SELINA Solutions for Class 9 Chemistry Chapter 4 - Atomic Structure and Chemical Bonding

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(E)

Question 1
  1. Define covalent bond.
  2. Give an example of the covalent bond formed by

(i) Similar atoms (ii) Dissimilar atoms 

Solution 1
  1. The chemical bond formed between two combining atoms by mutual sharing of one or more pairs of electrons is called a covalent bond.
  2. Covalent bond formed by
    1. Bond formed between two Cl atoms; Cl‒Cl
    2. Bond formed between hydrogen atom and chlorine atom; H‒Cl  
Question 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 2

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.

Question 3

Show number of bonds in

(i) ethene molecule  (ii) ethyne molecule

Solution 3
  1. Ethene molecule has one double covalent bond and four single covalent bonds.

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

  1. Ethyne molecule has one triple covalent bond and two single covalent bonds.

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 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 4

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.

Question 5

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

  1. Element which can form trivalent ion
  2. Element with four shells
  3. Element with 6 valence electrons
  4. Element which does not form ion 
Solution 5
  1. Atomic number 15
    Electronic configuration (15): 2,8,5
  1. Atomic number 19
    Electronic configuration (19): 2,8,8,1
  1. Atomic number 8
    Electronic configuration (8): 2,6
  1. Atomic number 10
    Electronic configuration (10): 2,8 
Question 6

If electrons are getting added to en element Y; then

(a) Is Y getting oxidized or reduced?

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

Solution 6
  1. Electrons are getting added to element Y, so, it is getting reduced.
  2. Y will migrate towards the positive charge.
Question 7
  1. Elements X,Y and Z have atomic numbers 6,9 and 12 respectively. Which one:

 i. Forms an anion

 ii. Forms a cation

  1. State the type of bond between Y and Z and give its molecular formula. 
Solution 7
  1.  

 i. Y = 9

 ii. Z = 12

  1. Ionic bond with molecular formula ZY2. 
Question 8

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

Solution 8

 

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.

 

Question 9

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

Solution 9

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.

Question 10

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

Solution 10

HCl and NH3

Question 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 11

Formula of compound when combined with sulphur - MS

Formula of compound when combined with chlorine - MCl2 

Question 12

Give orbital diagram of the following:

a) Magnesium chloride

b) Nitrogen

c) Methane

d) Hydrogen chloride

Solution 12

Orbital Diagram:

a) Magnesium chloride

 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

b) Nitrogen


Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

            Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 

c) Methane

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 

                   Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

d) Hydrogen chloride

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

          Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 13

State the type of bonding in the following molecules.

  1. Water,
  2. Calcium oxide
  3. Hydrogen chloride 
Solution 13
  1. Polar covalent bond
  2. Ionic bond
  3. Polar covalent bond 
Question 14

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 14

The bond formed between metal and nonmetal is ionic bond.

 

Bond formed between metal M and chlorine is ionic bond.

 

When metal M combines with sulphur – MgS

 

When metal M combines with  oxygen – MgO

When metal M combines with nitrogen – Mg3N2.

Question 15
  1. Mass of an atom is concentrated inside the nucleus of the atom.
  2. Atoms combine by transfer and sharing of electron(s).
  3. An element has atoms with different mass number.  
  4. Carbon-12 and carbon-14 both show similar chemical properties. 
Solution 15
  1. 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.
  2. 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.
  3. 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.
  4. 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.
Question 16
  1. 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.

 

  1. 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 16

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

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

Question 17

Identify the following reactions as either oxidation or reduction:

 i. O + 2e- → O2-

  1. K - e- → K+
  2. Fe3+ + e- → Fe2+
  3. Zn → Zn2+ + 2e- 
Solution 17
  1. Reduction
  2. Oxidation
  3. Reduction
  4. Oxidation 
Question 18
  1. Name the charged particles which attract one another to form electrovalent compounds.
  2. 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?
  3. 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?
  4. 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 18
  1. Cation and anion
  2. By mutual sharing of electrons
  3. Two
  4. Magnesium is oxidised and chlorine is reduced. 
Question 19

What is the term defined below?

