other elements are scandium (Sc) and germanium (Ge). In their gaps, the elements with names Eka-boron and Eka-silicon were placed.

Question 6.
What were the criteria used by Mendeleev in creating his periodic table?
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.

Question 7.
Why do you think the noble gases are placed in a separate group?
In Mendeleev’s 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’s 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’s Periodic Table. They were added later on.

Question 8.
How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
• 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

Question 9.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
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,

Mg (Z =12) 2 8 2
Ca (Z = 20) 2 8 8 2 2
Sr (Z = 38) 2 8 18 8 2

Question 10.

  1. three elements that have a single electron in their outermost shells.
  2. three elements that have two electrons in their outermost shells.
  3. three elements with filled outermost shells.


  1.  Lithium, sodium, potassium (alkali metals)
  2.  Beryllium, magnesium, calcium (alkaline earth metals)
  3.  Helium, neon, argon (noble gases).

Question 11.
(a) 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?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
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:

Li (Z = 3) 2 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 + 2H 2O ➝ 2LiOH + H 2
2Na + 2H 2O ➝ 2NaOH + H 2
2K+ 2H 2O ➝ 2KOH + H 2
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.

Question 12.
In the Modern Periodic Table, which are the metals among the first ten elements?
Metals among the first ten elements are lithium (Li) and beryllium (Be). These are placed towards the left of the table.

Question 13.
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
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

Question 1.
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?
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
(c) The atoms lose their electrons more easily.

Question 2.
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:
(a) Na
(b) Mg
(c) Al
(d) Si
(b) Mg

Question 3.
Which element has:
(a) two shells, both of, which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells with four electrons in the valence shell?
(d) a total of two shells with three electrons in the valence shell?
(e) twice as many electrons in the second shell – as in the first shell?
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 (Mg).
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 2, 3.
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.

Question 4.
(a) Which property do all elements in the same column of the periodic table as boron have in common?
(b) Which property do all elements in the same column of the periodic table as fluorine have in common?
(a) All elements in the same column of the Periodic Table as that boron belong to Group 13 and thus have three valence electrons.
(b) All these elements belong to Group 17 of the . Periodic Table and thus have seven valence

Question 5.
An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) 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)
(a) The atomic number of the element is 17 (2 + 8 + 7 = 17).
(b) It would be chemically similar to fluorine (F) which has also 7 electrons in its valence shell (2, 7).

Question 6.
The position of three elements A, B and C in the periodic table are shown below:

Group 16 Group 17


  1.  State whether A is a metal or a non-metal.
  2.  State whether C is more reactive or less reactive than A.
  3.  Will C be larger or smaller in size than B?
  4.  Which type of ion, cation or anion, will be formed by the element A?


  1.  Group 17 represents the halogen family. All the elements included in the family are non-metals. Therefore, element A is a non-metal.
  2.  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.
  3.  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.
  4.  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.

Question 7.
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?
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.

Question 8.
How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
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.

Question 9.
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?
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.

Question 10.
Compare and contrast the arrangement of elements in Mendeleev’s Period Table and the Modern Periodic Table.
The main points of distinction between Mendeleev’s Periodic Table and Modern Periodic Table are:

Mendeleev’s Periodic Table Modern Periodic Table
1. It regards the atomic masses of the elements as the basis of classification. 1. It regards the atomic number of the elements as the basis of classification.
2. No separate positions or slots have been allotted to the isotopes of an element since they have different atomic masses. 2. There is no need for separate slots for the isotopes since they have the same atomic numbers.
3. No justification is made for placing hydrogen in group IA along with alkali metals. 3. Justification has been made for placing hydrogen along with alkali metals in group 1 since both hydrogen and alkali metals have one valence electron.
4. Except for the elements, in group VIII, the remaining groups have been divided with sub-groups A and B. 4. There are no subgroups and all groups are independent in nature.
5. 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) 5. 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).
6. Electronic configurations and properties of the elements cannot be predicted from their positions in the table. 6. Both electronic configurations and certain properties of the elements can be predicted from their positions in the periodic table.
7. It is not very systematic and is difficult to remember. 7. It is quite systematic and is easy to remember.

NCERT Solutions