ICSE Solutions for Chapter 4 Atomic Structure and Chemical Bonding Class 9 Selina Chemistry

1. What is the contribution of the following in atomic structure?

(a) Maharshi Kanada

(b) Democritus

Solution

(a) According to Maharshi Kanada:

Matter consisted of indestructible particles called paramanus (now called atoms).
A paramanu does not exist in the free state, rather it combines with other paramanus to form a bigger particle called the anu (now called a molecule).

(b) Democritus called paramanu as atom which comes from the Greek word atoms, meaning indivisible.

2. State Dalton’s atomic theory.

Solution

Main postulates of Dalton’s atomic theory:

Matter consists of very small and indivisible particles called atoms.
Atoms can neither be created nor be destroyed.
The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
Atoms of an element combine in small numbers to form molecules.
Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
Atoms are the smallest units of matter which can take part in a chemical reaction.

3. What is an α (alpha) particle?

Solution

An α-particle is a doubly charged helium ion (He²⁺) containing two protons and two neutrons. It is formed by removing two electrons from the helium atom.

4. What are cathode rays? How are these rays formed?

Solution

Cathode rays are the beam of electrons which travel from the negatively charged end (i.e. cathode to anode) of a vacuum tube, across a voltage difference between the electrodes placed at each end.

Formation of cathode rays:

Generally gases are poor conductors of electricity. However, when a high voltage charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of mercury), the gases become good conductors of electricity and begin to flow in the form of rays.

These rays are called cathode rays and they travel from the cathode towards the anode.

5. What is the nature of the charge on

(i) Cathode rays and

(ii) Anode rays?

Solution

(i) Cathode rays are negatively charged as they consist of negatively charged particles called electrons.

(ii) Anode rays are made of positively charged particles.

6. How are X-rays produced?

Solution

When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are produced.

7. Why are anode rays also called as ‘canal rays’?

Solution

When perforated rays were used in the discharge tube, another set of rays travelling in a direction opposite to that of the cathode rays, i.e. from the anode towards the cathode, is seen, as in the figure below. These rays are called canal rays because they passed through the holes or canals in the cathode. These rays were named positive rays or anode rays.

8. How does cathode ray differ from as anode rays?

Solution

Cathode rays	Anode rays
These rays travel from the cathode to the anode.	These rays travel from the anode to the cathode.
They are made of negatively charged particles.	They are made of positively charged particles.
They produce a greenish yellow fluorescence on a soda-glass screen.	They produce fluorescence on a zinc sulphide screen.
They are affected by an electric field, i.e. they are inflected towards a positive field and deflected towards a negative field.	They are deflected by electric and magnetic fields but in a direction opposite to that of cathode rays.

9. State one observation which shows that atom is not indivisible.

Solution

Chadwick discovered neutral particles present in an atom by bombarding light nuclei like beryllium with alpha particles, i.e. helium nuclei.

₄Be⁹ + ₂He⁴→ ₆C¹² + ₀n¹

Discovery of the neutron also showed that an atom is divisible.

10. (a) Name an element which does not contain neutron

(b) If an atom contains one electron and one proton, will it carry any charge or not?

Solution

(a) Hydrogen does not contain any neutron.

(b) If an atom contains one electron and one proton, it will be neutral in charge.

11. On the basis of Thomson’s model of an atom explain how an atom as a whole is neutral.

Solution

Thomson’s Atomic Model:

According to Thomson’s atomic model, an atom is made of positively charged substances in the form of a sphere. Electrons are embedded into this sphere, and the total positive charge of the sphere is equal to the total negative charge of electrons, and hence, the atom remained electrically neutral. This model was not accepted because it was unable to explain how positively charged particles were shielded from negatively charged particles without getting neutralised.

12. Which sub-atomic particle was discovered by

(a) Thomson

(b) Goldstein

(c) Chadwick

Solution

(a) Thomson discovered particles.

(b) Goldstein discovered protons.

13. Name the sub-atomic particle whose charge is:

(a) +1

(b) -1

(c) 0

Solution

(a) Proton

(b) Electron

14. (a) Which metal did Rutherford select for his α particle scattering experiment and why?

(b) What do you think would be the observation of α particle scattering experiment if carried out on (i) heavy nucleus like platinum (ii) light nuclei like lithium.

Solution

(a) He selected a gold foil because he wanted as thin a layer as possible and gold is the most malleable metal.

(b) (i) Heavy metals such as platinum will show the same observation with alpha particle as shown by gold foil

(ii) Light nuclei like may even push the light nucleus aside and may not deflected back.

15. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Solution

According to Rutherford’s model of an atom, the subatomic particle in a nucleus is a proton.

