Structure of the Atom | Class 9 CBSE Guide for Chapter 4 Science (Chemistry) | NCERT Answers of In-text Questions


Chapter 4, Structure of the Atom  

Class 9, NCERT (CBSE) Science - Solutions of In-Text Questions   

NCERT Solutions - CBSE Guide

Question.1: What are canal rays?
Answer: Canal rays are positively charged radiations consisting of particles which have a charge equal in magnitude but opposite in sign to that of the electron. The mass of a canal ray particle is 2000 times as that of an electron. This particle is known as proton. Canal rays were discovered by Goldstein. 
Question.2: If an atom contains one electron and one proton, will it carry any charge or not?
Answer: Since the magnitude of the charge on an electron and proton is the same, the atom containing only one electron and one proton will carry no charge and will be electrically neutral. 
Question.3: On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Answer: As per Thomson’s model of an atom the negative and positive charges are equal in number and magnitude. So, they balance each other as a result of which atom as a whole is electrically neutral.
Question.4: On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?    
Answer: According to the Rutherford’s model of an atom, protons reside in the nucleus of an atom.
Question.5: Draw a sketch of Bohr’s Model of an atom with three shells.   
Answer: Refer to NCERT textbook.
Question.7: 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 α-particle scattering experiment is carried out using a foil of any metal as thin as gold foil used by Rutherford, there would be no change in observations. But since other metals are not so malleable so, such a thin foil is difficult to obtain. If we use a thick foil, then more α-particles would bounce back and no idea about the location of positive mass in the atom would be available with such a certainty.     
Question.8: Name the three sub-atomic particles of an atom.
Answer: Electron (0e–1), Proton (1p+1) and Neutron (1n0).
Question.9: Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Answer: The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Since helium atom has an atomic mass of 4u and it has two protons, the protons contribute 2u to atomic mass. Hence, it must contain 2 neutrons as each neutron contributes 1u to the nucleus.   
Question.10: Write the distribution of electrons in carbon and sodium atoms.
Answer: Carbon has 6 electrons with electronic configuration as given below -
K-shell – 2 electrons and L-shell – 4 electrons. That is 2, 4.
Sodium has 11 electrons with electronic configuration as given below -
K-shell – 2 electrons, L-shell – 8 electrons and M-shell – 1 electron. That is 2, 8, 1.   
Question.11: If K and L shells of an atom are full, then what would be the total number of electrons in the atom? 
Answer: K-shell can accommodate maximum 2 electrons and L-shell can accommodate 8 electrons and so, 10 is the total number of electrons provided K and L shells are full.   
Question.12: How will you find the valency of chlorine, sulphur and magnesium?
Answer: Electronic configurations:

Hence, valency of Chlorine = 8 – 7 = 1, valency of Sulphur = 8 – 6 = 2, valency of Magnesium = 2
Question.13: If the number of electrons in an atom is 8 and the number of protons are also 8, then (i) what is the atomic number of the atom? (ii) What is the charge on the atom?
Answer: (i) Atomic number = Number of Protons = 8. (ii) Charge on the Atom = 0.   
Question.14: For the symbol H, D and T tabulate three subatomic particles found in each of them.

Hydrogen (H) 1H1
Deuterium (D) 2H1
Tritium (T) 3H1

Question.15: Write the electronic configuration of any one pair of isotopes and isobars.  
Answer: 12C6 and 14C6 are isotopes, have the same electronic configuration as (2, 4). 22Ne10 and 22Ne11 are isobars. They have different electronic configuration as given below:
22Ne10 – 2, 8
22Ne11 – 2, 8, 1
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