8 - Classification of Elements 

In this post, you can check the solutions to the exercises of Classification of Elements of Oasis School Science Grade 9 by Jayananda Kapadi.

Group A

1. What is an element?
Answer: An element is a pure substance that cannot be broken down into two or more simpler forms by chemical means.

2. What is a compound?
Answer: A compound is a pure substance that is formed by the combination of two or more than two elements in a fixed proportion by weight.

3. What is electronic configuration?
Answer: Electronic configuration is the way of arrangement of electrons in different shells of an atom.

4. Write down the maximum number of electrons that can accommodate in the given sub-shells.
  1. s = 2 electrons
  2. p = 6 electrons
  3. d = 10 electrons
  4. f = 14 electrons
5. What are sub-atomic particles?
Answer: The smaller particles present in an atom are called sub-atomic particles.

6. What are proton and electron?
Answer: Proton and electron are sub-atomic particles bearing positive and negative charges, respectively.

7. What is an atom? Give one example.
Answer: An atom is the smallest particle of an element that can take part in a chemical reaction. An example of an atom is Sodium.

8. What is a molecule? Give one example.
Answer: A molecule is the smallest particle of a compound that can exist freely in nature. An example of a molecule is water ($H_2O$).

9. What is a diatomic molecule? Give one example.
Answer: A molecule containing exactly two atoms (either similar of different0 is called as a diatomic molecule. Example is CO.

10. What is a radical? How many types of radicals are there?
Answer: A radical is an atom or a group of atoms bearing certain charge that can take part in a chemical reaction as a single unit. On the basis of charges, there are two types of radicals: Electropositive radical and Electronegative radical.


11. What are electronegative radicals?
Answer: Radicals bearing negative charge are called electronegative radicals. Example: OH$^-$.

12. Name one electropositive and electronegative ion each.
Answer: One electropositive and electronegative ion are NH$_4^+$ and NO$_3^-$.

13. What is a chemical bond?
Answer: The force of attraction which is present in between the atoms of a molecule is called a chemical bond.

14. What are covalent compounds?
Answer: Compounds that are formed by the mutual sharing of valence electrons of combining atoms are called covalent compounds.

15. What type of compounds are called electrovalent compounds?
Answer: Such type of compounds that are formed by the gain or loss of valence electrons are called electrovalent compounds.

16. What are valence electrons? How many valence electrons are found in an oxygen atom?
Answer: Total number of electrons present in the outermost shell (or valence shell) of an atom are called valence electrons. There are 6 valence electrons in an oxygen atom.

17. What is molecular formula? Give one example.
Answer: Molecular formula is the expression that represents the molecules of an element or a compound, symbolically.

18. What is duplet state? Write with one example.
Answer: Duplet state is the condition where atoms gain stable state by having two valence electrons in K shell. Example: Helium has duplet state.

19. What is octet state? Give one example.
Answer: Octet state is the condition where atoms gain stable state by having eight electrons in their valence shell. Example: Argon has octet state.

20. What is octet rule?
Answer: The tendency of atoms to have eight electrons in their valence shell to attain a stable atomic state is called octet rule.


21. What in 2n$^2$ rule?
Answer: 2n$^2$ rule is the method used to determine the number of electrons in different shells. In the formula, 'n' denotes the number of shell.

22. What type of gases are called inert gases?
Answer: Gases that have stable atomic state in nature are called inert gases.

23. Write down the valency of the given radicals.
  • Nitrate - 1
  • Phosphate - 3
  • Chloride - 1
  • Silicate - 2
  • Sulphate  - 2
  • Carbonate - 2
  • Hydroxide - 1
  • Ammonium - 1
  • Bisulphate - 1
  • Bromide - 1
  • Nitrite - 1
  • Sulphite - 2
  • Iodide - 1
  • Chlorate - 1
  • Oxide - 2


Group B

1. Differentiate between atom and molecule in any two points.
Answer: Differentiation between atom and molecule is given below:
Atom Molecule
Atom is the smallest particle of an element. Molecule is the smallest particle of a compound.
It can take part in chemical reaction. It can exist independently in nature.

2. Sodium is called an element and water is called a compound. Why?
Answer: Sodium is called an element because it cannot be broken down into simpler forms by any chemical mean and water is called a compound because it is made up of two elements: Hydrogen and Oxygen.

