Request a Free Counselling Session from our Expert Mentor

Get Free NCERT Solutions for Class 10 Science Chapter 5 Periodic Classification of Elements, set up after careful examination by profoundly experienced Science instructors, at Swastik Classes. NCERT Solutions are extremely helpful while doing your homework and also for your Class 10 board exam preparation. We have given bit-by-bit answers to every one of the questions given in the NCERT class 10 Science course reading. This solution is free to download and the questions are systematically arranged for your ease of preparation and in solving different types of questions. To score good marks, students are encouraged to get familiar with these NCERT solutions of Chapter 5 Periodic Classification of Elements.

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

Answers of Science NCERT solutions for class 10 Chapter 5 Periodic Classification of Elements

Chapter -5

Periodic Classification of Elements

   Question 1:

Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Answer:

Only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.

   Dobereiner’s triads

   Li Ca Cl

Na Sr Br

K Ba I

Newlands’ octaves

HLiBeBCNO
FNaMgAlSiPS
ClKCaCrTiMnFe
Co and NiCuZnYInAsSe
BrRbSrCe and LaZr

Question 2:

What were the limitations of Dobereiner’s classification? 

Answer:

Limitation of Dobereiner’s classification:

All known elements could not be classified into groups of triads on the basis of their properties.

Question 3:

What were the limitations of Newlands’ Law of Octaves? 

Answer:

Limitations of Newlands’ law of octaves:

(i) It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.

(ii) Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.

(iii) The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.

(iv) Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.

Question 1:

Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements:

K, C, Al, Si, Ba.

Answer:

   K is in group 1. Therefore, the oxide will be K2O.

   C is in group 4. Therefore, the oxide will be CO2

   Al is in group 3. Therefore, the oxide will be Al2O3

   Si is in group 4. Therefore, the oxide will be SiO2

   Ba is in group 2. Therefore, the oxide will be BaO. 

   Question 2:

Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)

Answer:

Scandium and germanium

Question 3:

   What were the criteria used by Mendeleev in creating his Periodic Table? 

Answer:

Mendeleev’s periodic table was based on the observation that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties get repeated after regular intervals.

Question 4:

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

Answer:

Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.

Question 1:

How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Answer:

Mendeleev was unable to give fixed positions to hydrogen and isotopes in the periodic table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the elements is not always regular from one to its next. It was believed that a more fundamental property than atomic mass could explain periodic properties in a better manner.

It was Henry Moseley who demonstrated that atomic number of an element could explain periodic properties in a better way than atomic mass of an element and arranged the elements in increasing order of their atomic numbers. Then it was found that the various anomalies of Mendeleev’s periodic table were removed by the modern periodic table.

Question 2:

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

Answer:

Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements. And since chemical properties are due to valence electrons, they show same chemical reactions.

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) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.

(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.

(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.

Question 4:

  1. Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
  2. Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?

Answer:

  1. Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
  2. Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.

Question 5:

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

Answer:

Among the first ten elements, lithium (Li) and beryllium (Be) are metals.

Question 6:

By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?

Answer:

Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.

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.

  1. The elements become less metallic in nature.
  2. The number of valence electrons increases.
  3. The atoms lose their electrons more easily.
  4. The oxides become more acidic. 

 Answer:

    (c) The atoms lose their electrons more easily.

(On moving from left to right across the periods of the periodic table, the non-metallic character increases. Hence, the tendency to lose electrons decreases.)

Question 2:

Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as

  1. Na 

(b) Mg 

(c) Al 

(d) Si 

   Answer:

  1. X would most likely be in the same group of the Periodic Table as magnesium (Mg).

Question 3:

Which element has

  1. two shells, both of which are completely filled with electrons?
  2. the electronic configuration 2, 8, 2?
  3. a total of three shells, with four electrons in its valence shell?
  4. a total of two shells, with three electrons in its valence shell?
  5. twice as many electrons in its second shell as in its first shell? 

   Answer:

  1. Neon has two shells, both of which are completely filled with electrons (2 electrons in K shell and 8 electrons in L shell).
  2. Magnesium has the electronic configuration 2, 8, 2.
  3. Silicon has a total of three shells, with four electrons in its valence shell (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
  4. Boron has a total of two shells, with three electrons in its valence shell (2 electrons in K shell and 3 electrons in L shell).
  5. Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell).

Question 4:

  1. What property do all elements in the same column of the Periodic Table as boron have in common?
  2. What property do all elements in the same column of the Periodic Table as fluorine have in common?

