NCERT solution for class 9 science structure of the atom ( Chapter 4)

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Solution for Exercise Questions

1. Compare the properties of electrons, protons and neutrons.

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Answer :

Electrons Protons Neutrons
Negatively charged Positively charged No charge.
Located outside the nucleus Located within the nucleus Located inside the nucleus of an atom
Mass is negligible 1 a.m.u 1 a.m.u
Attracted towards positively charged Attracted towards negatively charged Do not get attracted to any charged particle

2. What are the limitations of J.J.Thomson’s model of the atom?

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Answer :

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

3. What are the limitations of Rutherford’s model of the atom?

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Answer :

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable. This Rutherford’s model of the atom could not explain.

4. Describe Bohr’s model of the atom.

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Answer :

Neil Bohr put forward the following postulates about the model of an atom:

(i) Atoms has nucleus in the centre.

(ii) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(iii) While revolving in discrete orbits the electrons do not radiate energy.

(iv) These orbits or shells are called energy levels. Energy levels in an atom are shown in Fig.These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

5. Compare all the proposed models of an atom given in this chapter.

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Answer :

Thomson Rutherford Bohr
1. Sphere is positively charged

2.Electrons are negatively charged and scattered all through the inside of the sphere.

3. Positively charged = negatively charged

4. The net charge in the atom is zero.

1. The nucleus is at the centre and is positively charged holding the entire mass.

2.Electrons are negatively charged revolving in a well-defined path

3. In comparison with the nucleus, the size of the atom is very large.

1. Nucleus is present at the centre and is positively charged

2. Electrons are negatively charged, revolving around but do not radiate energy.

3. The distinct orbits are labelled as K, L, M, N

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

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Answer :

The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) The maximum number of electrons present in a shell is given by the formula \( 2n^2 \) , where ‘n’ is the orbit number or energy level index, 1,2,3,….

Hence the maximum number of electrons in different shells are as follows:

first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42= 32, and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

7. Define valency by taking examples of silicon and oxygen.

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Answer :

The definite combining capacity of the atoms of each element is valency.

Example : To find the valency of silicon:

The atomic number of silicon is 14

Number of electrons is equal to the number of protons in silicon i.e., 14

The distribution of electrons in silicon atom is K – 2, L – 8, M – 4

Hence, from the distribution of silicon it is clearly evident that to fill the M shell 4 electrons are required. Therefore its valency is 8-4=4.

To find the valency of oxygen:

The atomic number of oxygen is 8

Number of electrons is equal to the number of protons in oxygen i.e., 8

The distribution of electrons in oxygen atom is K – 2, L – 6

Hence, from the distribution of oxygen it is clearly evident that to fill the M shell 6 more electrons are required. Therefore its valency is 8-6=2.

8. Explain with examples

(i) Atomic number,

(ii) Mass number,

(iii) Isotopes and

(iv) Isobars.

Give any two uses of isotopes.


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Answer :

(i) The number of positively charged protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z. Example: Hydrogen has one proton in its nucleus, hence its atomic number is one.

(ii) The total number of protons and neutrons present in the nucleus of an atom is known as the mass number. It is denoted by A. 20Ca40 . Mass number is 40. Atomic number is 20.

(iii) The atoms which have the same number of protons but different number of neutrons are referred to as isotopes. Hence the mass number varies.

Example: The most simple example is the Carbon molecule which exists as 6C12 and 6C14

(iv) Isobars: Isobars are atoms which have the same mass number but differ in the atomic number.

Examples are, 20Ca40and 18Ar40

Uses of isotopes:

1. The isotope of Iodine atom is used to treat goitre and iodine deficient disease.

2. In the treatment of cancer, an isotope of cobalt is used.

3. Fuel for nuclear reactors is derived from the isotopes of the Uranium atom.

9. Na+ has completely filled K and L shells. Explain.

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Answer :

The atomic number of sodium is 11. It has 11 electrons in its orbitals wherein the number of protons is equal to the number of electrons. Hence, 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 electrons, and obtains a positive charge, Na+ . The new electronic configuration is K-1 ; L-8 which is the filled state. Hence it is very difficult to eliminate the electron from a filled state as it is very stable.

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.

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Answer :

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 = 80.006 u

11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 8X16 and 8X18 in the sample?

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Answer :

Let the percentage of 8X16 be ‘a’ and that of 8X18 be ‘100-a’.

As per given data,

16.2u = 16 a / 100 + 18 (100-a) /100

1620 = 16a + 1800 – 18a

1620 = 1800 – 2a

a = 90%

Hence, the percentage of isotope in the sample 8X16 is 90% and that of

8X18 = 100-a = 100- 90=10%

12. If Z=3, what would be the valency of the element? Also, name the element.

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Answer :

Given: Atomic number, Z = 3

The electronic configuration of the element = K-2; L-1, hence its 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 numbers of X and Y. What is the relation between the two species?

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Answer :

Mass number of X: Protons + neutrons = 6+6 = 12

Mass number of Y: Protons + neutrons = 6+8 = 14

They are the same element as their atomic numbers are the same.

They are isotopes as they differ in the number of neutrons and hence their mass numbers.

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.


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Answer :

(a) False

(b) False

(c) True

(d) False

15. Rutherford’s alpha – particle scattering experiment was responsible for the discovery of

(a) Atomic nucleus

(b) Electron

(c) Proton

(d) Neutron


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Answer :

(a) Atomic nucleus

16. Isotopes of an element have

(a) The same physical properties

(b) Different chemical properties

(c) Different number of neutrons

(d) Different atomic numbers.


