LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 9 Chemistry Chapter 4 - Structure Of The Atom

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Chapter 4 - Structure Of The Atom Exercise 191

Question 1
Which subatomic particle is not present in an ordinary hydrogen atom ?
Solution 1
Neutron is not present in ordinary hydrogen atom.
Question 2
Name the scientists who described the arrangment of electrons in an atom.
Solution 2
J. J. Thomson
Question 3
What is the maximum number of electrons which can be accommodated in the K shell of an atom ?
Solution 3
Maximum of 2 e- can be accommodated in K shell of an atom.
Question 4
What is the maximum number of electrons which can be accommodated in the L shell of an atom ?
Solution 4
Maximum of 8 e- can be accommodated in L shell of an atom.
Question 5
What is the maximum number of electrons an M shell of the atom can accommodate ?
Solution 5
Maximum of 18 e- can be accommodated in M shell of an atom.
Question 6
What is the maximum number of electrons that can go into the N shell of an atom ?
Solution 6
Maximum of 32 e- can be accommodated in N shell of an atom.
Question 7
What is the maximum number of electrons which can be accommodated in the:
(a) innermost shell of an atom ?
(b) outermost shell of an atom ?
Solution 7
(a). Maximum of 2 e- can be accommodated in innermost shell of an atom.
(b). Maximum of 8 e- can be accommodated in outermost shell of an atom.

Question 8
 Name the three subatomic particles present in an atom.
Solution 8
Three subatomic particles present in atom are electrons, protons and neutrons.
Question 9
Name the negatively charged particle present in the atoms of all the elements.
Solution 9
Electron is negatively charged particle present in atoms of all the elements.
Question 10
Name the scientist who discovered electron.
Solution 10
J. J. Thomson discovered electron.
Question 11
What is the usual symbol for (a) an electron (b) a proton, and (c) a neutron?
Solution 11
(a). e-
(b). p+
(c). n

Chapter 4 - Structure Of The Atom Exercise 192

Question 12
12. State  whether the following statements are true or false :
(a) Thomson proposed that the nucleus of an atom contains protons and neutrons.
(b) The cathode ravs obtained from all the gases consist of negatively charged particles called  electrons.
(c) The anode rays obtained from all the gases consist of positively charged particles called protons.

Solution 12
(a). False
(b). True
(c). False

Question 13
Name the central part of an atom where protons and neutrons are held together.
Solution 13
Nucleus is the central part of an atom in which protons and neutrons are held together.
Question 14
What are the various letters used by Bohr to represent electron shell in an atom?
Solution 14
K, L, M, N were the letters used by Bohr to represent electron shells in an atom.
Question 15
Name the particles which actually determine the mass of an atom.
Solution 15
Protons and neutrons actually determine the mass of atom.
Question 16
Name the positively charged particle present in the atoms of all the elements
Solution 16
Proton is the positively charged particle present in atoms of all the elements.
Question 17
What is the electronic configuration of a hydrogen atom?
Solution 17
Electronic configuration of hydrogen : 1
Question 18
How many times is a proton heavier then an electron?
Solution 18
Proton is 1840 times heavier than electron.
Question 19
Name the gas which produces anode rays consisting of protons in the discharge tube experiment.
Solution 19
Hydrogen gas produces anode rays consisting of protons in the discharge tube experiment.
Question 20
Which part of an atom was discovered by Rutherford's alpha particle scattering experiment?
Solution 20
Nucleus was discovered by Rutherford in the alpha particle scattering experiment.
Question 21
What is the positive charge on the nucleus of an atom due to ?
Solution 21
Positive charge on the nucleus is due to presence of protons.
Question 22
State the number of electrons present in the outermost shell of the atoms of the following elements:
(a) Neon                      (b) Chlorine

Solution 22

(a). 8 electrons are present in outermost shell of Neon.

(b). 7 electrons are present in outermost shell of Chlorine.
Question 23
Which shell of an atom can accommodate a maximum of?
(a) 8 electrons?                        (b) 32 electrons?