Answer:

  1. All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3.
  2. All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1.

Question 5:

An atom has electronic configuration 2, 8, 7.

  1. What is the atomic number of this element?
  2. 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:

  1. The atomic number of this element is 17.
  2. It would be chemically similar to F(9) with configuration as 2, 7.

Question 6:

The position of three elements A, B and C in the Periodic Table are shown below −

Group 16Group 17
A
BC
  1. State whether A is a metal or 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 element A?

 Answer:

(a) A is a non-metal.

(b) C is less reactive than A, as reactivity decreases down the group in halogens.

(c) C will be smaller in size than B as moving across a period, the nuclear charge   increases and therefore, electrons come closer to the nucleus.

(d) A will form an anion as it accepts an electron to complete its octet.

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:

ElementK L M
Nitrogen2 5
Phosphorus2 8 5

Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.

Question 8:

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

Answer:

In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.

Elements across a period show an increase in the number of valence electrons.

Question 9:

In the Modern 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 element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).

Question 10:

Compare and contrast the arrangement of elements in Mendeleev’s periodic Table and the Modern Periodic Table.

Answer: Mendeleev’s periodic table   Modern periodic table

Mendeleev’s periodic table
Modern periodic table

1.
Elements are arranged in the increasing order of their atomic masses.
1.

Elements are arranged in the increasing order of their atomic numbers.

2.
There are a total of 7 groups (columns) and 6 periods (rows).
2.

There are a total of 18 groups (columns) and 7 periods (rows).


3.

Elements having similar properties were placed directly under one another.


3.
Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.

4.
The position of hydrogen could not be explained.
4.

Hydrogen is placed above alkali metals.

5.

No distinguishing positions for metals and non-metals.

5.
Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.

Access Answers of Science NCERT Class 10 Chapter 5 Periodic Classification Of Elements ( All In text and Exercise Questions Solved)

Questions Page number 81

1. Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Solution:

Döbereiner’s triads did exist in the columns of Newlands’ Octaves; For example, the elements

Lithium (Li), Potassium (K) and Sodium (Na) constitute a Dobereiner’s Triad but are also found in the second column of Newland’s Octaves.

2. What were the limitations of Döbereiner’s classification?

Solution:

(i) They were not applicable for very low mass or very high mass elements.

(ii) All the elements couldn’t fit into Dobereiner’s triads.

(iii) As the methods to calculate atomic mass improved, Dobereiner’s triads validity began to decrease. For example, in the triad of F, Cl and Br, the arithmetic mean of atomic masses of F and Br are not equal to the atomic mass of CI.

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

Solution:

Limitations of Newlands’ Law of Octaves are as follows

  • Newlands’ Law of Octaves applicable to elements up to Calcium
  • Newland assumed there are 56 elements in the nature and no more elements would be discovered in the future.
  • To fit elements into table Newland put two elements into one slot. Newland introduced unlike elements with different properties into one column.
  • Iron (Fe) was placed away from elements that resembles in properties. Ex: Nickel and cobalt

Questions Page number 85

1. Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, AI, Si, Ba.

Solution:

K- K2O

C-C2O4 or CO2

Al- Al2O3

Si-Si2O4 or SiO2

Ba2O2 or BaO

Oxygen is a member of group VI A 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 will help in writing the formulae of their oxides.

(i) Potassium (K) is a member of group IA. Its valency is 1. Therefore, the formula of it is K2O.

(ii) Carbon (C) is a member of group IV A. Its valency is 4. Therefore, the formula of it is C2O4 or CO2.

(iii) Aluminium (Al) belongs to groups III A and its valency is 3. The formula of its oxide is Al2O3.

(iv) Silicon (Si) is present in group IV A after carbon. Its valency is also 4. The formula oxide is Si2O4 or SiO2.

(v) Barium (Ba) belongs to group II A and the valency of the element is 2. The formula of oxide of the element is Ba2O2 or BaO.

2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)

Solution:

Germanium and Scandium are the element that are left by Mendeleev in his Periodic Table since its discovery.

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

Solution:

Mendeleev concentrated on various compounds formed by the elements with Hydrogen and Oxygen. Among physical properties, he observed the relationship between the atomic masses of various elements while creating his periodic table.

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

Solution:

Noble gases are placed in a separate group because of their inert nature and low concentration in our atmosphere. They are kept in a separate group called Zero group so that they don’t disturb the existing order.