3. Write any two differences between elements and compounds.
Answer: Differences between elements and compounds are:

Elements Compounds
Element is a pure substance that is made up of only one type of atom. Compound is a pure substance that is made up of two or more than two elements in a fixed proportion by weight.
It cannot be broken down into two or more simpler forms by any chemical mean. It can be broken down into two or more simpler forms by chemical mean.


4. Differentiate between protons and electrons in any two points.
Answer: Differentiation between protons and electrons are:

Protons Electrons
Protons are the electropositive sub-atomic particles. Electrons are the electronegative sub-atomic particles.
It is found within the nucleus of an atom. It is found in the different shells of an atom.

5. What is meant by duplet state and octet state?
Answer: Duplet state is the condition where atoms gain stable state by having two electrons in K shell. And, octet state is the condition where atoms gain stable state by having eight electrons in their valence shell.

6. Differentiate between atom and radical with one example of each.
Answer: Differentiation between atom and radical are:

Atom Radical
Atom is the smallest particle of an element. Radical is an atom or a group of atoms bearing a certain charge.
Example: Cl Example: Cl$^-$

7. Sodium atom cannot exist freely in nature but argon atom can. Give reason.
Answer: Sodium atom is not completely stable and is highly reactive. In nature, it reacts with other electronegative atoms and combines to form a molecule. Thus, it cannot exist freely in nature.
But, argon atom can exist freely in nature because it is stable, as it has eight valence electrons, and has very high ionization energy.

8. Write any two differences between electrovalent bond and covalent bond.
Answer: Differences between electrovalent bond and covalent bond are:

Electrovalent bond Covalent bond
Electrovalent bond is the chemical bond between two or more particles formed by the gain or loss of electrons. Covalent bond is the chemical bond formed by the mutual sharing of electrons by two or more particles.
Such bonds feature high melting and boiling point. Such bonds feature low melting and boiling point.

9. Differentiate between electropositive radicals and electronegative radicals with one example of each.
Answer: Differences between electropositive and electronegative radicals are:

Electropositive radicals Electronegative radicals
These radicals bear positive charge by losing electrons. These radicals bear negative charge by gaining electrons.
Example: NH$_4^+$ Example: Cl$^-$


10. Atoms are electrically neutral. Justify this statement.
Answer: Atoms are electrically neutral. It is because it contains three sub-atomic particles - electrons, protons, and neutrons. Electrons bear negative charge, neutron is charge less and protons bear positive charge. In a ground state atom, the number of electrons and protons is always equal. Thus, the positive and negative charges cancel each other. As a result, there is no charge in a ground state atom and it is electrically neutral.

11. Write any two differences between electrovalent compounds and covalent compounds.
Answer: Differences between electrovalent compounds and covalent compounds are:

Electrovalent compounds Covalent compounds
These compounds are formed by the gain and loss of electrons. These compounds are formed by mutual sharing of electrons.
These compounds can conduct electricity. These compounds cannot conduct electricity.

12. Sodium chloride is called electrovalent compound but ammonia is called covalent compound, why?
Answer: Sodium chloride is formed when sodium loses its valence electron and Chlorine receives that lost electron. Thus, it is called electrovalent compound.
But ammonia is formed when the valence electrons of are mutually shared by one Nitrogen and three Hydrogens. Thus, it is called covalent compound.

13. The valency of neon is zero but that of sodium is one, why?
Answer: Valency is the combining capacity of an atom. Neon has eight valence electrons and has stable state and it doesn't lose, gain or share its electrons so, its valency is zero. But, sodium has one valence electron which needs to be lost in order to gain octet state. Therefore, the valency of sodium is one.

14. The elements of group 1 are reactive but those of group 18 are inert, why?
Answer: Elements react in order to gain a stable atomic state by following duplet or octet rule. Elements of group 1 have one valence electron which needs to be lost to obtain stable state. Thus, they are reactive. However, the elements of group 18 have their outermost shell fulfilled and already have a stable atomic state and do not need to react. So, they are called inert.

15. Electrovalent compounds are electrolysed but covalent compounds are not electrolysed, why?
Answer: For compounds to get electrolysed, they must have ions. In electrovalent compounds, there are ions but there aren't any ions in covalent compounds. Thus, electrovalent compounds are elctrolysed but covalent compounds are not.