  1. A bond formed by a shared pair of electrons, each bonding atom contributing one electron to the pair.
  2. A bond formed by transfer of electron(s). 
Solution 19
  1. Single covalent bond
  2. Electrovalent bond 
Question 20

Name or state the following:

(a) An element having valency zero

(b) Metal with valency one

(c) Atoms of the same element differing in mass number

(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 20

(a) Helium

(b) Lithium

(c) Hydrogen: 11H, 21H, 31H

(d)  4018Ar and 4020Ca

(e) Ionic bond

(f) Anion

Question 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.

  1. Write the formula of the compound formed.
  2. Which nearest inert gas electronic configuration will element X and Y acquire.
  3. Show by orbital diagram the formation of compound between X and Y. 
Solution 21

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).

  1. MgCl2
  2. The nearest inert gas electronic configuration for element X is 2,8, while that for element Y is 2,8,8.
  3. Orbital diagram showing the formation of the compound between X and Y:

Magnesium chloride:

 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Question 22

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

  1. Electronic configuration of nearest inert gas attained.
  2. How many electrons are shared/transferred in bond formation
  3. Which type of bonds these compounds form?
  4. Draw their orbital diagrams. 
Solution 22

In the formation of

  1. Oxygen molecule
    1. Neon (10) 2,8
    2. Two pairs of electrons are shared.
    3. Covalent bond
    4. Orbital Diagram:

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

  1. Carbon tetrachloride molecule
    1. Neon (10) 2,8
    2. Four pair of electrons are shared.
    3. Covalent bond
    4. Orbital Diagram:

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(A)

Question 1

What is the contribution of the following in atomic structure?

a. Maharshi Kanada

b. Democritus

Solution 1
  1. According to Maharshi Kanada:
    1. Matter consisted of indestructible particles called paramanus (now called atoms).
    2. 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).
  2. Democritus called paramanu as atom which comes from the Greek word atomos, meaning indivisible. 
Question 2

State Dalton's atomic theory.

Solution 2

Main postulates of Dalton's atomic theory:

  1. Matter consists of very small and indivisible particles called atoms.
  2. Atoms can neither be created nor be destroyed.
  3. The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
  4. Atoms of an element combine in small numbers to form molecules.
  5. Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
  6. Atoms are the smallest units of matter which can take part in a chemical reaction. 
Question 3

What is an α (alpha) particle?

Solution 3

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.

Question 4

What are cathode rays? How are these rays formed?

Solution 4

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.

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 5

What is the nature of the charge on

  1. Cathode rays and
  2. Anode rays?
Solution 5
  1. Cathode rays are negatively charged as they consist of negatively charged particles called electrons.
  2. Anode rays are made of positively charged particles.
Question 6

How are X-rays produced?

Solution 6

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

Question 7

Why are anode rays also called as 'canal rays'?

Solution 7

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.

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 8

How does cathode ray differ from as anode rays?

Solution 8

 

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.

 

Question 9

State one observation which shows that atom is not indivisible.

Solution 9

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.

Question 10

a. Name an element which does not contain neutron

b. If an atom contains one electron and one proton, will it carry as a whole is neutral

Solution 10

a. Hydrogen does not contain any neutron.

b. No. It is neutral.

Question 11

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

Solution 11

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.

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Thomson's atomic model

Plum pudding

 

Question 12

Which sub-atomic particle was discovered by

a. Thomson

b. Goldstein

c. Chadwick

Solution 12

a. Thomson discovered particles.

b. Goldstein discovered protons.

c. Chadwick discovered neutrons.

