NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom
Chapter Name | NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom |
Topics Covered |
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Related Study |
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NCERT Intext Questions
1. What are canal rays?Solution
The rays coming from the side of anode which ever positively charged, were discovered by Goldstein. These rays were called canal rays as they pass through the holes in the perforated cathode.
Solution
An electron (negatively charged particle) and a proton (positively charged particle) neutralise each other.
Solution
According to the Thomson’s model,
- An atom contains a positively charged sphere in which the negatively charged electrons are inserted.
- Thus, an electron (negatively charged particle) and a proton (positively charged particle) neutralises each other.
4. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Solution
According to the Rutherford’s model of an atom, the positively charged protons reside in the mucleus in the atom.
5. Draw a sketch of Bohr’s model of an atom with three shells.
Solution
6. What do you think would be the observation if the a-particle scattering experiment is carried out using a foil of a metal other than gold?
Solution
In the a-particle scattering experiment, when any other metal foil is used instead of gold, the observation would remain the same. This is because the structure of an atom when considered individually remains the same. But, the other metal should be malleable enough to be hammered to extremely thin sheets.
7. Name the three subatomic particles of an atom.
Solution
An atom consists of three subatomic particles :
- Protons which are positively charged.
- Electrons which are negatively charged.
- Neutrons which are neutral in nature (no charge).
8. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Solution
Atomic mass = number of (protons + neutrons)
4 = 2 + number of neutrons
Number of neutrons = 4 – 2 = 2
Therefore, helium has 2 neutrons.
9. Write the distribution of electrons in carbon and sodium atoms.
Solution
Carbon atom : Atomic number = 6
Number of protons = 6 = Number of electrons
Distribution = K → 2 ; L → 4
Sodium atom : Atomic number = 11
Number of protons = 11 = Number of electrons
Distribution = K → 2; L → 8; M → 1
10. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Solution
Number of electrons K shell can hold = 2
Number of electrons L shell can hold = 8
Hence, when both the shells are full, the total number of electrons present = 2 + 8 = 10 electrons.
11. How will, you find the valency of chlorine, sulphur and magnesium?
Solution
The valency of an element is defined as its combining power with other atoms when it forms chemical compounds or molecules. It is measured by finding the number of electrons needed to complete the shell in which it is present or the excess electrons present after the filling is complete.
Chlorine :
Atomic number = 17
Number of protons = Number of electrons = 17
Distribution : K → 2; L → 8; M → 7
From the above observation, it is noticed that its valency is 1.
Sulphur :
Atomic number = 16
Number of protons = Number of electrons = 16
Distribution : K → 2; L → 8; M → 6
From the above observation, it is noticed that its valency is two.
Magnesium :
Atomic number = 12
Number of protons = Number of electrons = 12
Distribution : K → 2; L → 8; M → 2
From the above observation, it is noticed that its valency is two.
12. If the number of electrons in an atom is 8 and number of protons is also 8, then :
(a) What is the atomic number of the atom?
(b) What is the charge on the atom?
Solution
Given : Number of electrons = 8
Number of protons = 8
(a) Atomic number of the atom = Number of protons = 8
(b) As the number of electrons is equal to the number of protons in the atom, their charges neutralise each other. Therefore, the atom does not possess any charge.
13. With the help of given table, find out the mass number of oxygen and sulphur atom.
Table : Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells.
Solution
(a) Oxygen :
Number of protons = 8
Number of neutrons = 8
Atomic number = 8
Atomic mass number = Number of protons + neutrons = 8 + 8 = 16
(b) Sulphur :
Number of protons = 16
Number of neutrons = 16
Atomic number = 16
Atomic mass number = Number of protons + neutrons = 16 + 16 = 32
14. For the symbol H, D and T, tabulate three subatomic particles found in each of them.
Solution
(a) H (Protium) (11H)
Atomic number = 1
Mass number = 1
Number of proton = 1
Number of electron = 1
Number of neutron = Nil
(b) D (Deuterium) (12H)
Atomic number = 1
Mass number = 2
Number of proton = 1
Number of electron = 1
Number of neutron = 1
(c) T (Tritium) (13H)
Atomic number = 1
Mass number = 3
Number of proton = 1
Number of electron = 1
Number of neutron = 2
15. Write the electronic configuration of any one pair of isotopes and isobars.
Solution
(a) Isotopes : Isotopes are the atoms of same element which have same number of protons but different number of neutrons. Example : Carbon molecule which have 612C and 614C.
(b) Isobars : Isobars are the atoms of different elements which have same mass number but different atomic number. Example : 2040Ca and 1840Ar.