16. Which part of atom was discovered by Rutherford?

Solution

The nucleus was discovered by Rutherford.

17. How was it shown that atom has empty space?

Solution

Rutherford performed an experiment by allowing a stream of alpha particles to pass through a very thin gold foil. He observed that alpha particles pass through the metal foil without deviating from their path. This shows that an atom contains a large empty space called nuclear space.

18. State one major drawback of Rutherford’s model.

Solution

One major drawback of Rutherford’s model was the comparison of electrons with the planets in the solar system.

Thus, when an electron moved around the nucleus continually, it should radiate energy, i.e. lose energy. As a result it should be gradually pulled towards the nucleus and end up colliding with it. This should result in the total collapse of the atom.

However, we know that the atom is structurally stable. Thus, Rutherford’s model could not explain this stability.

19. In the figure given alongside

(a) Name the shells denoted by A, B and C. Which shell has least energy?

(b) Name X and state the charge on it

(c) The above sketch is of ……. Model of an atom

Solution

(a) (i) A is for K shell or I shell.

(ii) B is for L shell or II shell.

(iii) C is for M shell or III shell.

Shell K has the minimum amount of energy.

(b) X is a nucleus; it is positively charged.

20. Give the postulates of Bohr’s atomic model.

Solution

Atom has a central nucleus surrounded by electrons.
The electrons revolve around the nucleus in definite circular paths called orbits.
Each orbit has fixed energy. Therefore, these orbits are also known as energy levels or energy shells.
These orbits or energy levels are represented either by a number 1, 2, 3,… known as the principal quantum number (n) of the orbit or by K, L, M,….
There is no change of energy of electrons as long as they keep revolving in the same energy level and the atom remains stable. Electrons can jump from the higher orbit to the lower orbit when they lose energy and jump to the next when they gain energy.

Exercise 4B

1. (a) Name the three fundamental particles of an atom.

(b) Give the symbol and charge of each particle.

Solution

(a) The three fundamental particles of an atom are electron proton and neutron.

(b)

Particle	Symbol	Charge
Electron	e	-1
Proton	p	+1
Neutron	n	No charge

2. Complete the table given below by identifying P, Q, R and S.

Element	Symbol	No. of Protons	No. of neutrons	No. of Electrons
Sodium		11	P	11
Chlorine		Q	18	17
Uranium	R	92	146	92
S		9	10	9

Solution

Element	Symbol	No. of Protons	No. of Neutrons	No. of Electrons
Sodium		11	12	11
Chlorine		17	18	17
Uranium	²³⁸U₉₂	92	146	92
S		9	10	9

3. The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What are its atomic number and mass number?

Solution

Atomic number = Number of protons or number of electrons = 4

Mass number = Number of protons + Number of neutrons = 4 + 5 = 9

4. The atomic number and mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement?

Solution

Atomic number (11) of sodium conveys information that the number of protons and electrons is the same.

Mass number (23) of sodium indicates the sum of protons and neutrons.

5. Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached

₁p¹, ₀n¹, _-1e⁰

Solution

p = proton

n = neutron

e = electron

Superscript number : These superscript number show their mass number

Subscript number : These numbers show their atomic number

6. From the symbol, state the mass number, the atomic number and electronic configuration of magnesium.

Solution

Mass number = 24

Atomic number = 12

No. of electrons = 24 – 12 = 12

Electronic configuration = 2, 8, 2

7. Sulphur has an atomic number 16 and a mass of 32.

State the number of protons and neutrons in the nucleus of sulphur. Give a simple diagram to show the arrangement of electrons in an atom of sulphur.

Solution

Atomic number = 16

Atomic mass = 32

Number of protons = 16

Number of electrons = 16

Number of neutrons = 32 – 16 = 16

Electronic configuration = 2, 8, 6

8. Explain the rule according to which electrons are filled in various energy levels.

Solution

The maximum capacity of a shell to accommodate electrons is given by the general formula 2n², where n is the serial number of a shell.
The maximum number of electrons possible in the outermost shell is 8 and that in the penultimate shell is 18.
It is not necessary for an orbit to be completed before another is formed. In fact, a new orbit is formed when the outermost shell attains 8 electrons.

9. Draw the orbital diagram of ion and state the number of three fundamental particles present in it.

Solution

Orbital diagram of

Atomic number of Ca²⁺ is 18.

Electronic configuration of Ca²⁺ is 2, 8, 8.

Number of three fundamental particles of Ca²⁺:

Protons: 18

Electrons: 18

Neutrons: 40 – 18 = 22

10. (a) Write down the electronic configuration of the following:

(b) Write down the number of electrons in X and neutrons in Y.