Solution 23
(a). L shell can accommodate maximum of 8 e-.
(b). N shell can accommodate maximum of 32 e-.

Question 24
. Name the shell of an atom which can accommodate a maximum of :
(a) 2 electrons                 (b) 18 electrons

Solution 24
(a). K shell can accommodate maximum of 2 e-.
(b). M shell can accommodate maximum of 18 e-.

Question 25
Which subatomic particle was discovered by :
(i) Chadwick?          (ii)  Thomson?     (iii)  Goldstein?

Solution 25
(i). Chadwick discovered 'neutron'.
(ii). Thomson discovered 'electron'.
(iii). Goldstein discovered 'proton'.

Question 26
Name the subatomic particle whose relative charge is :
(a) +1       (b) -1           (c) 0

Solution 26
(a). Proton has relative charge of +1.
(b). Electron has relative charge of -1.
(c). Neutron has relative charge of 0.  

Question 27

 Fill in the blanks in the following statements:

(a) The number of protons in the nucleus of an atom is called its ______.
(b) The total number of protons and neutrons in the nucleus of an atom is called ______.
(c) An atom has atomic mass number 23 and atomic number 11. The atom has ______ electrons.
(d) An atom of an element has 11 protons, 11 electrons and 12 neutrons. The atomic mass of the atom is ______.
(e) If the nucleus of an atom has atomic number 17 , mass number 37 and there are 17 electrons outside the nucleus, the number of neutrons in it is ______.
(f) Almost all the mass of an atom is concentrated in a small region of space called the ______.
(g) Cathode rays are a beam of fast moving ______.
(h) The anode rays obtained from hydrogen gas consist of particles called ______.
(i) The maximum number of electrons that can be accommodated in L shell are ______.
(j) The maximum number of electrons that can go into the M shell is ______.
(k) The subatomic particle not present in a hydrogen atom is ______.
(l) The electron has ______ charge, the proton has ______ charge, and the neutron has ______ charge.

Solution 27
(a)    Atomic number
(b)    Mass number
(c)    11
(d)    23
(e)    20
(f)    Nucleus
(g)    Electrons
(h)    Protons
(i)    8
(j)    18
(k)    Neutron
(l)    Negative; Positive; No

Question 28
What is an electron? State its relative mass and charge.
Solution 28
Electron is a negatively charged particle found in the atoms of all elements.
The relative mass of an electron is 1/1840 u.
A charge of -1 is carried by an electron.

Question 29
What is the absolute mass and charge of an electron ?
Solution 29

Absolute mass of electron is 9 x 10-28 Kg.

Absolute charge on electron is 1.6 x 10-19 C.

Question 30
Give the evidence for the existence of nucleus in an atom.
Solution 30
The deflection of fast moving alpha- particles through small and large angles in Rutherford's scattering experiment is the evidence for the presence of nucleus in an atom.
Question 31
What important information is furnished about the nucleus of an atom by the alpha particle scattering experiment of Rutherford?
Solution 31

Important information furnished about nucleus in Rutherford's alpha- particle scattering experiment is:

(i). Nucleus of an atom is positively charged.
(ii). Nucleus of an atom is very hard and dense.
(iii). Nucleus of an atom is very small as compared to the size of an atom as a whole.

Question 32
How was it shown that an atom has a lot of empty space within it ?
Solution 32
Most of the alpha- particles passed straight through the gold foil without any deflection in Rutherford's alpha- particle scattering experiment, this shows that most of the space in an atom is empty.
Question 33
Why is an atom neutral inspite of the presence of charged particles in it ?
Solution 33
There are equal no. of positive and negative particles in an atom, so it is neutral as a whole.
Question 34
(a) Which of the nuclear particles is present in the same fixed number in the atoms of any particular element?
(b) What do we call this number which is characteristic of a particular element?

Solution 34
(a). Proton is present in same fixed no. in the atoms of any particular element.
(b). Atomic no. is characteristic for any particular element.

