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

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

Class 10 Science Chapter 5 Periodic Classification of Elements InText Questions and Answers

In-text Questions (Page 81)

Question 1.
Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
Answer:
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 1
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 2

Question 2.
What were the limitations of Dobereiner’s classification?
Answer:

  • Concept of triads could be applied only to a limited number of elements.
  • Dobereiner could identify only three triads from the elements known at that time. So this system was not found to be useful.

Question 3.
What were the limitations of Newlands’ Law of Octaves?
Answer:

  1. It was found that Law of Octaves was applicable only, up to calcium.
  2. It was assumed by Newland that only 56 elements existed in nature and no more elements would be discovered in the future. But, later on, a no. of new elements were discovered, whose properties did not fit into the Law of Octaves.
  3. In order to fit elements into his table, Newlands adjusted two elements in the same slot, but put also some unlike elements under the same note. For example iron, which resembles cobalt and nickel in properties, had been placed far away from these elements.

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

In-text Questions (Page 85)

Question 1.
Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.
Answer:
KO2, CO2, Al2O3, SiO2, Ba2O

Question 2.
Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table?
Answer:
Scandium, germanium.

Question 3.
What were the criteria used by Mendeleev in creating his Periodic Table?
Answer:

  • Mendeleev proposed that the physical and chemical proportion of an elements are the periodic function of then atomic masses.
  • The formulae of the hydrides and oxides formed by element were treated as one of the basic properties of an element for its classification.

Question 4.
Why do you think the noble gases are placed In a separate group?
Answer:
Inert gases like He, Ne, Ar, Kr, Xe were discovered very late because they are very invert and present in extremely low concentrations in our atmosphere. One of the strengths of Mendeleev’s periodic table was that, when these gases were discovered they could be placed in a new group without disturbing the existing order. Because inert gases have 2, or 8 electrons in their outer most shell and do not reach with other elements in normal conditions. So a new group can only justify their properties.

In-text Questions (Page 90)

Question 1.
How could the Modem Periodic Table remove various anomalies of Mendeleev’s Periodic Table?
Answer:
The Modern Periodic Table is based on the atomic number which is more fundamental property of the elements. The Modern Periodic Table rectify the various anomalies of Mendeleev’s Periodic Table; which are given below.

  • Since this classification is based on the atomic number, the position of placing isotopes at one place is justified.
  • It separate metals and non-metals and metalloids clearly.
  • The position of some elements which were misfit on the basis of atomic mass is justified on the basis of atomic no. For example, argon proceeds potassium because argon has atomic number 18 and potassium has 19.
  • The lanthanoids and actinoids which have properties different from other groups are placed separately at the bottom of the periodic table.
  • It so removed the other anomalies found in Mendeleev’s periodic table such as grouping of the chemically dissimilar elements, and separation of the chemically similar elements.
  • It separates very soft elements (Na, K…) from very hard elements (Cu, Ag, Au).
  • This periodic table does not has sub-groups but it has only groups starting from 1 to 18.

Question 2.
Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis of your choice.
Answer:
Calcium and Stronsium. Because they belongs to same group. Group no. 2.

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

Question 3.
Name
(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Answer:
(a) Li (3) = 2, 1
Na (11) = 2, 8, 1
K (19) = 2, 8, 8, 1

(b) Be (4) = 2, 2
Mg (12) = 2, 8, 2

(c) He (2) = 2
Ne (10) = 2, 8
Ar (18) = 2, 8, 8

Question 4.
(a) Lithium, sodium, 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?
Answer:
(a) Atoms of lithium, sodium, and potassium has one electron in their outermost or valence shell.
(b) The atoms of ‘He’ and ‘Ne’ have complete octet. That is why they are inert.

Question 5.
In the Modem Periodic Table which are the metals among the first ten elements?
Answer:
Lithium (Li)
Beryllium (Be)

Question 6.
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:
Be.