Solution

Electronic configuration: ²⁷X₁₃ = 2, 8, 3

Number of electrons in X = 13

Number of neutrons in X = 27 – 13 = 14

Electronic configuration of ³⁵Y₁₇ = 2, 8, 7

Number of electrons in Y = 17

Number of neutrons in Y = 35 – 17 = 18

Hence, formula ²⁷X₁₃ stands for ²⁷Na₁₃ and ³⁵Y₁₇ for ³⁵Cl₁₇

Compound formula = NaCl

Exercise 4C

1. How does the Modern atomic theory contradict and correlate with Dalton’s atomic theory?

Solution

The latest research on the atom has proved that most of the postulates of Dalton’s atomic theory contradict. However, Dalton was right that atoms take part in chemical reactions.

Comparisons of Dalton’s atomic theory with the modern atomic theory.

Dalton’s atomic theory:

Atoms are indivisible.
Atoms of the same element are similar in every respect.
Atoms combine in a simple whole number ratio to form molecules.
Atoms of different elements are different.
Atoms can neither be created nor be destroyed.

Modern atomic theory:

Atoms are no longer indivisible and consist of electrons, protons, neutrons and even more sub-particles.
Atoms of the same element may differ from one another called isotopes.
Atoms of different elements may be similar called isobars.
Atoms combine in a ratio which is not a simple whole number ratio; e.g. in sugar, the C₁₂H₂₂O₁₁ ratio is not a whole number ratio.

2. (a) What are inert elements?

(b) Why do they exist as monoatoms in molecules?

(c)What are valence electrons?

Solution

(a) The elements have a complete outermost shell, i.e. 2 or 8 electrons. They ordinarily do not enter intoany reaction.

(b) These exist as monoatoms because molecules of these elements contain only one atom.

3. In what respects do the three isotopes of hydrogen differ? Give their structures.

Solution

The three isotopes differ only due to their mass number which is respectively 1, 2 and 3 and named protium, deuterium and tritium.

4. Match the atomic numbers 4, 14, 8, 15 and 19 with each of the following:

(a) A solid non-metal of valency 3.

(b) A gas of valency 2.

(c) A metal of valency 1.

(d) A non-metal of valency 4.

Solution

Atomic Number	Name with valency
(a) 15	A solid non-metal of valency 3
(b) 8	A gas of valency 2
(c) 19	A metal of valency 1
(d) 14	A non-metal of valency 4

5. Draw diagrams representing the atomic structures of the following:

(a) Sodium atom

(b) Chlorine ion

(c) Carbon atom

(d) Oxygen ion

Solution

(a) atomic structures of Sodium atom

(b) atomic structures of Chlorine ion

(d) atomic structures of Oxygen ion

6. What is the significance of the number of protons found in the atoms of different elements?

Solution

The number of protons in the nucleus of an atom determines the element type of the atom.

7. Elements X, Y and Z have atomic numbers 6, 9 and 12 respectively. Which one:

(a) Forms an onion

(b) Forms a cation

(c) Has four electrons in its valence shell?

Solution

Element	X	Y	Z
Atomic numbers	6	9	12
Electronic configuration	2, 4	2, 7	2, 8, 2

(a) Y(2, 7) forms an anion.

(b) Z(2, 8, 2) forms a cation.

8. Element X has electronic configuration 2,8, 18, 8,1. Without identifying X,

(a) Predict the sign and charge on a simple ion of X.

(b) Write if X will be an oxidizing agent or a reducing agent. Why?

Solution

(a) X¹⁺

(b) Oxidising agent, because it has the ability to donate electrons.

9. Define the terms:

(a) Mass number

(b) Ion

(c) Cation

(d) Anion

(e) Element

(f) orbit

Solution

(a) Mass number is the sum of the number of protons and the number of neutrons in the nucleus of an atom.

(b) Ion is an atom or molecule which carries a positive or negative charge because of loss or gain of electrons.

(c) Cation is a positively charged ion which is formed when an atom loses one or more electrons; for example, Na⁺, Hg²⁺ and Ca²⁺.

(d) Anion is a negatively charged ion which is formed when an atom gains one or more electrons; for example, Cl^–and I^–.

(e) Element is a substance which cannot be split up into two or more simple substances by usual chemical methods of applying heat, light or electric energy; for example, hydrogen, oxygen and chlorine.

(f) Orbit is a circular path around the nucleus in which electrons of the atom revolve.

10. From the symbol ₂He⁴for the element helium, write down the mass number and the atomic number of the element.

Solution

Atomic number = 2

Mass number = 4

Question 11

Five atoms are labeled A to E

Atoms	Mass No	Atomic No.
A	40	20
B	19	9
C	7	3
D	16	8
E	14	7

11. (a) Which one of these atoms:

(i) contains 7 protons

(ii) has electronic configuration 2,7

(b) Write down the formula in the compound formed between C and D

(c) Predict: (i) metals (ii) non-metals

Solution

(a) (i) Atom E contains 7 protons.