Chapter 4 - Structure Of The Atom Exercise 193

Question 35
What is a proton ? State its relative mass and charge.
Solution 35
Protons are positively charged particle found in the atoms of all the elements.
Relative mass of proton is 1u.
Relative charge of proton is +1 C.

Question 36
What is the absolute mass and charge of a proton ?
Solution 36
Absolute mass of proton - 1.6x 10-27 Kg
Absolute charge of proton - 1.6 x 10-19 C

Question 37
How does a proton differ from an electron ?
Solution 37

Difference between proton and neutron-

(1). Proton is positively charged while electron is negatively charged.
(2). Proton is much heavier than electron.

Question 38
State two observations which show that atom is not indivisible.
Solution 38

Two observations which shows that atom is not indivisible are-

(1). In J. J. Thomson's experiment, the stream of cathode rays in the gas discharge tube shows the presence of negatively charged subatomic particles called electrons.
(2). In Goldstein's experiment, the faint red glow in the gas discharge tube shows the presence of positively charged subatomic particles called protons.

Question 39
All the gases from cathode rays and anode rays when electricity is passed through them:
(i) What does the formation of cathode rays tell us about the atoms?
 (ii) What does the formations of anode rays tell us about the atoms?

Solution 39
(i). Formation of cathode rays tells about the presence of negatively charged electrons in all the atoms.
(ii). Formation of anode rays tells about the presence of positively charged protons in all the atoms.

Question 40
What do you understand by the term "electronic configuration" of an element?  Write down the electronic configuration of oxygen (At. No. =8).
Solution 40
The arrangement of electrons in various shells of an atom of the element is known as electronic configuration of the element.
Electronic configuration of oxygen (atomic no. = 8) is (2,6)

Question 41
An elements has an atomic number 12.How many electrons will be present in the  K,L and M energy shells of its atom ?
Solution 41
Electronic configuration of element with atomic no. 12- (2,8,2)
             So, K-2 ; L-8 ; M-2

Question 42

(a) What is the nucleus of an atom and  what is the nature of charge on it?

(b) Name the scientist who discovered the nucleus of atom.

Solution 42

(a). Nucleus is a small positively charged part at the center of an atom. Nucleus is positively charged.

(b). Rutherford discovered nucleus of an atom.
Question 43
Name the particles used by Rutherford in his experiment on the discovery of nucleus. Also state the charge on these particles.

Solution 43
Alpha - particles were used by Rutherford in his experiment on the discovery of nucleus.
Alpha - particles have +2 units of charge.

Question 44
An element has atomic number 13 and an atomic mass of 27.

(a) How many electrons are there in each atom of the element?
(b) How are these electrons distributed in the various energy levels?
Solution 44
(a). There are 13 e- in each atom of the element.
(b). Electronic configuration of given element- (2,8,3)
       K-2 ; L-8 ; M-3

 
Question 45
Write the distribution of electrons in an atom of element whose atomic number is 18. What is special about the outermost electron shell (or valence shell) of the atom of this element ?

Solution 45
Atomic No. - 18
Electronic configuration - (2,8,8)
The special thing about the outermost shell is that it is completely filled with the electrons.

Question 46
What is neutron? State its relative mass and charge.
Solution 46
The neutron is a neutral particle found in the nucleus of an atom. Its relative mass is 1 u. It has no charge.

Question 47
Compare an electron, a proton and a neutron in respect of their relative masses and charges.
Solution 47
Electron has relative mass of 1/1840 u, proton has 1u and neutron also has 1u.
Electron has relative charge of -1u, proton has +1u and neutron has 0 relative charge.

Question 48
What is a proton ?How does it differ from a neutron ?
Solution 48
Protons are positively charged particle found in the atoms of all the elements whereas neutron is a neutral particle found in the nucleus of an atom.
Question 49
Compare an electron and a proton in respect of mass and charge.

Solution 49
Electron has relative mass of 1/1840 u and proton has relative mass of 1u.
Electron has relative charge of -1u while proton has +1u of relative charge.

Question 50
Compare a proton and a neutron in respect of mass and charge.

