4. Structure of the Atom Class 9 Science Chapter 4 NCERT Solutions (CBSE)
CBSE Class 9 Science Chapter 4: Structure of the Atom ā Full NCERT Textbook Solutions
Chapter 4: Structure of the Atom
Internal Questions :
1. What are canal rays?
Answer: Canal rays are the positively charged radiation which ultimately led to the discovery of another sub-atomic particles .
2. If an atom contains one electron and one proton, will it carry any charge or not ?
Answer: Yes, an atom containing one electron and one proton will have a net charge of zero. The positive charge of the proton cancels out the negative charge of the electron, resulting in a neutral atom.
Internal Questions :
1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: According to Thomson's model of the atom,
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude.
So, the atom as a whole is electrically neutral.
2. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus of an atom?
Answer: According to Rutherford's model of the atom, the nucleus of an atom contains positively charged protons and uncharged neutrons. Electrons orbit around the nucleus in specific energy levels or shells.
3. Draw a sketch of Bohr’s model of an atom with three shells.
Answer: The sketch of Bohr’s model of an atom with three shells is shown below:
4. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
Answer: If the α-particle scattering experiment is conducted using a foil of a metal other than gold, the observations would likely be similar but with variations. The scattering pattern and angles of deflection would depend on the atomic structure and density of the metal used.
Internal Questions :
1. Name the three sub-atomic particles of an atom.
Answer: Name of the three sub-atomic particles of an atom are proton , electron and neutrons .
2. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Answer: We know that, Atomic mass = Number of protons + Number of neutrons
The atomic mass of helium = 4 u
Number of protons = 2
Number of neutrons = Atomic mass - Number of protons
Number of neutrons = 4 – 2 = 2
So, a helium atom has 2 neutrons in its nucleus.
Internal Questions :
1. Write the distribution of electrons in carbon and sodium atoms.
Answer: Atomic number of carbon = 6
Carbon has 6 electrons since its atomic number is 6.
Electron configuration = 2. 4
The K shell (n=1) = 2 electrons
The L shell (n=2) = 4 electrons
Atomic number of sodium = 11
Sodium has 11 electrons since its atomic number is 11.
Electron configuration = 2.8.1
The K shell (n=1) = 2 electrons
The L shell (n=2) = 8 electrons
The K shell (n=3) = 1 electrons
2. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Answer: We know that, maximum number of electrons in a shell = 2n² , where 'n' is number the shell.
For the K shell (n = 1), the maximum capacity = 2 × 1² = 2 electrons.
For the L shell (n = 2), the maximum capacity = 2 × 2² = 8 electrons.
Total number of electrons = Number of electrons in K shell + Number of electrons in L shell
Total number of electrons = 2 + 8
Total number of electrons = 10
So, if both K and L shells of an atom are full, the atom would have a total of 10 electrons.
Internal Questions :
1. How will you find the valency of chlorine, sulphur and magnesium?
Answer: Atomic number of chlorine = 17
Electron configuration (Cl) = 2.8.7
The valency of Cl = 8 – 7 = 1
Atomic number of sulphur = 16
Electron configuration = 2.8.6
The valency of S = 8 – 6 = 2
Atomic number of magnesium = 12
Electron configuration = 2.8.2
The valency of Mg = 2
Internal Questions :
1. If 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?
Answer: (i) The atomic number of an atom is equal to the number of protons in its nucleus. In this case, the number of protons is given as 8. Therefore, the atomic number of the atom is also 8.
(ii) The charge on an atom is determined by comparing the number of protons (positive charge) and electrons (negative charge). If the number of protons and electrons are equal, the atom is neutral .
2. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Answer: The number of proton in oxygen atom = 8
The number of neutrons in oxygen atom = 8
The mass of atom = Proton + neutron
So, the mass number of oxygen atom = 8 + 8 = 16 u
The number of proton in sulphur atom = 16
The number of neutrons in sulphur = 16
So, the mass number of sulphur atom = 16 + 16 = 32 u
Internal Questions :
1. For the symbol H,D and T tabulate three sub-atomic particles found in each of them.
Answer:
|
Name (Symbol) |
Protons |
Neutrons |
Electrons |
|
Hydrogen(H) |
1 |
0 |
1 |
|
Deuterium (D) |
1 |
1 |
1 |
|
Tritium (T) |
1 |
2 |
1 |
2. Write the electronic configuration of any one pair of isotopes and isobars.
Answer: Isotopes are atoms of the same element, which have different mass numbers.