Class 10 Science Chapter 5 Periodic Classification of Elements Textbook Questions and Answers

Page No. 91

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 becomes more acidic.
Answer:
(c) The atoms lose their electrons more easily.

Question 2.
Element X forms a chloride with the formula XCI2, which is a solid with a high melting point. X would most likely be in the same group of the periodic table as.
(a) Na
(b) Mg
(c) Al
(d) Si
Answer:
(b) Mg

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

Question 3.
Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electrons configuration is 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Answer:
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 3

Question 4.
(a) What property do all elements in the same column of the periodic table as boron have in common?
(b) What property do all elements in the same column of periodic table as fluorine have in common?
Answer:
(a) Boron belongs to group no. 13 of the modern periodic table. All the elements of this group has similar valency as boron has. The valency of Boron in Z, because it has three electrons in its valency shell. So all the elements of G-13 have three valency.

(b)Valency of all the elements of the same group will be same. So all the elements of this group shows one valence like fluorine.

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 number are given in parentheses).
N (7), F (9), F (15), Ar (18)
Answer:
(a) 17
(b) F (9)

Question 6.
The position of three elements A, B, and C in the periodic table are shown below:
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 4
(a) State whether ‘A’ is a metal or non-metal.
(b) State whether ‘C’ is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer:
(a) ‘A’ is a non-metal because group no. 17 is present in the right side of periodic table and in right side of the periodic. Table non-metals are present.
(b) ‘A’ is more reactive than ‘C’ because ‘A’ has higher position in the group so it has high electronegativity than C
(c) ‘C’ will be smaller in size than B because across the period from left to right (G-16 to G-17) size decreases.
(d) A form anion (A) due to its high el ectronegativity.

Question 7.
Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Answer:
N (7) = 2, 5
P (15) = 2, 8, 5
Nitrogen is more electronegative than phosphorus because the size of nitrogen is smaller than phosphorus. So nitrogen has high effective nuclear charge. The electron negativity depend upon size and effective nuclear charge. Smaller the size higher will be the electronegativity and higher the effective nuclear charge, again in favour of high electronegative.

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

Question 8.
How does the electronic configuration of an atom relate to its position in the Modem Periodic Table?
Answer:
Modern Periodic Table is based on the atomic number or electronic configuration of elements. The elements of a group have same electronic configuration of the outer most valence shell. After a certain interval repetition of similar outer electronic configuration of elements of atoms causes periodicity.
e.g.,
Li (3) = 2, 1
Na (11) = 2, 8, 1
K (19) = 2, 8, 8, 1
Rb (37) = 2, 8, 18, 8, 1
Cs (55) = 2, 8, 18, 18, 8, 1
Fr (87) = 2, 8, 18, 18, 32, 8, 1
Thus, it is because of similarity in electronic configuration that all the elements have similar properties, and all the atom have the same no. of electrons in their valence shell so they belong to the same group. So the electronic configuration of an atom justify its position in the periodic table.

Question 9.
In the Modem Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12,19,21 And 38. Which of these have physical and chemical properties Resembling calcium?
Answer:
The electronic configuration of ‘Ca’ (20) = 2, 8, 8, 2
The electronic configuration of elements with atomic numbers 12, 19, 21 and 38 are as follows:
= 2, 8, 2
= 2, 8, 8, 1
= 2, 8, 8, 3
= 2, 8, 18, 8, 2
The electronic configuration of elements with atomic numbers 12 and 38 is similar to calcium. So the physical and chemical properties of these elements is similar to calcium.

Question 10.
Compare and contrast the arrangement of elements in mendeleev’s periodic table and the Modem Periodic Table.
Answer:
Comparison in Mendeleev’s Periodic Table and the modern Periodic Table:

  • Both the tables gives a systematic study of elements.
  • In both the table elements are arranged in rows and columns.
  • The columns are known as groups and the rows are known as periods.
  • Both the tables have metals in left hand side and non-metals in right hand side.
  • In both the tables after a certain interval the elements of similar properties are arranged in a same group and dissimilar elements are separated.
  • The variation in the valence of elements with respect to hydrogen and oxygen across the period remains same for both the periodic tables.
  • The metallic character decreases across the period from left to right and increases down the group in a similar way in both the periodic tables.