Solution 50
Proton has relative mass of 1u and neutron also has relative mass of 1u.
Proton has relative charge of +1u and neutron has no relative charge.

Question 51
How does an electron differ from a neutron ?
Solution 51
Electron has relative charge of -1 u whereas neutron has no relative charge.
Also, electron has relative mass of 1/1840 u and neutron has relative mass of 1 u.

Question 52
State the location of electrons, protons and neutrons in an atom.
Solution 52
Protons and neutrons are collectively present in the nucleus at the center while electrons revolve rapidly round the nucleus in fixed circular orbits called energy levels.
Question 53

Fill in the following blanks :

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Solution 53
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Question 54

Fill in the following blanks in respect of an atom of an element:

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Solution 54
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Question 55

(a) What are cathode rays? What is the nature of charge on cathode rays ?

(b) Explain how, cathode rays are formed from the gas taken in the discharge tube.

(c) What conclusion is obtained from the fact that all gases form cathode rays ?

Solution 55
(a). The stream of particles coming out from cathode (negative electrode) are called cathode rays. Cathode rays are negatively charged.
(b). When electricity at high voltage is passed through a gas at very low pressure taken in discharge tube, stream of minute particles are given out by the cathode. These stream of particles are called cathode rays.
(c). The conclusion is that all the atoms contain negatively charged particles called electrons.

Question 56
(a) Describe Thomson's model of the atom. Which subatomic particle was not present in Thomson's model of the atom?

(b) The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?

Solution 56

(a). According to Thomson model of atom- An atom consists of a sphere of positive charge with negatively charged electrons embedded in it. The positive and negative charges in an atom are equal in magnitude.

Neutron was not present in the Thomson model of atom.

(b). When mass no. is 18 and no. of electrons is 7 then
    (i). No. of protons = 7
    (ii). No. of neutrons = 18 - 7 = 11
    (iii). Atomic no. = 7

Question 57
(a) Describe the Rutherford's model of an atom. State one drawback of Rutherford's model of the atom.   
(b)The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it  ? What is the the atommic number of the element ?
Solution 57

(a).Rutherford's model of atom-

1. An atom consists of positively charged, dense and very small nucleus containing all the protons and neutrons. Almost all the mass of atom is concentrated in the nucleus.

2. The nucleus is surrounded by negatively charged electrons. The electrons are revolving at very high speed round the nucleus in fixed circular orbits.

3. The electrostatic attraction between the positively charged nucleus and negatively charged electrons keep the atom held together.

4. An atom is electrically neutral.

5. Most of the space in an atom is empty.

The major drawback of Rutherford model of atom is that it does not explain the stability of the atom.

(b).Given: Mass no. = 23
               No. of electrons = 11

Then, no. of protons = 11
         No. of neutrons = 23 - 11 = 12
         Atomic no. = 11

Question 58

(a) Describe Bohr's model of the atom. How did Neils Bohr explain the stability of atom ?     

(b) An element has an atomic number of 11 and its mass number is 23. What is the arrangement of electrons in the shells? State nuclear composition of an atom of the element.

Solution 58

(a). Bohr's model of atom-
1. An atom is made up of three particles, namely electrons, protons and neutrons.
2.The protons and neutrons are located in the small nucleus at the center of atom.
3. Electrons revolve round the nucleus in fixed circular orbits.
4. Maximum no. of electrons for any given shell is fixed. Any shell cannot exceed that maximum value.
5. Each given shell is associated with fixed amount of energy.
6. There is no change in energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable.

(b).Given: Atomic no. = 11
                 Mass no. = 23

Then, electronic configuration - (2,8,1)
         Nuclear composition is - 11 protons and 12 neutrons

Chapter 4 - Structure Of The Atom Exercise 194

Question 59
(a) What is meant by (i) atomic number, and (ii) mass number, of an element ? Explain with the help of an example.
(b) What is the relation between the atomic number and mass number of an element?
(c) If an element M has mass number 24 and atomic number 12, how many neutrons does its atom contain?
Solution 59

(a). (i). Atomic no. is the number of protons in one atom of an element.
     (ii). Mass no. is the total number of protons and neutrons present in one atom of the element.
Example- The total no. of protons in a carbon atom is 6, so its atomic no. is 6.
Also, one atom of Na contains 11 protons and 12 neutrons, so its mass no. is 23.

