Class 9 Science Chapter 4 – Structure of the Atom NCERT Intext Questions and Answers are provided here in simple, clear, and exam-oriented English. These solutions help students understand important concepts such as electrons, protons, neutrons, atomic models, isotopes, and isobars in an easy and step-by-step manner. Each intext question is answered accurately and to the point, keeping the CBSE board exam pattern in mind. This content is specially designed to improve concept clarity and strengthen students exam preparation.
Ch:- 4 – STRUCTURE OF THE ATOMS
INTEXT QUESTION-ANSWER
Intext Question - Page 39
Ques 1 :- What are canal rays ?
Ans :- Canal rays are positively charged radiations discovered by E. Goldstein in a discharge tube. These rays led to the discovery of the proton.
• These rays are called canal rays because these rays passed through the holes or canals in the cathode.
Ques 2:- If an atom contains one electron and one proton, will it carry any charge or not?
Ans: No, it will not carry any charge. (The charge will cancel each other).
Intext Question - Page 41
Ques 1 :- On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Ans:- In 1899, Thomson proposed that :
• An atom consists of a positively charged sphere and the electrons are embedded in it.
• The negative and positive charges balance each other and so the atom as a whole is neutral.
Ques 2 :- On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans: On the basis of Rutherford’s model of an atom, the proton is the subatomic particle present in the nucleus of an atom.
• Rutherford proposed that the atom has a small, dense, positively charged nucleus at its center.This positive charge is due to the presence of protons, while electrons revolve around the nucleus.
Ques 3 :- Draw a sketch of Bohr’s model of an atom with three shells.
Ans:
Fig. :- Energy shells around the nucleus of an atom.
Ques 4 :- 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?
Ans: If the a-particle scattering experiment is performed using a foil of a metal other than gold, the observations would be similar to those with a gold foil.
• This is because atoms of all metals have a small, dense, positively charged nucleus.
• Gold was used in Rutherford’s experiment mainly because it is highly malleable and can be beaten into an extremely thin foil.
• Other metals are less malleable, so their foils would be thicker, causing increased scattering or absorption of a-particles, but the basic result of the experiment would remain the same.
Intext Question - Page 41
Ques 1 :- Name the three sub-atomic particles of an atom.
Ans: The three sub-atomic particles of an atom are :-
• electrons (Negatively Charged),
• protons (Positively Charged),
• Neutrons (No Charged).
Ques 2:- Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have ?
Ans: Atomic mass of Helium atom = 4u
Atomic mass = Number of Proton + Number of Neutrons
Number of Neutrons = Atomic mass - Number of Proton
Number of Neutrons = 4-2 = 2
Hence Helium has 2 neutrons.
Intext Question - Page 42
Ques 1 :- Write the distribution of electrons in carbon and sodium atoms.
Ans: (i) Atomic number of carbon = 6
Distribution of electrons =
| K | L |
| 2 | 4 |
(ii) Atomic number of Sodium = 11
Distribution of electrons =
| K | L | M |
| 2 | 8 | 1 |
Ques 2:- If K and L shells of an atom are full, then what would be the total number of electrons in the atom ?
Ans: If K and L shells of an atom are full then the total number of electrons in the atom would be 10.
(K shell can maximum 2 electrons and L shell can have maximum 8 electrons)
Distribution of electrons =
| K | L |
| 2 | 8 |
Intext Question - Page 44
Ques 1:- How will you find the valency of Chlorine, Sulphur and Magnesium ?
| Name of element | Atomic number | Electronic Configuration | Valence electrons | Valency | ||
|---|---|---|---|---|---|---|
| K | L | M | ||||
| Chlorine | 17 | 2 | 8 | 7 | 7 | 8 - 7 = 1 |
| Sulphur | 16 | 2 | 8 | 6 | 6 | 8 - 6 = 2 |
| Magnesium | 12 | 2 | 8 | 2 | 2 | 2 |
Intext Question - Page 44
Ques 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?
Ans: Atomic number = Number of protons = 8.
(ii)
As number of protons(+ve charge) = number of electrons(-ve charge).
So, both the charges positive (+ve) and negative(-ve) neutralise each other.
Therefore, the atom does not possess any charge.
Ques 2 :- With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
| Element | Atomic Number | No. Of Protons | No. Of Neutrons | No. Of Electrons | Valency |
|---|---|---|---|---|---|
| Oxygen | 8 | 8 | 8 | 8 | 2 |
| Sulphur | 16 | 16 | 16 | 16 | 2 |
Ans: - Atomic mass number = No. of protons + No. of neutrons
Atomic mass number of
oxygen = 8 + 8 = 16
Atomic mass number of
Sulphur = 16 + 16 =32.
Intext Question - Page 45
Ques 1 :- For the symbol H,D and T tabulate three sub-atomic particles found in each of them.
Ans :-
Ques 2 :- Write the electronic configuration of any one pair of isotopes and isobars.
Ans: - (i) * Isotopes: - same atomic number butdifferent mass numbers.
| Electronic configuration | K | L |
|---|---|---|
| Carbon (C) | 2 | 4 |
* Isobars :- same mass numbers but different atomic number.
| Electronic configuration | K | L | M | N |
|---|---|---|---|---|
| Argon (Ar) | 2 | 8 | 8 | |
| Calcium (Ca) | 2 | 8 | 8 | 2 |
Conclusion :- In this chapter, Structure of the Atom, we studied the basic structure of an atom including electrons, protons, and neutrons. The NCERT Intext Questions and Answers help students understand important concepts such as atomic number, mass number, isotopes, electronic configuration, and valency in a simple and clear manner. These solutions are strictly based on the CBSE Class 9 Science syllabus and are very useful for exam preparation. By practicing these questions regularly, students can strengthen their fundamental concepts and score better marks in examinations.
Frequently Asked Questions (FAQs)
Ques 1: - What are isotopes?
Ans : - Isotopes are atoms of the same element having the same atomic number but different mass numbers.
Ques 2: - Why do isotopes have similar chemical properties?
Ans: - Because isotopes have the same electronic configuration, their chemical properties are similar.
Ques 3: - What are isobars?
Ans: - Isobars are atoms of different elements having the same mass number but different atomic numbers.
Ques 4: - Write the electronic configuration of chlorine.
Ans: - The electronic configuration of chlorine (atomic number 17) is 2, 8, 7.
Ques 5: - What is meant by structure of the atom.?
Ans: - The structure of the atom refers to the arrangement of subatomic particles such as electrons, protons, and neutrons inside an atom.
Ques 6: - Are NCERT intext questions important for Class 9 exams?
Ans : - Yes, NCERT intext questions are very important as many exam questions are directly asked from them.


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