(ii) Atom B has an electronic configuration 2, 7.

(b) Atom C stands for ₇Li³, Atom D stands for ₈O¹⁶.

Hence, compound formula is Li₂O.

Non-metals: B, D, E

12. An atom of an element has two electrons in the M shell.

What is the (a) atomic number (b) number of protons in this element?

Solution

Number of electrons in the M shell = 2

So, the number of electrons in the K and L shells will be = 2, 8

Hence, atomic number = 2 + 8 + 2 = 12

Number of protons = 12

13. ₁₂Mg²⁴ and ₁₂Mg²⁶ are symbol of two isotope of magnesium.

(a) Compare the atom of these isotope with respect to:

(i) Composition of their Nuclei.

(i) Their electronic configuration.

(b) Give reason why two isotopes of magnesium have different mass numbers.

Solution

(a) (i)

	₁₂Mg²⁴	₁₂Mg²⁶
No. of electrons	12	12
No. of protons	12	12
No. of neutrons	24 – 12 = 12	26 – 12 = 14

Hence, composition of nuclei

(ii) Electronic configuration = 2, 8, 2

(b) Mass numbers of two isotopes of magnesium are different because of different number of neutrons, i.e. 12 and 14, respectively.

14. What are nucleons? How many nucleons are present in phosphorus? Draw its structure.

Solution

Nucleons: Particles which constitute the nucleus are called nucleons.

Protons and neutrons are the nucleons.

Atomic mass of phosphorus = 31

Atomic number = 15

15. What are isotopes? With reference to which fundamental particle do isotopes differ? Give two uses of isotopes.

Solution

Isotopes: Atoms of the same element having the same atomic number but different mass numbers.

Isotopes differ with reference to neutrons.

Uses of isotopes:

Some isotopes are radioactive, i.e. isotopes of cobalt are used for treating cancer and other diseases.

An isotope of ²³⁵U is used as a fuel in a nuclear reactor.

16. Why do ₁₇Cl³⁵ and ₁₇Cl³⁷have the same chemical properties? In what respect do these atoms differ?

Solution

Only electrons take part in chemical reactions. Chemical properties depend on the electronic configuration. Isotopes of the element ₁₇Cl³⁵ and ₁₇Cl³⁷ have the same atomic number, and hence, the same configuration. So, they have the same chemical properties. These differ only in physical contents and weights because neutrons contribute to the mass of an atom. ₁₇Cl³⁵ and ₁₇Cl³⁷ have different number of neutrons 18 and 20, respectively.

17. Explain fractional atomic mass. What is the fractional mass of chlorine?

Solution

Atomic masses of the isotopes of chlorine are 35 and 37. However, in any given sample of chlorine gas, the isotopes occur in the approximate ratio 3:1, 75% of Cl³⁵ and 25% of Cl³⁷. So, the relative atomic mass or atomic weight of chlorine is 35.5.

18. (a) What is meant by ‘atomic number of an element”?

(b) Complete the table given below

	No. of protons	No. of electrons	No. of Neutrons	Atomic Number	Mass number
₁₇Cl³⁵
₁₇Cl³⁷

(c) Write down the electronic configuration of (i) chlorine atom (ii) chlorine ion

Solution

(a) Atomic number of an element is the number of protons in the nucleus of an atom.

(b)

	No. of protons	No. of electrons	No. of neutrons	Atomic number	Mass number
₁₇Cl³⁵	17	17	18	17	35
₁₇Cl³⁷	17	17	20	17	37

(ii) Electronic configuration of chlorine ion = 2, 8, 8

19. Name the following:

(a) The element which does not contain any neutron in its nucleus.

(b) An element having valency ‘zero’

(c) Metal with valency 2

(d) Two atoms having the same number of protons and electrons but different number of neutrons.

(e) The shell closest to the nucleus of an atom

Solution

(a) Hydrogen

(b) Helium

(d) Hydrogen and carbon

(e) K

20. Give reasons

(a) Physical properties of isotopes are different.

(b) Argon does not react.

(c) Actual atomic mass is greater than mass number.

(d)₁₇Cl³⁵ and ₁₇Cl³⁷ do not defer in their chemical reaction

Solution

(a) Physical properties depend on atomic mass, and isotopes have different mass number, i.e. they have different number of neutrons. So, isotopes have different physical properties.

(b) Argon does not react as it has the outermost orbit complete, i.e. 8 electrons in the outermost shell.

(c) Actual atomic mass is greater than the mass number because the mass number is a whole number approximation of atomic mass unit. In fact, neutrons are slightly heavier than protons, and an atom has over 200 sub-atomic particles.