(i) Chlorine: 
and 
Atomic number of Cl = 17
Electronic configuration of Cl = 2.8.7
Isobars are atoms having the same mass number but different atomic numbers.
(i) Argon 
and Calcium 
Atomic number of Ar = 18
Electronic configuration of Ar = 2.8.8
Atomic number of Ca = 20
Electronic configuration of Ca = 2.8.8.2
Exercise:
1. Compare the properties of electrons, protons and neutrons.
Answer: The comparison of the properties of electrons, protons, and neutrons is :
|
Particle |
Nature of Charge |
Mass (relative to electron) |
Location in Atom |
|
Electron |
Negative (-1 unit) |
1/1840 u (negligible) |
Outside the nucleus |
|
Proton |
Positive (+1 unit) |
1 u (approx. 1840 times an electron) |
Inside the nucleus |
|
Neutron |
No charge (Neutral) |
1 u (approx. same as proton) |
Inside the nucleus |
2. What are the limitations of J.J. Thomson’s model of the atom?
Answer: J.J. Thomson’s model could not explain the results of experiments done later, especially Rutherford’s scattering experiment. It failed to explain how positive charge is arranged inside the atom and why most alpha particles passed straight while some were deflected or bounced back. It also could not explain the stability of electrons inside the atom.
3. What are the limitations of Rutherford’s model of the atom ?
Answer: The limitations of Rutherford’s model of the atom are :
(i) The revolution of the electron in a circular orbit is not expected to be stable.
(ii) During acceleration, charged particles would radiate energy.
(iii) The revolving electron would lose energy and finally fall into the nucleus.
4. Describe Bohr’s model of the atom.
Answer: Bohr’s model of the atom is described as follows:
(i) Only certain special orbits, known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in these discrete orbits, the electrons do not radiate energy.
These orbits are called energy levels or shells (K, L, M, N…). Energy is radiated only when an electron jumps from a higher to a lower energy level.
5. Compare all the proposed models of an atom given in this chapter.
Answer: The proposal of the models of an atom given below :
J.J. Thomson’s model :
(i) An atom consists of a positively charged sphere and the electrons are embedded in it.
(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
Rutherford’s model :
(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in circular paths.
(iii) The size of the nucleus is very small as compared to the size of the atom.
Bohr’s model :
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Answer: The distribution of electrons in various shells for the first eighteen elements follows specific rules based on the principles of electron configuration:
The maximum number of electrons present in a shell = 2n² , where n is number of shell (orbit) .
The K shell (n=1) can hold a maximum of 2 electrons (= 1.1²).
The second shell (n=2) can hold a maximum of 8 electrons (= 2.2² ).
The third shell (n=3) can hold a maximum of 18 electrons (= 2.3² ).
The fourth shell (n=4) can hold a maximum of 32 electrons (= 2. 4²).
The elements fill their electron shells in increasing order of atomic number.
7. Define valency by taking examples of silicon and oxygen.
Answer: Valency is the combining capacity of an atom of an element, which tells how many electrons an atom can lose, gain or share to become stable.
Example: Silicon (Si) has electronic configuration 2, 8, 4. It has 4 electrons in its outer shell, so its valency is 4 (= 8 - 4).
Oxygen (O) has electronic configuration 2, 6. It needs 2 electrons to complete its octet, so its valency is 2 (= 8 - 6).
8. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.
Answer: (i) The atomic number is defined as the total number of protons present in the nucleus of an atom .
(ii) The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.
(iii) Isotopes are atoms of the same element, which have different mass numbers.
(iv) Isobars are atoms having the same mass number but different atomic numbers.
Two uses of isotopes are :
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in the treatment of goiter .
9. 
has completely filled K and L shells. Explain.