Contrast in Mendeleev’s and Modem Periodic Table.

Mendeleev’s Periodic table Periodic Table
1. It is based on atomic mass of the elements. 1. It is based on atomic numbers of the elements.
2. It has eight groups. 2. It has 18 groups.
3. The position of the hydrogen is not justified. 3. The position of hydrogen has been justified.
4. There are subgroups (a and b) in each group. 4. Groups are not further subdivided into subgroups.
5. This is quite difficult to reproduce. 5. This is simple and easy to reproduce.
6. No separate position is allotted to the Lanthanoides and actinoides. 6. Lanthaniods and actinoids are kept in two separate rows and placed under the periodic table.

Class 10 Science Chapter 5 Periodic Classification of Elements Textbook Activities

Activity 5.1 (Page 84)

  • Looking at its resemblance to alkali metals and the halogen family, try to assign hydrogen a correct position in Mendeleev’s Periodic Table.

Question 1.
To which group and period should hydrogen be assigned?
Answer:
Electronic configuration of hydrogen resemblances of that alkali metals. Like alkali metals hydrogen combines with halogens (F, Cl, Br, …….) Oxygen and sulphur to form compounds having similar formula i.g. HCl, H2O, H2S like NaCl, Na2O, Na2S.

On the other hand. just, like halogens, hydrogen also exists as diatomic molecules and it combines with metals and non-metals to from covalent compounds.

Certainly, no fixed position can be given to hydrogen in the periodic table. This was the first limitation of Mendelev’s periodic table. He could not assign a correct position to hydrogen in his table.

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

Activity 5.2 (Page 85)

  • Consider the isotopes of chlorine, Cl-35 and Cl-37

Question 1.
Would you place them in different slots because their atomic masses are different?
Or would you place them in the same position because their chemical properties are the same?
Answer:
According to Mendeelev’s periodic table elements of different atomic masses should be placed at different positions.

Because the chemical properties of Cl-35 and Cl-37 are same so we would like to place them in the same position in the periodic table.

Activity 5.3 (Page 85)

Question 1.
How were the positions of cobalt and nickel resolved in the Modern Periodic Table?
Answer:
The Mendeleev’s Periodic Table contain eight groups. In eighth group, there are there rows of elements namely Fe, Co, Ne and Ru, Rh, Pd and Os, Ir, Pt. In modern periodic table these three rows has been converted into three groups. The modern periodic table is based upon atomic no. not upon atomic mass. So the problem of atomic mass of Co, and Ni resolved by considering their atomic number.

Question 2.
How were the positions of isotopes of various elements decided in the Modem Periodic liable?
Answer:
The positions of isotopes of various elements decided in the periodic table by considering their atomic no. in place of atomic mass because isotopes of an element have same atomic no. So they are placed at the same position in the periodic table.

Question 3.
Is it possible to have an element with atomic number 1.5 placed between hydrogen and helium?
Answer:
No, it is not possible that an element with atomic mass 1.5 be placed between hydrogen and helium.

Question 4.
Where do you think should hydrogen be placed in the Modem Periodic Table?
Answer:
The modem periodic table is based on the atomic number of the element. Hydrogen shows resemblance with first group elements (Li, Na, K……) and 17 group elements (F, Cl, Br. ) also. So it is better that hydrogen should be placed with alkali metals on the basis of its electronic configuration. But the best way in this, that it should be placed at a separate portion in the periodic table neither with alkali metals nor with halogens.