NCERT Exercises Solutions
1 Compare the properties of electrons, protons and neutrons.
Solution
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Electrons |
Protons |
Neutrons |
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They ar negatively charged particle. |
They are positively charged. |
They have no charge. |
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They are present in outer shells within an atom. |
They are present within the nucleus. |
They are present inside the mucleus of an atom. |
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The mass of an electron is about 1/2000 times the mass of an hydrogen atom. |
The mass of a proton is taken as one unit and equals to neutron. |
The mass of a neutron is taken as one unit and equals to that of proton. |
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Attracted towards positve charged. |
Attracted towards negative charged. |
Do not get attracted to any charged particle. |
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An electron is represented as ‘e- ‘. |
A proton is represented as ‘P+’. |
A neutron is represented as ‘n’ . |
2. What are the limitations of J.J. Thomson’s model of the atom?
Solution
- The model of an atom proposed by J.J. Thomson states that negatively charged electrons are embedded in a positive sphere.
- But he did not give any experimental evidence in his support.
- The model was not able to explain the scattering of light in experiment also.
3. What are the limitations of Rutherford’s model of the atom?
Solution
In the model of the atom, Rutherford states that the electrons revolve around the nucleus of an atom in the same way the Earth revolves around the Sun. But, whenever this situation prevails, the revolving particle gains acceleration and radiates its energy outwards. With continuous radiation of energy, the electron would lose its energy and fall into the nucleus which when happens makes the atom highly unstable. But on the contrary, we find the atom as a highly stable particle which disproves the assumption.
4. Describe Bohr’s model of the atom.
Solution
- The nucleus of an atom is present in the centre.
- Around this nucleus, the negatively charged electrons revolve.
- Discrete orbits of electrons are present inside the atom.
- While in this orbits, the electrons do not radiate energy.
- These discrete orbits are represented as K, L, M, N ..... orbits or denoted with the numbers n = 1, 2, 3, 4 .....
5. Compare all the proposed models of an atom given in this chapter.
Solution
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Sl. No. |
Thomson’s model |
Rutherford’s Model |
Bohr’s Model |
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1. |
An atom is a sphere of uniform positive charge in which electrons are embedded. |
An atom consists of a heavy, rigid and positively charged part at the centre of the atom. It is called nucleus. |
At atom consists of positively charged nucleus at its centre. |
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2. |
The total positive charge in the sphere is equally balanced by s u f f i c i e n t number of e l e c t r o n s embedded in it. |
The total positive charge and almost the entire mass of the atom is concentrated in the nucleus. |
Out of the several possible orbits, electrons move only in certain discrete circular orbits Called stationary states or energy levels or shells. |
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3. |
The mass and charge are uniformly distributed. |
E l e c t r o n s move around the nucleus in circular orbits. |
While moving in an allowed orbit, an electron is associated with a definite amount of energy. |
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4 |
The total positive charge on the nucleus is due to the presence of protons. It is balanced by equal number of electrons. |
Electron is not allowed to emit out energy so long as it is moving in the allowed orbit. |
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5. |
Limitations : It failed to explain the observations of Rutherford’s s c a t t e r i n g experiment. |
Limitations : Rutherford’s model failed to explain the stability of the atom. |
Energy is radiated out when an electron jumps from higher orbit to lower orbit but energy is absorbed when the electron jumps from lower orbit to higher orbit. Advantage : Bohr’s theory s u c c e s s f u l l y explains the stability of atom. |
6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Solution
- (The maximum number of electrons that can be accommodated in a shell is given by the formula : 2n2, where n = 1, 2, 3...
- Maximum number of electrons in different shells are :
K shell – n = 1; 2n2 = 2(1)2 = 2
L shell – n = 2; 2n2 = 2(2)2 = 8
M shell – n = 3; 2n2 = 2(3)2 = 18
N shell – n = 4; 2n2= 2(4)2 = 32 - Given number of electrons is 18.
- The electrons are not taken unless the inner shells are filled.
- Hence, the highest element has K – 2; L – 8; M – 8.
7. Define valency by taking examples of silicon and oxygen.
Solution
The valency of an element is defined as the number of electrons lost or gained by an atom to complete its outermost shell. It can also be defined as the combining capacity of an element. For example : Silicon and oxygen.
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Oxygen |
Silicon |
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Atomic number : 8 |
Atomic Number : 14 |
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Electronic configuration : K – 2; L – 6. |
Electronic configuration : K – 2; L – 8; M – 4 . |
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Valence electrons : 6 |
Valence electrons : 4 |
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Valency : 8 – 6 = 2 |
Valency : 8 – 4 = 4 |
8. Explain with examples :
(a) Atomic number
(b) Mass number
(c) Isotopes
(d) Isobars
Give any two uses of isotopes.