(b). Mass No. = Atomic no. + No. of neutrons
(c). No. of neutrons = Mass No. - Atomic no.
                            = 24 - 12 = 12

Question 71
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
 (i) mass number of phosphorus,
 (ii) atomic number of phosphorus, and
 (iii) electron configuration of phosphorus

Solution 71
(i). Mass no. = 31
(ii). Atomic no. = 15
(iii). E.C. = (2,8,5)

Question 72
The atom of an element X has 7 electrons in its M shell.
 (a) Write the electronic configuration of element X.
 (b) What is the atomic number of element X ?
 (c) Is it a metal or a non-metal ?
 (d) What type of ion will be formed by an atom of element X ? Write the  symbol of the ion formed.
 (e) What could  element X be ?

Solution 72

(a). E.C. - (2,8,7)

(b). Atomic No. = 17

(c). Non-metal

(d). Anion; X-

(e). X must be Chlorine
Question 73
An atom of element E contains 3 protons, 3 electrons and 4 neutrons :
    (a) What is its atomic number?
    (b) What is its mass number?
    (c) Write the electronic configuration of the element E.
    (d) State whether element E is a metal or non-metal. Why?
    (e) What type of ion, cation or anion, will be formed by an atom of element E? Why?
    (f) Write the symbol of the ion formed by an atom of element E.
    (g) What could element E be?
Solution 73
(a). Atomic no. = 3
(b). Mass no. = 3 + 4 = 7
(c). E.C. - (2, 1)
(d). Metal
(e). Cation will be formed; because outermost single electron can be easily donated.
(f). E+
(g). Lithium

Chapter 4 - Structure Of The Atom Exercise 195

Question 74
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
  (a) What does the figure 9 indicate ?
  (b) What does the figure 4 indicate ?
  (c) What is the number of protons in atom X ?
  (d) What is the number of neutrons in atom X ?
  (e) What is the number of electrons in atom X ?
  (f) How many electrons are there in the outermost shell of an atom of element X ?
  (g) Write the symbol of ion formed by an atom of element X .
Solution 74
(a). Mass number
(b). Atomic number
(c). No. of protons = 4
(d). No. of neutrons = 9 - 4 = 5
(e). No. of electrons = 4
(f). Electrons in outermost orbit = 2
(g). X2+

Question 75
The electronic configuration of an element Z is 2, 8, 8.
  (a) What is the atomic number of the element ?
  (b) State whether element Z is a metal or a non-metal.
  (c) What type of ion (if any) will be formed by an atom of element Z ? Why ?
  (d) What is special about the outermost electron shell of the atom of this element ?
  (e) Give the name and symbol of element Z.
  (f) Name the group of element to which Z belongs.
Solution 75
(a). Atomic no. = 18
(b). Element Z is non-metal
(c). As the outermost shell of element Z is completely filled so, it will not form any ion.
(d). Outermost electronic shell is completely filled with electrons.
(e). Name of element 'Z' = Argon
      Symbol is Ar
(f). Z belongs to the group 'Noble gases'.

Chapter 4 - Structure Of The Atom Exercise 210

Question 1
The total number of electrons in nitrogen atom is 7.Find the number of valence electrons in it.

Solution 1

E.C of Nitrogen = 2, 5

So, no. of valence electrons in Nitrogen atom = 5

Question 2
What is the general name of the element having 8 electrons in the valence shell of their atoms ?
Solution 2
Noble gases
Question 3
Which noble gas has less then 8 electrons in the valence shell of its atom ? What is this number ?
Solution 3
Helium has less than 8 electrons in the valence shell of an atom. Its atomic no. is 2
Question 4
State one use of radioactive isotopes in medicine.
Solution 4
Radioactive isotopes are used in the treatment of cancer.
One such isotope is Cobalt-60.