Question 13

Name the sub-atomic particle whose charge is:

a. +1

b. -1

c. 0

Solution 13

a. Proton

b. Electron

c. Neutron

Question 15

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

Solution 15

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

Question 16

Which part of atom was discovered by Rutherford?

Solution 16

The nucleus was discovered by Rutherford.

Question 17

How was it shown that atom has empty space?

Solution 17

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.

Question 18

State one major drawback of Rutherford's model

Solution 18

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.

Question 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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Solution 19

a)  A is for K shell or I shell.

      B is for L shell or II shell.

      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.

c)  The above sketch is of Bohr model of an atom. 

Question 20

Give the postulates of Bohr's atomic model

Solution 20
  1. Atom has a central nucleus surrounded by electrons.
  2. The electrons revolve around the nucleus in definite circular paths called orbits.
  3. Each orbit has fixed energy. Therefore, these orbits are also known as energy levels or energy shells.
  4. 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,....
  5. 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. 

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(C)

Question 1

How does the Modern atomic theory contradict and correlate with Dalton's atomic theory?

Solution 1

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:

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

 

Modern atomic theory:

  1. Atoms are no longer indivisible and consist of electrons, protons, neutrons and even more sub-particles.
  2. Atoms of the same element may differ from one another called isotopes.
  3. Atoms of different elements may be similar called isobars.
  4. 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.

 

Question 2
  1. What are inert elements?
  2. Why do they exist as monoatoms in molecules?
  3. What are valence electrons? 
Solution 2
  1. The elements have a complete outermost shell, i.e. 2 or 8 electrons. They ordinarily do not enter into any reaction.
  2. These exist as monoatoms because molecules of these elements contain only one atom.
  3. Valence electrons: The number of electrons present in the valence shell is known as valence electrons. 
Question 3

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

Solution 3

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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 4

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

  1. A solid non-metal of valency 3.
  2. A gas of valency 2.
  3. A metal of valency 1.
  4. A non-metal of valency 4. 
Solution 4

 

 

Atomic Number

Name with valency

  1. 15

A solid non-metal of valency 3

  1. 8

A gas of valency 2

  1. 19

A metal of valency 1

  1. 14

A non-metal of valency 4

 

Question 5

Draw diagrams representing the atomic structures of the following:

  1. Sodium atom
  2. Chlorine ion
  3. Carbon atom
  4. Oxygen ion 
Solution 5
  1. Sodium atom

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

  1. Chlorine atom

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

  1. Carbon atom

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

  1. Oxygen ion 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Question 6

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

Solution 6

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

Question 7

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

  1. Forms an onion
  2. Forms a cation
  3. Has four electrons in its valence shell? 
Solution 7

Atomic numbers of X          Y            Z

                               6          9           12

                            (2, 4)   (2, 7)     (2, 8, 2)

  1. Y(2, 7) forms an anion.
  2. Z(2, 8, 2) forms a cation.
  3. X(2, 4) has four electrons in the valence shell.

 

Question 8

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

  1. Predict the sign and charge on a simple ion of X.
  2. Write if X will be an oxidizing agent or a reducing agent. Why? 
Solution 8
  1. X1+
  2. Oxidising agent, because it has the ability to donate electrons. 
Question 9

Define the terms:

  1. Mass number
  2. Ion
  3. Cation
  4. Anion
  5. Element
  6. orbit 
Solution 9
  1. Mass number is the sum of the number of protons and the number of neutrons in the nucleus of an atom.
  2. Ion is an atom or molecule which carries a positive or negative charge because of loss or gain of electrons.
  3. Cation is a positively charged ion which is formed when an atom loses one or more electrons; for example, Na+, Hg2+ and Ca2+.
  4. Anion is a negatively charged ion which is formed when an atom gains one or more electrons; for example, Cland I-.
  5. 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.
  6. Orbit is a circular path around the nucleus in which electrons of the atom revolve. 
Question 10

From the symbol Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding for the element helium, write down the mass number and the atomic number of the element.