Questions Page number 90

1. How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Solution:

(a) In the Modern Periodic table elements are arranged in the increasing order of their atomic number. This removes the anomaly regarding certain pairs of elements in Mendeleev’s periodic table.

(b) Atomic number of cobalt is 27 and nickel is 28. Hence, cobalt will come before nickel even though its atomic mass is greater.

c) All isotopes of the same elements have different atomic masses, but same atomic number; therefore they are placed in the same position in the modern periodic table.

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

Solution:

Calcium and Beryllium are similar to Magnesium because all the three elements belong to the same group and have 2 valence electrons in their outer shell.

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

Solution:

  1. Lithium (Li), Sodium (Na) and potassium (k) have a single electron in their outermost shells.
  2. Magnesium (Mg) and Calcium (Ca) have two electrons in their outermost shells
  3. Neon (Ne), Argon (Ar and Xenon (Xe) filled outermost shells

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?

Solution:

They’ve one valence electron in their outermost shells and as a result of this, they are very unstable. So, they readily react with water to liberate hydrogen. They are also called alkali metals.

Their outermost shells are full leading to high stability. They react only in extreme circumstances and hence are called noble gases.

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

Solution:

Lithium and Beryllium are the metals among the first ten elements in Modern Periodic Table.

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

Solution:

Among the elements listed in the question. Be and Ga are expected to be most metallic. Out of Be and Ga, Ga is bigger in size and hence has a greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.

Exercise questions Page number 91-92

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

Solution:

Correct answer is c . The atoms lose their electrons more easily.

The atoms lose their electrons more easily is a wrong statement because as we move from left to right across the periods of the periodic table, the non-metallic character increases.

Therefore, tendency to lose an electron decreases.

2. Element X forms a chloride with the formula XCl2, 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) AI (d) Si

Solution:

Answer is Magnesium because Mg has the valency 2 which is same as the group (a) Na (b) Mg (c) AI (d) Si

Also, Mg when combines chloride forms MgCl2.

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 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?

Solution:

a) Neon has two shells which are completely filled.

b) Magnesium has electronic configuration 2, 8, 2

c) Silicon has a total of three shells, with four electrons in its valence shell

d) Boron a total of two shells, with three electrons in its valence shell

e) Carbon has twice as many electrons in its second shell as in its first shell

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 the Periodic Table as fluorine have in common?

Solution:

(a) All the elements which lie in me same column as that of boron belong to group 13. Therefore, they have three electrons in their respective valence shells. Except, boron which is a non-metal, all other elements (i.e., aluminum, gallium, indium and thallium) in this group are metals.

(b) All elements in the same column of the Periodic Table as fluorine have in common three electrons in their valence shell and they all are belong to group thirteen.

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)

Solution:

(a)The element with electronic configuration (2+8+7) 17 is chlorine.

The no. of atomic number = no. of electrons

Therefore, atomic number is 17.

(b) An atom with electronic configuration 2, 8, 7 would be chemically similar to F (9)

6. The position of three elements A, B and C in the Periodic Table are shown below–

Group 16          Group 17

–                                –

–                                A

–                                –

B                               C

(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?

Solution:

  1. Element A is a non-metal
  2. Element C is less reactive than Element A
  3. C is smaller in size than B
  4. A will form anion

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?

Solution:

Atomic number of Nitrogen is 7 hence Electronic configuration of Nitrogen is 1s2 2s2 2p3

Atomic number of Nitrogen is 15 hence Electronic configuration of Phosphorous is 1s2 2s2 2p6 3s2 3p3

On moving down a group in the periodic table, the number of shell increases. Due to this, valence electrons move away from the electrons and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electro negativity decreases. Because of all these reasons Nitrogen is more electronegative than phosphorus.

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

Solution:

The number of valence electrons decides an atom’s position in the periodic table while the electronic configuration decides the number of valence electrons.

9. In the Modern 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?

Solution:

Calcium has an atomic number of 20, and thus has an electronic configuration of 2, 8, 8, 2. Thus, calcium has 2 valence electrons. The electronic configuration of the element having atomic number 12 is 2, 8.2. Thus, this element with 2 valence electrons resemble calcium the most.

10. Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.

Solution:

Mendeleev’s Periodic TableModern Periodic Table
Elements are arranged in the increasing order of their atomic masses.Elements are arranged in the increasing order of their atomic numbers.
There are 8 groupsThere are 18 groups
Each groups are subdivided into sub group ‘a’ and ‘b’Groups are not subdivided into sub-groups.
Groups for Noble gas was not present as noble gases were not discovered by that timeA separate group is meant for noble gases.
There was no place for isotopes.This problem has been rectified as slots are determined according to atomic number.