Question 5
Give one example of a radioactive isotope which is used as a fuel in the reactors of nuclear power plants.

Solution 5
Uranium-235 isotope is used as a fuel in the reactors of nuclear power plants for generating electricity.
Question 6
Name the radioactive isotope which is used in the treatment of cancer.
Solution 6
Cobalt-60 radioisotope is used in the treatment of cancer.
Question 7
Which radioactive isotope is used to determine the activity of thyroid gland ?
 
Solution 7
Iodine-131 radioisotope is used to determine the activity of thyroid gland.
Question 8
State one use of radioactive isotopes in industry.
Solution 8
Radioactive isotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipes.
Question 9
State whether the following statement is true or false :
Radioactive isotope of iodine is used for making the medicine called tincture iodine.

Solution 9
The given statement is false.
Question 10
What name is given to those atoms which contain the same number of protons and electrons but different number of neutrons :?

Solution 10
Atoms containing same number of protons and electrons but different number of neutrons are called ISOTOPES.
Question 11
What is the relationship between an atom containing 11 protons, 11 electrons, and 11 neutrons, and another atom containing 11 protons, 11 electrons, and 12 neutrons?
Solution 11
Isotopes
Question 12
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Solution 12
The given pair are isotopes.
Question 13
What name is given to those isotopes which have unstable unclei and emit various types of radiations ?

Solution 13
Radioactive isotopes have unstable nuclei and emit various types of radiations.
Question 14

Fill in the following blanks in respect of an atom of an element :
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Solution 14
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Question 15

Complete the following statements:

(a) Magnesium has 2 valence electrons in the ..........shell.
(b) The valency of nitrogen in N2 molecule is ..........
(c) Isotopes have different mass number because their nuclei contain different number of .........
(d) Some boron atoms have mass number 10 and some have mass number 11. These boron atoms with
     different mass numbers are called.........

Solution 15

(a). M

(b). 3

(c). neutrons

(d). isotopes

Question 16
The nucleus of an atom has 5 protons and 6 neutrons. What would be the (a) atomic number, (b) mass  number, (c) the number of electrons, and (d) the number of valence electrons, per atom of this element?
Solution 16
(a). Atomic no. = 5
(b). Mass no. = 6 + 5 = 11
(c). No. of electrons = 5
(d). No. of valence electrons, per atom = 3

Question 17
Write the electronic configuration of the element with atomic number 17. Indicate the valency of the element.
Solution 17
Atomic No. = 17
E.C. = (2, 8, 7)
Valency = 8 - no. of valence electrons = 8 - 7 = 1

Question 18
The atomic number of an element X is 16.
  (a) Write down the electronic configuration of X.
  (b) What will be the valency of X ?
Solution 18

Atomic No. of X = 16

E.C. of X = (2, 8, 6)

Valency of X =  8 - no. of valence electrons = 8 - 6 = 2

Chapter 4 - Structure Of The Atom Exercise 211

Question 19
What valencies will be shown by the elements A, B, C, D and E having atomic numbers 2, 4, 8, 10 and 13 respectively.

Solution 19
Valency shown by A (atomic no. 2) -  0
Valency shown by B (atomic no. 4) -  2
Valency shown by C (atomic no. 8) -  2
Valency shown by D (atomic no. 10) -  0
Valency shown by E (atomic no. 13) -  3

Question 20

Give one use of each of the following radioactive isotopes :

(a) Uranium-235 (b) Cobalt-60

Solution 20
Uranium-235 : This isotope is used as a fuel in the reactors of nuclear power plants for generating electricity.
Cobalt-60 : This is used in the treatment of cancer.

Question 21
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Solution 21
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom
Question 22
What is the reason for the different atomic masses of the isotopes of an element ?

Solution 22
The difference in the masses of isotopes of an element is due to different number of neutrons in their nuclei.
Question 23
What is the reason  for the identical chemical properties of all the  isotopes of an element ? Explain with the help of  an  example.
Solution 23
Because all the isotopes of an element have identical atomic configuration containing same number of valence electrons, therefore, all the isotopes of an element show identical chemical properties.
For example- Cl-35 and Cl-37, show identical chemical properties as they have same no. of 7 valence electrons.