(d) ₁₇Cl³⁵ and ₁₇Cl³⁷ are isotopes of chlorine element which differ in the number of neutrons, whereas chemical properties are determined by the electronic configuration of an atom. Isotopes of an element are chemically alike.

21. An element A atomic number 7 mass numbers 14

B electronic configuration 2, 8, 8

C electrons 13, neutrons 14

D Protons 18 neutrons 22

E Electronic configuration 2, 8, 8, 1

State:
(i) Valency of each element
(ii) which one is a metal
(iii) which is non-metal
(iv) which is an inert gas

Solution

(i) Element A

Atomic number = 7 = Number of electrons = 2, 5

Valency of A = 8 – 5 = 3

Element B

Electronic configuration 2, 8, 8

Valency of B = Zero

Element C has 13 electrons

Electronic configuration = 2, 8, 3

Valency of C = 3

Element D

Protons = 18 = Electrons = 2, 8, 8

Valency of D = Zero

Element E

Electronic configuration = 2, 8, 8, 1

Valency of E = 1

(ii) C and E are metals.

(iii) A is a non-metal.

(iv) A, C and E are not inert gases.

22. Choose the correct option

(a) Rutherford’s alpha-particle scattering experiment discovered

(i) Electron

(ii) Proton

(iii) Atomic nucleus

(iv) Neutron

(b) Number of valence electrons in O^2- is :

(i) 6

(ii) 8

(iii) 10

(iv) 4

(c) Which of the following is the correct electronic configuration of potassium?

(i) 2,8,9

(ii) 8,2,9

(iii) 2,8,8,1

(iv) 1,2,8,8

Solution

(a) (iii) Atomic nucleus

(b) (i) 6

23. Explain

(a) Octet rule for formation of sodium chloride

(b) Duplet rule for formation of hydrogen

Solution

Elements tend to combine with one another to attain the stable electronic configuration of the nearest inert gas.

(a) Octet rule for formation of Sodium chloride

Sodium atom has 1 electron in the valence shell which it donates to the chlorine atom with 7 electrons in the valence shell to attain the stable electronic configuration of the nearest inert gas, i.e. 8 electrons in the valence shell. This is known as the octet rule. These elements combine to form sodium chloride.

(b) Duplet rule for formation of Hydrogen

Hydrogen atom has one electron in the valence shell which it shares with another hydrogen atom having one electron to complete its duplet state, i.e. two electrons in the valence shell and resulting in the formation of hydrogen.

24. Complete the following table relating to the atomic structure of some elements.

Element Symbol	Atomic Number	Mass Number	Numbers of neutrons	Number of Electrons	Number of Protons
Li	3	6
Cl	17		20
Na			12		11
Al		27			13
S		32	16

Solution

Element Symbol	Atomic Number	Mass Number	Numbers of Neutrons	Number of Electrons	Number of Protons
Li	3	6	4	3	3
Cl	17	37	20	17	17
Na	11	23	12	11	11
Al	13	27	14	13	13
S	16	32	16	16	16

Exercise 4D

1. How do atoms attain noble gas configurations?

Solution

Combining atoms attain the noble gas configuration by transferring or sharing electrons.

2. Define electrovalent bond.

Solution

The chemical bond formed due to the electrostatic force of attraction between a cation and an anion is called an electrovalent bond.

3. Elements are classified as metals, non-metal, metalloids and inert gases. Which of them form electrovalent bond?

Solution

Metals have a tendency to lose their valence electrons (1, 2 or 3). So, they combine with non-metals which have 7, 6 or 5 electrons in their valence shell to form an electrovalent bond.

4. (a) An atom X has three electrons more than the noble gas configuration. What type of ion will it form?

(b) Write the formula of its (X)

(i) sulphate

(ii) nitrate

(iii). phosphate

(iv) carbonate

(v) hydroxide.

Solution

(a) An atom X will lose its three electrons to acquire the noble gas configuration and form a positive ion (a cation).

(b) The formulae are

(i) X(SO₄)₃

(ii) X(NO₃)₃

(iii) XPO₄

(iv) X₂(CO₃)₃

(v) X(OH)₃

5. Mention the basic tendency of an atom which makes it to combine with other atoms

Solution

Besides chaos, everything in this world wants stability. The same is the case with atoms. For atoms, stability means having the electron arrangement of an inert gas, i.e. octet in the outermost shell. Helium has two electrons (duplet), while all other inert gases, i.e. neon, argon, krypton, xenon and radon have eight electrons (octet) in the outermost shell.

6. What type of compounds are usually formed between metals and non-metals and why?

Solution

Electrovalent compounds are usually formed between metals and non-metals.