NCERT Solutions for Class 10 Science Chapter 5- Periodic Classification Of Elements

Chapter 5 Periodic Classification of Elements is categorized under Unit Chemical Substances – Nature and behavior. This unit, on the whole, comprises 25 marks in the board exam, most compared to any chapter in the syllabus, hence all chapters in Unit Chemical Substances needs to be studied thoroughly. Periodic classification of elements chapter alone holds up 5 marks in the board exam as expected and from a careful analysis of previous question papers.

List of subtopics covered in Chapter 5 Periodic Classification Of Elements:

5.1 Making Order Out of Chaos – Early Attempts At the Classification of Elements

We have been learning how various things or living beings can be classified on the basis of their properties. Even in other situations, we come across instances of organisations based on some properties. The topic discusses concepts of Dobereiner’s Triads and Newlands’ Law of Octaves. Both concepts are explained with suitable examples.

5.2 Making Order Out of Chaos – Mendeleev’s Periodic Table

The topic explains Mendeleev’s Periodic table, achievements of Mendeleev’s Periodic Table and Limitations of Mendeleev’s Classification. The concepts are presented along with the periodic table and Dmitri Ivanovich Mendeleev’s creation of periodic table.

5.3 Making Order Out of Chaos – The Modern Periodic Table

The Modern Periodic Table stated as Properties of elements are a periodic function of their atomic number. The topic further discusses Position of Elements in the Modern Periodic Table, trends in the Modern Periodic table such as valency, Atomic Size, Metallic and Non – metallic Properties.

List of Exercise with questions

Number – 5.1 – Making order out of chaos 3 Question ( 2 long, 1 short)

-Early attempts at the classification of elements

Number – 5.2 – Making order out of chaos

– Mendeleev’s Periodic Table 4 Question ( 1 long, 3 short)

Number – 5.3 – Making order out of chaos

-The Modern Periodic table 6 Question ( 1 long, 5 short)

Exercise Solutions – 10 Question ( 2 long, 8 short)

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

Periodic Classification of elements gives a brief idea about how elements are classified based on the similarities in the properties shared by different elements. It briefly discusses how Dobereiner classified various elements into triads and how Newland proposed the Law of Octaves. The chapter briefly discusses arrangement of elements in increasing order of their atomic numbers and chemical properties as suggested by Mendeleev, as he even predicted the existence and presence of some more elements that are to be discovered yet which he arrived at due to gaps in the Periodic table.

The chapter also talks about the limitations of Mendeleev’s classification and hence the modern periodic table. It gives an idea of the positioning of elements in the modern periodic elements and also the vertical and horizontal arrangement known as groups and periods, respectively.

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

  • NCERT Class 10 science solutions are answered considering chapterwise marks distribution and question type aligning with the CBSE blueprint
  • Solutions have been elaborated enabling students to better understand
  • Numerical have been solved giving a step-by-step explanation
  • Tabular columns have been used wherever necessary, which enables students to learn at a faster pace
  • The language used in the chapter is simplified and logical

Frequently Asked Questions on NCERT Solutions for Class 10 Science Chapter 5

What are the concepts covered in the Chapter 5 of NCERT Solutions for Class 10 Science?

The concepts covered in the Chapter 5 of NCERT Solutions for Class 10 Science are –
5.1 Making Order Out of Chaos – Early Attempts At the Classification of Elements
5.2 Making Order Out of Chaos – Mendeleev’s Periodic Table
5.3 Making Order Out of Chaos – The Modern Periodic Table


swc google search e1651044504923
2021 Result Highlight of Swastik Classes

NCERT Solutions Class 10 Science Chapters

  • Chapter 1 Chemical Reaction and equation
  • Chapter 2 Acid, Base and Salts
  • Chapter 3 Metals & Non-Metals
  • Chapter 4 Carbon and its Compounds
  • Chapter 5 Periodic Classification of Elements
  • Chapter 6 Life Processes
  • Chapter 7 Control and Coordination
  • Chapter 8 How do Organisms Reproduce
  • Chapter 9 Heridity
  • Chapter 10 Light, Reflection & Refraction
  • Chapter 11 Human Eye
  • Chapter 12 Electricity
  • Chapter 13 Magnetic effects of electric current
  • Chapter 14 Sources of Energy
  • Chapter 15 Environment

Start Pop Up