Question 24
What is the reason for the slightly different physical properties of all isotopes of an  element ?
Solution 24
Due to slight difference in the masses of the isotopes of an element, the physical properties of the isotopes are slightly different.
Question 25
Explain  why , the atomic masses of many elements are in  fractions and not whole number.
Solution 25
The fractional atomic masses of elements are due to the existence of their isotopes having different masses.
Question 26

Which of the following are isotopes and which are isobars?

Argon, Deuterium, Calcium, Tritium, Protium

Solution 26

Deuterium, Protium and Tritium are isotopes.

Argon and Calcium are isobars.

Question 27

Hydrogen has three isotopes written as :

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Explain why:
  (i) these isotopes have almost identical chemical properties.
  (ii) they are electrically neutral.

Solution 27
(i). Due to identical electronic configuration containing the same no. of valence electrons, these isotopes have almost identical chemical properties.
(ii). All of them have 1 electron and 1 proton, so, they are electrically neutral.

Question 28

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Solution 28
Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Question 29
What are isobars? Explain with an example.

Solution 29
Isobars are the atoms of different elements having different atomic numbers but the same mass number (or same atomic mass).
Question 30
For the symbols H, D and T,  write the subatomic  particles (protons, neutrons and electrons) found in each one of them.

Solution 30
H - 1 proton, 1 electron and no neutron.
D - 1 proton, 1 electron and 1 neutron.
T - 1 proton, 1 electron and 2 neutrons.

Question 31
An element has Z = 7. What is the valency of the element? Also name the element.
Solution 31

Atomic No. = 7

E.C = 2, 5

Valency of given element = 3

Given element is NITROGEN.

Question 32
 (a) What are valence electrons ? Where are valency electrons situated in an atom ?
 (b) What is the number of valence electrons in the atoms of an element having atomic number 13 ? Name the valence shell of this atom.
Solution 32
(a). The number of electrons present in the valence shell are called valence electrons.
 Valence electrons are situated in the outermost shell.
(b). There are 3 valence electrons present in the element with atomic no. 13.
Valence shell of this atom is M.

Question 33

 (a) What are isotopes ? Explain by giving an example.
 (b) Give one similarity and one difference between a pair of isotopes.

 (c) Give the number of protons, neutrons and electrons per atom in the two isotopes of chlorine 35Cl17 and 37Cl17 .


   

Solution 33

(a). Isotopes are the atoms of the same element having the same atomic number but different mass numbers.
For example - 35Cl17 and 37Cl17 are isotopes of chlorine.

(b). Similarity - A pair of isotopes have same atomic number.
     Difference - A pair of isotopes have different mass numbers.

(c). In 35Cl17 - 17 protons, 17 electrons and 18 neutrons.

      In 37Cl17 - 17 protons, 17 electrons and 20 neutrons.

Question 34

 (a) What are radioactive isotopes ? Give two examples of radioactive isotopes.

 (b)  Give any two uses of radioactive isotopes.

  (c) An element Z contains two naturally occuring isotopes 35Z17 and 37Z17. If the average atomic mass of this element be 35.5 u, calculate the percentage of two isotopes.
 

Solution 34

(a).The isotopes which are unstable due to presence of extra neutrons in their nuclei and emit various types of radiations, are called radioactive isotopes or radioisotopes.
For example: Carbon - 14 , Arsenic - 74

(b). Uses of isotopes-
     (i). They are used in the treatment of cancer.
     (ii). Radioactive isotopes are used as 'tracers' in medicine to detect the presence of tumors and blood clots in human body.

(c). Average atomic mass = 35.5 u
Let % amount of 35Z17 be y, then amount of 37Z17 is (100 - y).

Then-
 Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Question 35
(a) Define valency of an element. What valency will be shown by an element having atomic number 14?
(b) What is the relation between the valency of an element and number of valence electrons in its atoms? Explain with examples.