Solution
(a) Atomic number of an atom is defined as the number of positively charged protons present in the nucleus. Example : Hydrogen has one proton in its nucleus; so, its atomic number is one.
(b) Mass number of an atom is defined as the total number of protons and neutrons. This takes into account the elements that contribute to the atomic mass of the atom. The mass of electrons are negligible as compared to the protons and neutrons of the corresponding element.
(c) Isotopes : Isotopes are atoms which have the same number of protons but the number of neutrons differs or that have same atomic number but different mass number. Example : The most simple example is the element carbon which exists as 612C and 614 C.
(d) Isobars : Isobars are atoms which have the same mass number but differ in the atomic number. Examples are : 2040Ca and 1840Ar.
Uses of isotopes :
- The isotopes of iodine atom is used to treat goitre.
- Uranium isotopes is used as fuel for nuclear reactors.
9. Na+ has completely filled K and L shells. Explain.
Solution
A sodium atom has 11 electrons in its orbits, hence its atomic number is also 11 and has the same number of protons and electrons. Thus, its electronic configuration is K-2; L-8; M-1; The one electron in the M shell is lost and it obtains a positive charge since it has one more proton than the electrons, and obtains a positive charge, Na+. Now, the configuration is K-1; L-8 and it is a filled state. Hence, it is very stable so it is difficult to remove the electron from a filled state.
10. If bromine atom is available in the form of say, two isotopes 35Br79 (49.7%) and 35Br81 (50.3%). Calculate the average atomic mass of bromine atom.
Solution
The atomic masses of two isotopic atoms are 79 (49.7%) and 81(50.3%)
Thus, total mass = [79 × (49.7/100)] + [81 × (50.3/100)]
= 39.263 + 40.743 = 80.006u
11. The average atomic mass of a sample of an element X is 16.2u. What are the percentages of isotopes 816X and 818X in the sample?
Solution
Let the percentage of 816X be ‘p’ and than of 818X be ‘100-p’.
16.2 = 16× P/100 + 18 × (100- p)/100
= 16p/100 + 1800/100 = 18p/100
1620 = -2p – 1800
p = 90
On solving the equation, we obtain ‘p’ to be 90.
Therefore, 816X = 90%
And, 818X = 10%
12. If Z = 3, what would be the valency of the element? Also, name the element.
Solution
Z = atomic number = 3 (given)
Electronic configuration = K-2; L-1
Thus, valency = 1
The element with atomic number 3 is lithium.
13. Composition of the nuclei of two atomic species X and Y are given as under X-Y.
Protons = 6-6
Neutrons = 6-8
Give the mass number of X and Y. What is the relation between the two species?
Solution
Mass number of X : 6 + 6 = 12
Mass number of Y : 6 + 8 = 14
Since, the atomic numbers of both the species are the same, they are the same element. Also since they have different number of neutrons, their mass number is different and they are the isotopes.
14. For the following statements, write T for true and F for false.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Solution
(a) False
(b) False
(c) True
(d) False
DIRECTION : Put tick against correct choice and cross against wrong choice in questions 15, 16, 17 and 18.
15. Rutherford’s a-particle scattering experiment was responsible for the discovery of :
(a) Atomic nucleus
(b) Proton
(c) Electron
(d) neutron
Solution
(a) Atomic nucleus
16. Isotopes of an element have :
(a) The same physical properties
(b) Different number of neutrons
(c) Different number of protons
(d) Different atomic number
Solution
(b) Different number of neutrons
17. Number of valence electrons in Cl– ion are :
(a) 16
(b) 8
(c) 17
(d) 18
Solution
(b) 8
18. Which one of the following is a correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
Solution
(d) 2, 8, 1
19. Complete the following table :
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Atomic number |
Mass number |
Number of neutrons |
Number of protons |
Number of electrons |
Name of the atomic elements |
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9 |
- |
10 |
- |
- |
- |
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16 |
32 |
- |
- |
- |
Sulphur |
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- |
24 |
- |
12 |
- |
- |
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- |
2 |
- |
1 |
- |
- |
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- |
1 |
0 |
1 |
0 |
- |
Solution
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Atomic number |
Mass number |
Number of neutrons |
Number of protons |
Number of electrons |
Name of the atomic elements |
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9 |
19 |
10 |
9 |
9 |
Fluorine |
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16 |
32 |
16 |
16 |
16 |
Sulphur |
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12 |
24 |
12 |
12 |
12 |
Magnesium |
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1 |
2 |
1 |
1 |
1 |
Hydrogen Deuterium |
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1 |
1 |
0 |
1 |
0 |
Hydrogen |