16. Identify electropositive and electronegative radicals from the following:
  • Sulphate - Electronegative radical
  • Ammonium - Electropositive radical
  • Nitrate - Electronegative radical
  • Carbonate - Electronegative radical



Group C

1. What is a chemical bond? How is it formed? Write with an example.
Answer: A chemical bond is the force of attraction that binds two or more elements together to form a molecule.
It is formed when there is gain of electrons or loss of electrons or mutual sharing of electrons or sharing of lone pair of electrons by two or more atoms. All these electrons must be valence electrons.
Example: During the formation of NaCl from Sodium and Chlorine, Sodium loses its one valence electron which is gained by Chlorine and hence, a chemical bond (ionic bond) is formed. 

2. How is covalent bond formed? Write in brief.
Answer: Covalent bond is formed when there is mutual sharing of valence electrons by two or more elements to form a covalent compound. Here, mutual sharing refers to equal sharing of electrons by both of the elements. For example: in the formation of NH$_3$, one atom of Nitrogen shares one each valence electron with 3 atoms of Hydrogen and so do the hydrogen atoms with Nitrogen. Thus, such bond is called a covalent bond.

3. Calculate the maximum number of electrons present in shell L and N by using the 2n$^2$ rule.
Answer: In the formula $2n^2$, n refers to the number of shell.
For L shell, n = 2
For N shell, n = 4
Now,
Putting value of n of L shell in the formula, we get,
number of electrons = $2*2^2 = 2*4 = 8$ electrons
And,
Putting value of n of N shell in the formula, we get,
number of electrons = $2*4^2 = 2*16 = 32$ electrons
Hence, the maximum number of electrons present in shell L is 8 electrons and that in shell N is 32 electrons.

4. Describe the formation of sodium chloride with a neat figure.
Answer: Sodium chloride is an electrovalent or ionic compound formed the gain and loss of valence electrons by Cl and Na, respectively.
See Page Number 150 of your book for the figure
As we can see, the valence electron of Sodium is gained by Chlorine and both of them become stable and Sodium Chloride is formed.

5. Describe the formation of carbon dioxide with a neat figure.
Answer: Carbon dioxide is a covalent compound formed by the mutual sharing of valence electrons by Carbon and Oxygen.


Group D

1. Draw the molecular structure of the following molecules:
i) Sodium chloride
Answer: See page number 150 of your book for the molecular structure

ii) Carbon dioxide

2. In a part of the periodic table given below, some elements are denoted by symbols A,B,C,D and E. Answer the following questions on this basis:
i) Name the elements 'B' and 'D' and state their valency also.
Answer: Information is mentioned below:
  • Element B is Sodium (Na) and its valency is 1.
  • Element D is Oxygen (O) and its valency is 2.
ii) Name the compound formed by the combination of elements 'A' and 'C' and also write down the type of bond present between them.
Answer: Element 'A' is Aluminum (Al) and Element 'C' is Chlorine (Cl). On reaction between them, AlCl$_3$ (Aluminum Chloride) compound is formed which has ionic bond present in it.

3. Answer the following questions on the basis of given figures.

i) What is the valency of B and C? Why?
Answer: The valency of B is 1 and that of C is also 1.

ii) Name the compound formed by the combination of the elements A and B. Also write down the type of the bond with reason.
Answer: Element A = Oxygen and Element B = Sodium

$ 1    2$
$Na  O$

$\implies Na_2O$

Hence, Sodium Oxide is formed by the combination of the elements A and B. It is electrovalent bond because it is formed when Sodium loses two electrons and that is gained by oxygen atom.


4. Name any four elements having variable valency. Also, write their valencies.
Answer: Any four elements having variable valency are:
  • Iron: 2,3
  • Copper: 1,2
  • Gold: 1,3
  • Mercury: 1,2

5. Explain the method of writing molecular formula with examples.
Answer: The method of writing molecular formula is given below:
See page number 154 of the book.


6. Calculate the molecular weight of the given compounds.

i) Calcium carbonate = $CaCO_3$

$\implies 1 * Ca + C + 3*O$

$\implies 40 + 12 + 3*16$

$\implies 52 + 48$

$\implies 100$


ii) Ammonium nitrate = $NH_4(NO_3)$

$\implies 1 * N + 4* H + 1*N + 3* O$

$\implies 14 + 4*1 + 14 + 3*16$

$\implies  32 + 48$

$\implies 80$


About this book:

Oasis School Science Grade 10
Author: Jayananda Kapadi
Editor: Dr. Rameshwar Adhikari

About Oasis Publication:

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