Solution 35

(a). The capacity of an atom of an element to form chemical bonds is known as its valency.
Valency of an atom with atomic no. 14 is 4.

(b). The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.

Valency of metal = No. of valence electron in its atom
Valency of a non-metal = 8 - No. of valence electron in its atom

For example- Valency of sodium (metal) is 1 and that of chlorine (non-metal) is also 1.

Chapter 4 - Structure Of The Atom Exercise 212

Question 52
What is the number of valence electrons in :
(a) Sodium ion, Na+   (b) oxide ion, O2-

Solution 52

(a). E.C of Na+ = 2, 8

So, no. of valence electrons in sodium ion, Na+ = 8

(b). E.C of O2- = 2,8

So, no. of valence electrons in oxide ion, O-2 = 8

Question 53

Atom A has a mass number 209 and atomic number 82.
Atom B has a mass number 209 and atomic number 83.

  (i) How many protons atom A has ?
  (ii) How many protons atom B has ?
  (iii) Are atoms A and B isotopes of the same element ?

Solution 53

Atom A - 209A82
Atom B - 209B83

(i). A has 82 protons.
(ii). B has 83 protons.
(iii). No, A and B are not isotopes.

Chapter 4 - Structure Of The Atom Exercise 213

Question 54

Which of the following pairs are isotopes ? Give reasons for your choice:

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Solution 54
(i). 58A26 and 58B28 - these are not isotopes because they have different atomic numbers.
(ii). 79X35 and 80Y35 - these are isotopes as they have same atomic number.

Question 55

Three different atoms of oxygen are represented as :

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

(i) What do the subscripts (lower figures) and superscripts (upper figures) represent?

(ii) What factor is responible for the change in the superscripts 16, 17 and 18, though the element is the same?

(iii) What is the usual name for such atoms of an element?

(iv) Give the nuclear composition of 18O8

Solution 55

(i). Subscripts represent atomic number whereas superscripts represent atomic mass.

(ii). Number of neutrons is responsible for the change in the superscripts.

(iii). Isotopes is the usual name for the given atoms of the element.

(iv). Nuclear composition of 18O8 is:-

       No. of protons = 8

       No. of neutrons = 18 - 8 = 10

Question 56

The atomic species A and B have different number of protons but the same number of nucleons. On the other hand, the atomic species X and Y have the same number of protons but different number of nucleons. Which pair is an example of isobars? Why?

Solution 56

A and B are the example of isobars. This is because they have same number of nucleons.

Question 57
Composition of the nuclei of two atomic species A and B is give as under :

                     A              B
Protons :      18             20

Neutrons:     22             20

Give the mass number of A and B. What is the relation between the two species and which element or elements they represent ?

Solution 57
Mass no. of A and B is 40.
The two species are isobars.
A represents Argon (Atomic no. = 18) while B represents Calcium (Atomic no. = 20).

Question 58

Which of the following pairs are isobars?

Lakhmir-singh-and-manjit-kaur Solutions Cbse Class 9 Chemistry Chapter - Structure Of The Atom

Give reasons for your choice.

Solution 58
58A26 and 58B28 are isobars because they have same number of nucleons.
Question 59
The number of protons, neutrons and electrons in particles A to E are given below :
 
   Particle   Protons    Neutrons   Electrons
   A           17             18            17
   B           3               4              2
   C           18             22            18
   D           17             20            17
   E            9              10            10

 Giving reasons, find a pair of isotopes from the abave particles.

Solution 59
A and D are isotopes as they have the same number of protons.
Question 60

The composition of two atomic particles is given below:

                    X            y
   
   Protons       8           8

   Neutrons     8           9
   
   Electrons     8           8
 
   (i) Which is the mass number of X?
   (ii) Which is the mass number of Y?
   (iii) What is the relation between X and Y?
   (iv) Which element/elements do they represent?

Solution 60
(i). Mass number of X = 8 + 8 = 16
(ii). Mass number of Y = 8 + 9 = 17
(iii). X and Y are isotopes.
(iv). X and Y represent Oxygen.