Activity 5.4 (Page 87)

Question 1.
Look at the group 1 of the Modem Periodic Table, and name the elements present in it.
Answer:
H, Li, Na, K, Rb, Cs

Question 2.
Write down the electronic configuration of the first three elements of group J.
Answer:
H = 1
Li = 2, 1
Na = 2, 8, 1

Question 3.
What similarity do you find in their electronic configurations?
Answer:
All the elements have equal no. of electrons m their outer most shell.

Question 4.
How many valence electrons are present in these three elements?
Answer:
One.

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

Activity 5.5 (Page 87)

Question 1.
If you look at the Modern Periodic table, you will find that the elements Li, Be, B, C, N, O, E, and Ne are present in the second period. Write down their electronic configurations.
Answer:
Li (3) = 2, 1
Be (4) = 2, 2
B (5) = 2, 3
C (6) = 2, 4
N (7) = 2, 5
O (8) = 2, 6
F (9) = 2, 7
Ne (10) = 2, 8

Question 2.
Do these elements also contain the same number of valence electrons?
Answer:
No, these elements contain different no. of valence electrons.

Question 3.
Do they contain the same number of shells?
Answer:
Yes they contain the same no. of shells, two.

Activity 5.6 (Page 88)

  • How do you calculate the valency of an element from its electronic configuration?
  • What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?
  • Similarly find out the valencies of the first twenty elements.
  • How does the valency vary in a period on going from left to right?
  • How does the valency vary in going down a group?
  • If the no. of electrons in the valence shell is upto four, then its valence is equal to the no. of valence shell electrons. If the no. of valence electrons is more than five then its valence = 8 – No. of valence electrons.
  • Mg (12) = 2, 8, 2
    So the valence of Mg = 2
    S (16) = 2, 8, 6
    So the valence of ‘S’ = 8 – 6 = 2
  • Valency of the first twenty elements are as follows:
  • Valency remain constant in going down a group.

NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 5
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 6

Activity 5.7 (Page 88)

Atomic radii of the elements of the second period are given below:
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 7

Question 1.
Arrange them in decreasing order of their atomic radii.
Answer:
Li > Be > B > C > N > O

Question 2.
Are the elements now arranged in the pattern of a period in the Periodic Table?
Answer:
Yes the elements now are arranged in the pattern of a period in the periodic table.

Question 3.
Which elements have the largest and the smallest atoms?
Answer:
Lithium largest and oxygen has the smallest atoms.

Question 4.
How does the atomic radius change as you go from left to right in a period?
Answer:
Atomic radii decreases going from left to right in a period.

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

Activity 5.8 (Page 89)

Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.
NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 8

Question 1.
Name the elements which have the smallest and the largest atoms.
Answer:
Na < Li < K < Rb < Cs
(Na) Sodium – Smallest atoms.
(Cs) Ceisium – Largest atoms.

Question 2.
How does the atomic size vary as you go down a group?
Answer:
Generally atomic size increases, down a group because the no. of shells increases.

Activity 5.9 (Page 89)

Question 1.
Examine elements of the third period and classify them as metals and non-metals.
Answer:
Elements of third period.
Na, Mg, Al, Si, P, S, Cl, Ar
Metals : Na, Mg, Al
Metalloids : Si
Non-metals : P, S, Cl, Ar

Question 2.
On which side of the Periodic Table do you find the metals?
Answer:
In the left hand side of the periodic table, we find metals.

Question 3.
On which side of the Periodic Table do you find the non-metals?
Answer:
In the right hand side of the periodic table, we find non-metals.

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

Activity 5.10 (Page 89)

Question 1.
How do you think the tendency to lose electrons changes in a group?
Answer:
Down the group, the effective nuclear charge experienced by valence electrons is decreasing because the outermost electrons are farther away from the nucleus. So the tendency to lose electrons decrease down the group.

Question 2.
How will this tendency change in a period?
Answer:
As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to loss electrons will decreases.