Solution 10

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

  1. Which one of these atoms:
    1. contains 7 protons
    2. has electronic configuration 2,7
  1. Write down the formula in the compound formed between C and D
  2. Predict : (i) metals (ii) non-metals 
Solution 11

a.  

  1. Atom E contains 7 protons.
  2. Atom B has an electronic configuration 2, 7.

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

    Hence, compound formula is Li2O.

c. Metals: A and C, Non-metals: B, D, E 

Question 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 12

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

Question 13

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Solution 13

 

  1.  

 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

12Mg24 Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 ii. Electronic configuration = 2, 8, 2

  1. Mass numbers of two isotopes of magnesium are different because of different number of neutrons, i.e. 12 and 14, respectively. 
Question 14

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

Solution 14

Nucleons: Particles which constitute the nucleus are called nucleons.

Protons and neutrons are the nucleons.

Atomic mass of phosphorus = 31

Atomic number = 15

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Question 15

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

Solution 15

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

Isotopes differ with reference to neutrons.

Uses of isotopes:

  1. Some isotopes are radioactive, i.e. isotopes of cobalt are used for treating cancer and other diseases.
  2. An isotope of 235U is used as a fuel in a nuclear reactor.
Question 16

Why do Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding have the same chemical properties? In what respect do these atoms differ?

Solution 16

Only electrons take part in chemical reactions. Chemical properties depend on the electronic configuration. Isotopes of the element Cl Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 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. Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding have different number of neutrons 18 and 20, respectively.

Question 17

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

Solution 17

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.

Fractional atomic mass of chlorine

At. Mass = 3(35) + 1(37) = 105 + 37

                          2                    2

Question 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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 

 

 

 

 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 

 

 

 

 

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

Solution 18

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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

17

17

18

17

35

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

17

17

20

17

37

c.  

 i. Electronic configuration of chlorine = 2, 8, 7

 ii. Electronic configuration of chlorine ion = 2, 8, 8 

Question 19

Name the following:

  1. The element which does not contain any neutron in its nucleus.
  2. An element having valency 'zero'
  3. Metal with valency 2
  4. Two atoms having the same number of protons and electrons but different number of neutrons.
  5. The shell closest to the nucleus of an atom 
Solution 19
  1. Hydrogen
  2. Helium
  3. Magnesium
  4. Hydrogen and carbon
  5. K 
Question 20

Give reasons

  1. Physical properties of isotopes are different.
  2. Argon does not react.
  3. Actual atomic mass is greater than mass number.
  4. Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 
Solution 20
  1. 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.
  2. Argon does not react as it has the outermost orbit complete, i.e. 8 electrons in the outermost shell.
  3. 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.
  4. Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bondingare 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. 

 

Question 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 21

(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.

Question 22

Choose the correct option

  1. Rutherford's alpha-particle scattering experiment discovered

A. Electron

B. Proton

C. Atomic nucleus

D. Neutron

  1. Number of valence electrons in O2- is :

A. 6

B. 8

C. 10

D. 4

  1. Which of the following is the correct electronic configuration of potassium?

A. 2,8,9

B. 8,2,9

C. 2,8,8,1

D. 1,2,8,8 

Solution 22
  1. Atomic nucleus
  2. [6]
  3. [2, 8, 8, 1] 
Question 23

Explain

a. Octet rule for formation of sodium chloride

b. Duplet rule for formation of hydrogen 

Solution 23

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

 

(a) 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) 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.

Question 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 24

 

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

 

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(D)

Question 1

How do atoms attain noble gas configurations?

Solution 1

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

Question 2

Define electrovalent bond.

Solution 2

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

Question 3

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

Solution 3

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.

Question 4
  1. An atom X has three electrons more than the noble gas configuration. What type of ion will it form?
  2. Write the formula of its (X)  

 i. sulphate

 ii. nitrate

 iii. phosphate

 iv. carbonate

 v. hydroxide. 