Atoms of metallic elements which have 1, 2 or 3 valence electrons can lose electron(s) to atoms of non-metallic elements which have 5, 6 or 7 valence electrons and thereby form an electrovalent bond.

7. (a) In the formation of the compound XY₂, an atom X gives one electron to each Y atom. What is the nature of bond in XY₂?

(b) Draw the electron dot structure of this compound.

Solution

(a) Nature of bond in XY₂ is an ionic bond.

(b) Electron dot structure of XY₂:

8. An atom X has 2, 8, 7 electrons in its shell. It combines with Y having 1 electron in its outermost shell.

(a) What type of bond will be formed between X and Y?

(b) Write the formula of the compound formed.

Solution

(a) Ionic bond

(b) XY

9. Draw orbit structure diagram of

(a) sodium chloride (NaCl) and

(b) calcium oxide (CaO).

Solution

(a) orbit structure diagram of NaCl

(b) orbit structure diagram of CaO

10. Compare :

(a) Sodium atom and sodium ion

(b) Chlorine atom and chloride ion, with respect to

(i) Atomic structure

(ii) Electrical state

Solution

(a) Comparison of Sodium atom and sodium ion

Sodium atom	Sodium ion
Sodium atom is electrically neutral.	Sodium ion is positively charged.
In sodium atom, there are 11 protons and 11 electrons, i.e. equal number of protons and electrons.	In sodium ion, there are 11 protons but 10 electrons, i.e. sodium ion contains lesser number of electrons.
Sodium atom has only one electron in its valence shell.	Sodium ion has 8 electrons in its valence shell.
Size of a sodium atom is larger than a sodium ion.	Size of a sodium ion is smaller than a sodium atom.

(b) In chlorine atom, the number of protons (17) is equal to the number of electrons (17).

In chloride ion, there are 17 protons but 18 electrons.

(i) Chlorine atom is electrically neutral. Chloride ion is negatively charged.

(ii) Chlorine ion is reactive. It reacts with sodium vigorously forming sodium chloride. Chloride ion is unreactive. It does not react with sodium.

Chlorine (Cl₂) is a poisonous, toxic, corrosive gas which is used in the manufacture of bleaching agents and disinfectants. It is non-toxic and readily adsorbed by plants.

11. The electronic configuration of fluoride ion is the same as that of neon atom. What is the difference between the two?

Solution

Fluoride ion is a negatively charged ion with 9 protons and 10 electrons.

Neon atom is electrically neutral with 10 protons and 10 electrons.

12. (a) What do you understand by redox reactions?

(b) Explain oxidation and reduction in terms of loss or gain of electrons.

Solution

(a) Transfer of electron(s) is involved in the formation of an electrovalent bond. The electropositive atom undergoes oxidation, while the electronegative atom undergoes reduction. This is known as a redox reaction.

(b) Oxidation: In the electronic concept, oxidation is a process in which an atom or ion loses electron(s).
Zn → Zn²⁺+ 2e^–Reduction: In the electronic concept, reduction is a process in which an atom or ion accepts electron(s).
Cu²⁺ + 2e^– → Cu

13. Potassium (at No.19) and chlorine (at No.17) react to form a compound. Explain on the basis of electronic concept:

(a) Oxidation

(b) Reduction

(c) Oxidizing agent

(d) Reducing agent

Solution

Atomic Structure and Chemical Bonding

2K + Cl₂ → 2KCl

(a) Oxidation: In the electronic concept, oxidation is a process in which an atom or ion loses electron(s).

K → K⁺+ e^–

(b) Reduction: In the electronic concept, reduction is a process in which an atom or ion accepts electron(s).

Cl₂ + 2e^–→ 2Cl^–

(c) Oxidising agent

An oxidising agent oxidises other substances either by accepting electrons or by providing oxygen or an electronegative ion, or by removing hydrogen or an electropositive ion.

Cl₂ + 2e^–→ 2Cl^–

(d) Reducing agent

A reducing agent reduces other substances either by providing electrons or by providing hydrogen or an electropositive ion, or by removing oxygen or an electronegative ion.

K → K⁺+ e^–

Exercise 4E

1. (a) Define covalent bond.

(b) Give an example of the covalent bond formed by

(i) Similar atoms (ii) Dissimilar atoms

Solution

(a) The chemical bond formed between two combining atoms by mutual sharing of one or more pairs of electrons is called a covalent bond.

(b) Covalent bond formed by

(i) Bond formed between two Cl atoms; Cl‒Cl

(ii) Bond formed between hydrogen atom and chlorine atom; H‒Cl

2. Covalent bonds can be single, double or triple covalent bonds. How many electrons are shared in each? Give an example of each type.