Activity 5.11 (Page 90)

Question 1.
How would the tendency to gain electrons change as you go from left to right across a period?
How would the tendency to gain electrons change as you go down a group?
Answer:
The electronegativity of the elements increases across the period from left to right so the tendency to gain electrons increase from left to right across a period. The electronegativity of the elements decreases down the group. So the tendency to gain electrons decreases down the group.

Class 10 Science Chapter 5 Periodic Classification of Elements Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Who gave (i) Law of triads (ii) Law of octaves?
Answer:
(1) Dobereiner
(2) Newland.

Question 2.
To which elements Mendeleev gave the name Eka-aluminium and Eka-silicon?
Answer:
Galium and Germanium respectively.

Question 3.
Which important property did Mendeleev use to classify the elements in his periodic table?
Answer:
Atomic masses

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

Question 4.
What are horizontal rows and vertical columns of the periodic table called?
Answer:
Horizontal rows are called periods and vertical rows are called groups

Question 5.
What is the basis of long form of the periodic table called?
Answer:
Atomic number.

Short Answer Type Questions

Question 1.
What was the need for classification of elements?
Answer:
When a large number of elements discovered, the detailed study of each and every element became very time consuming and Highly reactive elements formed a large number of compounds. So, it would be very difficult to study the chemistry after all these elements and their large number of compounds individually. For the systematic study of elements, it led to the need for classifying elements into families or groups having somewhat similarities in their chemical and physical properties.

Question 2.
What is the basis of Mendeleev’s Periodic Table?
Answer:
Mendeleev’s Periodic Table is based on the fundamental periodic law which was proposed by Mendeleev. This law is stated as The physical and chemical properties of the elements are the periodic function of their atomic masses. It suggested that when the elements are arranged in order of increasing atomic masses, the elements with similar properties are placed in the same group.

Question 3.
What is periodic table? What do you mean by classification of elements?
Answer:
It is defined as the arrangement of all the known elements starting from atomic no. 1 to 114, according to their physical and chemical properties in a tabular form. This method of arranging similar elements and separating them from dissimilar elements is called classification of elements. The main aim of classification is to make the study of the elements and their compounds systematic and easier. The elements are arranged in groups and periods. The chemical and physical properties of an element can be predicted by knowing its position in periodic table.

Long Answer Type Question

Question 1.
Give a brief account in periodicity of valence.
Answer:
The electrons present in the outermost shell or valence shell of the elements are called valence electrons and they normally represent the valency of the element.

The valence is either equal to the number of electrons in the outermost shell or eight minus the number of outermost electrons.

Variation along a period; The number of electrons in the valence shell increases from one to eight as we move from left to right in the periodic table. The valency with respect to Hydrogen initially increases from 1 to 4 and then decreases from 4 to zero.

NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements 9
In a similar manner, the valency with respect to chlorine also Initially increases from 1 to 4 and then decreases from 4 zero.

Variation along a group; The valency of the element depends upon its valence shell configuration. Since a group, the valence shell configuration of the elements remains almost the same, they exhibit a common valency. For example, all the elements of group 1 have valency one and those of group 2 have valency two.

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

Multiple Choice Questions

Question 1.
The Dobereiner’s Triad is
(a) Li, Na, K
(b) Li, Na, Ca
(c) Na, K, Ca
(d) Li, Na, Mg
Answer:
(a) Li, Na, K

Question 2.
The valency of alkali metals is/are
(a) 2
(b) 3
(c) 4
(4) 1
Answer:
(4) 1

Question 3.
The Modem periodic table is based on
(a) Atomic no.
(b) Atomic mass
(c) Valency
(d) Inner shell electrons
Answer:
(a) Atomic no.

Question 4.
The Octave rule is given by
(a) Newland
(b) Dobereiner
(c) Moselay
(d) Mendeleev
Answer:
(a) Newland

Question 5.
The Mendeleev’s periodic table is based on
(a) Atomic no.
(b) Atomic mass
(c) Valency
(d) Inner shell electrons
Answer:
(b) Atomic mass

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