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Answer :

(c) Different number of neutrons

17. Number of valence electrons in Cl– ion are:

(a) 16

(b) 8

(c) 17

(d) 18


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Answer :

(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


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Answer :

(d) 2, 8, 1

19. Complete the following table.


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Answer :

Solution for Intext Question

1. What are the canal rays?

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Answer :

Canal rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.

2. If an atom contains one electron and one proton, will it carry any charge or not?

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Answer :

No, the atom will be neutral as a proton is a positively charged particle and an electron is a negatively charged particle, the net charge becomes neutral as both the particles neutralizes each other.

1. On the basis of Thompson’s model of an atom, explain how the atom is neutral as a whole.

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Answer :

(i) An atom contains a positively charged sphere in which the negatively charged electrons are implanted.

(ii) Electrons and protons are equal in magnitude hence an atom on the whole is electrically neutral.

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

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Answer :

According to Rutherford’s model of an atom, the positively charged protons are present in the atom.

3. Draw a sketch of Bohr’s model of an atom with three shells.

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Answer :

4. What do you think would be the observation if the ?– particle scattering experiment is carried out using a foil of a metal other than gold?

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Answer :

The observation would remain the same even when a metal other than gold was used in the ?– particle scattering experiment as the structure of an atom when considered individually remains the same.

1. Name the three subatomic particles of an atom.

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Answer :

The three subatomic particles of an atom are:
1.Protons which are positively charged

2. Electrons which are negatively charged

3. Neutrons which are neutral in nature ( no charge )

2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

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Answer :


Given, atomic mass of helium atom = 4 u and 2 protons in helium nucleus

We know that, atomic mass = number of protons + number of neutrons

So, 4 = 2 + number of neutrons

=> Number of neutrons = 4 – 2 = 2

Hence, Helium has 2 neutrons.

1. Write the distribution of electrons in Carbon and Sodium atoms.

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Answer :

The atomic number of Carbon is 6

Number of electrons is equal to the number of protons in carbon atom i.e., 6

The distribution of electrons in carbon atom is K-2, L-4

The atomic number of Sodium is 11

Number of electrons is equal to the number of protons in sodium atom i.e., 11

The distribution of electrons in sodium atom is K-2, L-8, M-1

2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

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Answer :

K shell can hold 2 electrons

L shell can hold 8 electrons

Hence, when both the shells are full, the total number of electrons present in the atom = 2+8 = 10 electrons.

1. How will you find the valency of chlorine, sulphur and magnesium?

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Answer :

To find the valency of chlorine:

The atomic number of chlorine is 17

Number of electrons is equal to the number of protons in chlorine i.e., 17

The distribution of electrons in chlorine atom is K-2, L-8, M-7

Hence, from the distribution of chlorine it is clear that to fill the M shell only one electron is required. Therefore its valency is -1.

To find the valency of sulphur:

The atomic number of sulphur is 16

Number of electrons is equal to the number of protons in sulphur i.e., 16

The distribution of electrons in sulphur atom is K-2, L-8, M-6

Hence, from the distribution of sulphur it is clear that to fill the M shell two more electrons are required. Therefore its valency is -2.

To find the valency of magnesium:

The atomic number of magnesium is 12

Number of electrons is equal to the number of protons in magnesium i.e., 12

The distribution of electrons in magnesium atom is K-2, L-8, M-2

Hence, from the distribution of magnesium it is clearly evident that to fill the M shell six more electrons are required. But it will be difficult to gain 6 electron. So, it will loss two electrons instead.

Thus, its valency is +2.

1. If the number of electrons in an atom is 8 and number of protons is also 8, then

(i) What is the atomic number of the atom? and

(ii) What is the charge on the atom?


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Answer :

Given: Number of electrons = 8

Number of protons = 8

(a) The atomic number of an atom is the same as the number of protons in that atom, hence its atomic number is 8.

(b) In an atom, the number of protons is equal to the number of electrons. Hence both the charges – positive and negative neutralize each other. Therefore, the atom does not possess any charge.

2. With the help of given Table, find out the mass number of oxygen and sulphur atom.

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Answer :

(a) To find the mass number of Oxygen:

Number of protons = 8

Number of neutrons = 8

Atomic number = 8

Atomic mass number = Number of protons + number of neutrons = 8 + 8 = 16

Therefore, mass number of oxygen = 16

(b) To find the mass number of Sulphur:

Number of protons = 16

Number of neutrons = 16

Atomic number = 16

Atomic mass number = Number of protons + number of neutrons = 16 + 16 = 32

1. For the symbol H, D and T, tabulate three subatomic particles found in each of them.

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Answer :

Isotope Symbol Mass no. Atomic no. No. of electrons No. of protons No. of neutrons
Hydrogen H 1 1 1 1 0
Deuterium D 2 1 1 1 1
Tritium T 3 1 1 1 2

2. Write the electronic configuration of any one pair of isotopes and isobar.

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Answer :

(a) Isotopes: Isotopes are atoms which have the same number of protons but different number of neutrons. Example: Carbon molecule exists as 6C12 and 6C14 but when their electronic configuration is K-2; L-4

(b) Isobars: Isobars are atoms which have the same mass number but different atomic number. Example: Electronic configuration of 20Ca40is – K-2; L-8; M-8; N- 2 and of 18Ar40 is – K-2; L-8; M-8