Solution

A single covalent bond is formed by sharing of one pair of electrons between atoms, each atom contributing one electron.
A double bond is formed by sharing of two pairs of electrons between two atoms.
A triple bond is formed by sharing of three pairs of electrons between two atoms.

3. Show number of bonds in

(i) ethene molecule

(ii) ethyne molecule

Solution

(i) Ethane molecule has one double covalent bond and four single covalent bonds.

(ii) Ethyne molecule has one triple covalent bond and two single covalent bonds.

4. An element A has 1 electron in its first shell. It combines with element B having 7 electrons in its third shell. What type of bond is formed?

Solution

Element A with 1 electron in its first shell is hydrogen, and element B with 7 electrons in its third shell is chlorine.

So, a single covalent bond is formed between hydrogen and chlorine by sharing one pair of electrons.

5. Match the atomic numbers 4, 8, 10, 15 and 19 with each of the following:

(a) Element which can form trivalent ion

(b) Element with four shells

(c) Element with 6 valence electrons

(d) Element which does not form ion

Solution

(a) Atomic number 15
Electronic configuration (15): 2,8,5

(b) Atomic number 19
Electronic configuration (19): 2,8,8,1

(d) Atomic number 10
Electronic configuration (10): 2,8

6. If electrons are getting added to an element Y; then

(a) Is Y getting oxidized or reduced?

(b)What charge will Y migrate to during the process of electrolysis?

Solution

(a) Electrons are getting added to element Y, so, it is getting reduced.

(b) Y will migrate towards the positive charge.

7. (a) Elements X, Y and Z have atomic numbers 6,9 and 12 respectively. Which one:

(i) Forms an anion

(ii) Forms a cation

(b) State the type of bond between Y and Z and give its molecular formula.

Solution

(a) (i) Y = 9

(ii) Z = 12

(b) Ionic bond with molecular formula ZY₂.

8. Atomic Structure and Chemical Bonding

Taking MgCl₂ as an electrovalent compound, CCl₄ as a covalent compound, give four difference between electrovalent and covalent compounds

Solution

Difference between electrovalent and covalent compounds

MgCl₂ – Electrovalent compound	CCl₄ – Covalent compound
They are hard crystalline solids consisting of ions.	These are gases or liquids or soft solids.
They have high melting and boiling points.	They have low melting and boiling points.
They conduct electricity in the fused or aqueous state.	They do not conduct electricity in the solid, molten or aqueous state.
These are soluble in inorganic solvents but insoluble in organic solvents.	These are insoluble in water but dissolve in organic solvents.

9. Potassium chloride is an electrovalent compound, while hydrogen chloride is covalent compound, But, both conduct electricity in their aqueous solutions. Explain.

Solution

Potassium chloride is an electrovalent compound and conducts electricity in the molten or aqueous state because the electrostatic forces of attraction weaken in the fused state or in aqueous solution.

Polar covalent compounds like hydrogen chloride ionise in their solutions and can act as an electrolyte. So, both can conduct electricity in their aqueous solutions.

10. Name two compounds that are covalent when taken pure but produce ions when dissolved in water.

Solution

HCl and NH₃

11. An element M burns in oxygen to form an ionic compound MO. Write the formula of the compounds formed if this element is made to continue with chlorine and sulphur separately.

Solution

Formula of compound when combined with sulphur – MS

Formula of compound when combined with chlorine – MCl₂

12. Give orbital diagram of the following:

(a) Magnesium chloride

(b) Nitrogen

(c) Methane

(d) Hydrogen chloride

Solution

(a) orbital diagram of the Magnesium chloride

(b) orbital diagram of the Nitrogen

(d) orbital diagram of the Hydrogen chloride

13. State the type of bonding in the following molecules.

(a) Water,

(b) Calcium oxide

(c) Hydrogen chloride

Solution

(a) Polar covalent bond

(b) Ionic bond

14.Atomic Structure and Chemical Bonding

Element M forms a chloride with the formula MCl₂ which is a solid with high melting point. What type of bond is in MCl₂. Write the formula of the compound when M combine with sulphur, oxygen and nitrogen.

Solution

The bond formed between metal and nonmetal is ionic bond.

Bond formed between metal M and chlorine is ionic bond.

Sulphur: When metal M combines with sulphur – MgS

Oxygen: When metal M combines with oxygen – MgO

Nitrogen: When metal M combines with nitrogen – Mg₃N₂.

15. (a) Mass of an atom is concentrated inside the nucleus of the atom.

(b) Atoms combine by transfer and sharing of electron(s).

(c) An element has atoms with different mass number.

(d) Carbon-12 and carbon-14 both show similar chemical properties.