Solution 4
  1. An atom X will lose its three electrons to acquire the noble gas configuration and form a positive ion (a cation).
  2. The formulae are

 i. X(SO4)3

 ii. X(NO3)3

 iii. XPO4

 iv. X2(CO3)3

 v. X(OH)3 

Question 5

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

Solution 5

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.

Question 6

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

Solution 6

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.

Question 7
  1. 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?
  2. Draw the electron dot structure of this compound. 
Solution 7
  1. Nature of bond in XY2 is an ionic bond.
  2. Electron dot structure of XY2:

 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Question 8

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

  1. What type of bond will be formed between X and Y?
  2. Write the formula of the compound formed. 
Solution 8
  1. Ionic bond
  2. XY 
Question 9

Draw orbit structure diagram of sodium chloride (NaCl) and calcium oxide (CaO).

Solution 9

NaCl

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

MgCl2

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

CaO

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

 

Question 10

Compare :

  1. Sodium atom and sodium ion
  2. Chlorine atom and chloride ion, with respect to

           i. Atomic structure

          ii. Electrical state

Solution 10
  1.  

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. 

 

  1.  
    1. 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.

    1. Chlorine atom is electrically neutral. Chloride ion is negatively charged.
    2. Chlorine atom is reactive. It reacts with sodium vigorously forming sodium chloride. Chloride ion is unreactive. It does not react with sodium.
    3. 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. 
Question 11

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

Solution 11

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

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

Question 12
  1. What do you understand by redox reactions?
  2. Explain oxidation and reduction in terms of loss or gain of electrons.
Solution 12
  1. 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.
  2. 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


Question 13

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

 i. Oxidation

 ii. Reduction

 iii. Oxidizing agent

 iv. Reducing agent 

Solution 13

2K + Cl2 → 2KCl

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

K → K+ e-

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

Cl2 + 2e-→ 2Cl-

 iii. 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-

 iv. 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-

Chapter 4 - Atomic Structure and Chemical Bonding Exercise Ex. 4(B)

Question 1
  1. Name the three fundamental particles of an atom. 
  2. Give the symbol and charge of each particle. 
Solution 1
  1. The three fundamental particles of an atom are electron proton and neutron. 
  2.  

Particle

Symbol

Charge

Electron 

e

-1

Proton

p

+1

Neutron

n

No charge

 

Question 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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

11

P

11

Chlorine

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Q

18

17

Uranium

R

92

146

92

S

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

9

10

9

Solution 2

 

Element

Symbol

No. of Protons

No. of Neutrons

No. of Electrons

Sodium

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

11

12

11

Chlorine

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

17

18

17

Uranium

238U92

92

146

92

S

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

9

10

9

 

Question 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 3

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

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

Question 4

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

Solution 4

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.

Question 5

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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Solution 5

p = proton

n = neutron

e = electron

Superscript number : These superscript number show their mass number

Subscript number : These numbers show their atomic number

Question 6

From the symbolSelina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding, state the mass number, the atomic number and electronic configuration of magnesium.

Solution 6

Mass number = 24

Atomic number = 12

No. of electrons = 24 - 12 = 12

Electronic configuration = 2, 8, 2

Question 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 7

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

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Question 8

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

Solution 8
  1. 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.
  2. The maximum number of electrons possible in the outermost shell is 8 and that in the penultimate shell is 18.
  3. 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.
Question 9

Draw the orbital diagram of Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bondingion and state the number of three fundamental particles present in it.

Solution 9

Orbital diagram of Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

Atomic number of Ca2+ is 18.

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

 

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding 

 

Number of three fundamental particles of Ca2+:

Protons: 18

Electrons: 18

Neutrons: 40 - 18 = 22

Question 10

Write down the electronic configuration of the following:

Selina Solutions Icse Class 9 Chemistry Chapter - Atomic Structure And Chemical Bonding

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

Solution 10

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