Answer: The atomic number of Na = 11
The electronic configuration of Na = 2.8.1
The electronic configuration of = 2.8
10. If bromine atom is available in the form of, say, two isotopes 
(49.7%) and 
(50.3%), calculate the average atomic mass of bromine atom.
Answer: Given, two isotopes of bromine are (49.7%) and (50.3%) .
The isotopes of the mass of bromine are 79 u and 81 u .
The average atomic mass of bromine atom 
u
The average atomic mass of bromine (Br) atom is 80.006 atomic mass units (u)
11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 
and 
in the sample?
Answer: let, the percentages of be y .
And the percentages of be 
.
A/Q, 
-
Therefore, the percentages of is 
.
And the percentages of is 
.
12. If Z = 3, what would be the valency of the element? Also, name the element.
Answer: The atomic number (Z) of an element = 3
The electron configuration Z = 2.1
The valency of lithium (Z = 3) is 1 .
The element with atomic number 3 is lithium (Li).
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?
Answer: We know that, Atomic mass = proton + neutron
The mass numbers of X = 6+ 6 = 12 u
The mass numbers of Y = 6 + 8 = 14 u
Both X and Y have the same number of protons (6) but different number of neutrons (6 and 8).
Therefore, X and Y are isotopes of the same element (carbon).
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 
times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Answer: 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.→ False
[ J.J. Thomson proposed the "plum pudding model" (electrons embedded in a positive sphere). The nucleus was discovered later by Rutherford. Nucleons (protons and neutrons) were not proposed by Thomson.]
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.→ False
[ A neutron is not formed by an electron and a proton combining. It is a separate subatomic particle present in the nucleus, and it is neutral.]
(c) The mass of an electron is about times that of proton.→ True
[ Mass of electron = 1/1837ā (approx. 1/2000ā) times the mass of a proton.]
[ A neutron is not formed by an electron and a proton combining. It is a separate subatomic particle present in the nucleus, and it is neutral.]
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.→ False
[ Tincture iodine (an antiseptic) contains iodine (Iā) dissolved in alcohol — not an isotope of iodine. The isotope of iodine (I-131) is used in medical treatment of goiter and cancer, not in tincture iodine.]
Put tick (right Sign) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17
15. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic Nucleus (b) Electron (c) Proton (d) Neutron
Answer : (a) Atomic Nucleus .
[ Rutherford’s alpha-particle scattering experiment showed that most of the atom is empty space and there is a small, dense, positively charged centre called the 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.
Answer: (c) different number of neutrons
[ Isotopes are atoms of the same element, so they have the same number of protons (same atomic number), but they differ in the number of neutrons. This is why their mass numbers are different while chemical properties remain almost the same.]
17. Number of valence electrons in 
ion are:
(a) 16 (b) 8 (c) 17 (d) 18
Answer: (d) 18
[ Atomic number of Cl = 11
The electronic configuration of Cl = 2. 8 .1 . Therefore, The number of valence electrons of is 18 (= 17+1) ]
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.
Answer : (d) 2.8.1
[ Atomic number of Na = 11
The electronic configuration of sodium = 2. 8 .1 ]
19. Complete the following table.
|
Atomic Number |
Mass Number |
Number of Neutrons |
Number of Protons |
Number of Electrons |
Name of the Atomic Species |
|
9 |
-------- |
10 |
--------- |
------- |
------ |
|
16 |
32 |
--------- |
------- |
--------- |
Sulphur |
|
-------- |
24 |
-------- |
12 |
--------- |
------- |
|
-------- |
2 |
-------- |
1 |
-------- |
------ |
|
-------- |
1 |
0 |
1 |
0 |
------- |
Answer: The following tables are :
|
Atomic Number |
Mass Number |
Number of Neutrons |
Number of Protons |
Number of Electrons |
Name of the Atomic Species |
|
9 |
19 |
10 |
9 |
9 |
Fluorine |
|
16 |
32 |
16 |
16 |
16 |
Sulphur |
|
12 |
24 |
12 |
12 |
12 |
Magnesium |
|
1 |
2 |
1 |
1 |
1 |
Deuterium |
|
1 |
1 |
0 |
1 |
1 |
Hydrogen |
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