Solution

(a) Mass of an atom is contributed by mass of protons and mass of neutrons present inside the nucleus of an atom. Electrons present outside the nucleus are of negligible mass. Therefore, the mass of an atom is concentrated inside the nucleus of an atom.

(b) Atoms combine because they have incomplete valence shells and tend to attain a stable electronic configuration. So, to attain a stable electronic configuration, atoms of elements gain or share their electrons with other atoms.

(c) Mass of an atom is contributed by mass of protons and mass of neutrons present inside the nucleus of an atom. Atoms of the same element contain the same number of protons but may differ in their number of neutrons. Therefore, an element has atoms with different mass number.

(d) Carbon-12 and carbon-14 are isotopes of carbon. They have the same atomic number (6) and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. So, carbon-12 and carbon-14 show similar chemical properties.

16. (i) The characteristic of an electrovalent compound is that:

(A) They are formed by sharing of electrons.
(B) They are formed between metals and non-metals.
(C) They are formed between two non-metals.
(D) They often exist as a liquid.

(ii) When a metal atom becomes an ion:

(A) It loses electrons and is oxidised.
(B) It gains electrons and is reduced.
(C) It gains electrons and is oxidised.
(D) It loses electrons and is reduced.

Solution

(i) B. They are formed between metals and non-metals.

(ii) A. It loses electrons and is oxidized.

17. Identify the following reactions as either oxidation or reduction:

(i) O + 2e^– → O^2-

(ii) K – e^– → K⁺

(iii) Fe³⁺ + e^– → Fe²⁺

(iv) Zn → Zn²⁺+ 2e-

Solution

(i) Reduction

(ii) Oxidation

(iii) Reduction

(iv) Oxidation

18. (a) Name the charged particles which attract one another to form electrovalent compounds.

(b) In the formation of electrovalent compounds, electrons are transferred from one element to another. How are electrons involved in the formation of a covalent compound?

(c) The electronic configuration of nitrogen is (2, 5). How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule?

(d) In the formation of magnesium chloride (by direct combination between magnesium and chlorine), name the substance that is oxidized and the substance that is reduced.

Solution

(a) Cation and anion

(b) By mutual sharing of electrons

(d) Magnesium is oxidized and chlorine is reduced.

19. What is the term defined below?

(a) A bond formed by a shared pair of electrons, each bonding atom contributing one electron to the pair.

(b) A bond formed by transfer of electron(s).

Solution

(a) Single covalent bond

(b) Electrovalent bond

20. Name or state the following:

(a) An element having valency zero

(b) Metal with valency one

(d) Elements having same mass number but different atomic number

(e) Bond formed by transfer of electron(s)

(f) Ion formed by gain of electron(s)

Solution

(a) Helium

(b) Lithium

(d) ⁴⁰₁₈Ar and ⁴⁰₂₀Ca

(e) Ionic bond

(f) Anion

21. An element X has 2 electrons in its M shell, it forms bond with an element Y which has 7 electrons in its third orbit.

(a) Write the formula of the compound formed.

(b) Which nearest inert gas electronic configuration will element X and Y acquire.

(c) Show by orbital diagram the formation of compound between X and Y.

Solution

Element X has 2 electrons in its M shell.

The electronic configuration would be 2, 8, 2.

So, element X is Mg (12).

Element Y has 7 electrons in its third orbit.

The electronic configuration would be 2, 8, 7.

So, element Y is Cl (17).

(a) MgCl₂

(b) The nearest inert gas electronic configuration for element X is 2,8, while that for element Y is 2,8,8.

Magnesium chloride:

22. In the formation of (i) oxygen molecule (ii) carbon tetrachloride molecule, state the following:

(a) Electronic configuration of nearest inert gas attained.

(b) How many electrons are shared/transferred in bond formation?

(c) Which type of bonds these compounds form?

(d) Draw their orbital diagrams.

Solution

In the formation of Oxygen molecule

(i) oxygen molecule

(a) Neon (10) 2,8

(b) Two pairs of electrons are shared.

(d) Orbital Diagram of Oxygen molecule

(ii) Carbon tetrachloride molecule

(a) Neon (10) 2,8

(b) Four pair of electrons are shared.

(d) Orbital Diagram:

ICSE Solutions for Chapter 4 Atomic Structure and Chemical Bonding Class 9 Selina Chemistry

Exercise 4B

Exercise 4C

Exercise 4D

Exercise 4E

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Selina Concise Chapter 4 Atomic Structure and Chemical Bonding ICSE Solutions Class 9 Chemistry

ICSE Solutions for Chapter 4 Atomic Structure and Chemical Bonding Class 9 Selina Chemistry

Exercise 4B

Exercise 4C

Exercise 4D

Exercise 4E

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