Class 9 Science Chapter 4 Structure of the Atom
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Structure of the Atom
Answers to Questions Within the Lesson(1)
1. Q. What are canal rays?
Ans: Canal rays are streams of positively charged radiations that pass through the perforations (holes) of the cathode. These rays are composed of positively charged particles (protons).
2. Q. If an atom contains one proton and one electron, will it carry any charge?
Ans: An electron carries a negative charge and a proton carries an equal positive charge. Since their charges are equal and opposite, they cancel each other. Therefore, the atom will be electrically neutral and will not carry any charge.
Answers to Questions Within the Lesson(2)
1. Q. According to Thomson’s model of an atom, explain how the atom is neutral as a whole.
Ans: According to Thomson’s atomic model, the positive and negative charges in an atom are equal in magnitude. Hence, the atom as a whole is electrically neutral.
2. Q. According to Rutherford’s model of an atom, which subatomic particle is present in the nucleus?
Ans: Proton.
3. Q. Draw a diagram of Bohr’s model of an atom with three shells.
Ans: A diagram should show three concentric circular shells (K, L, M)around the nucleus with electrons distributed in them.
4. Q. What would you expect to observe if in the alpha-particle scattering experiment, a foil of a metal other than gold is used?
Ans: Similar results would be observed. However, not all metals are as malleable as gold, so it is difficult to make such thin foils. Very thin foils are necessary for proper scattering of particles.
Answers to Questions Within the Lesson(3)
1. Q. Write the names of the three subatomic particles of an atom.
Ans: The three subatomic particles are neutron, proton and electron.
2. Q. The atomic mass of helium is 4 u and it contains two protons in its nucleus. How many neutrons are present?
Ans: Number of neutrons = Atomic mass – Number of protons = 4 – 2 = 2 neutrons
Answers to Questions Within the Lesson(4)
1. Q. Show the distribution of electrons in carbon and sodium atoms.
Ans:
Carbon (Atomic number 6): 2, 4
Sodium (Atomic number 11): 2, 8, 1
2. Q. If the K and L shells of an atom are completely filled, what will be the total number of electrons in the atom?
Ans: K shell can hold 2 electrons and L shell can hold 8 electrons.
Total electrons = 2 + 8 = 10
Answers to Questions Within the Lesson(5)
1. Q. How will you find the valency of chlorine, sulphur and magnesium?
Ans: Valency depends on the number of electrons in the outermost shell.
Chlorine: Electronic configuration = 2, 8, 7 →Valency = 1
Sulphur: Electronic configuration = 2, 8, 6 →Valency = 2
Magnesium: Electronic configuration = 2, 8, 2 →Valency = 2
2. Q. If an atom has 4 electrons and 4 protons, then—
(i) What is its atomic number?
(ii) What is its charge?
Ans:
(i) Atomic number = number of protons = 4
(ii) Since number of electrons equals number of protons, the atom is neutral and its charge is zero.
3. Q. Using Table 4.1, find the mass numbers of oxygen and sulphur.
Ans:
Oxygen = 16
Sulphur = 32
Answers to Questions Within the Lesson(5)
1. Q. List the subatomic particles present in H, D and T.
Ans:
H (Protium): 1 proton, 0 neutron, 1 electron
D (Deuterium): 1 proton, 1 neutron, 1 electron
T (Tritium): 1 proton, 2 neutrons, 1 electron
2. Q. Write the electronic configuration of one pair each of isotopes and isobars.
Ans:
Isotopes (e.g., Carbon-12 and Carbon-14): Both have electronic configuration 2, 4
Isobars (e.g., Calcium-40 and Argon-40):
Calcium (20): 2, 8, 8, 2
Argon (18): 2, 8, 8
Exercise: Questions and Answers
1. Compare the properties of electron, proton and neutron.
Ans: Compare the properties of electron, proton and neutron.
2. What are the limitations of J. J. Thomson’s atomic model?
Ans: Although Thomson’s model could explain that atoms are electrically neutral, it failed to produce results like those obtained in Ernest Rutherford’s experiment. There was no experimental evidence supporting Thomson’s model.
3. What are the limitations of Rutherford’s atomic model?
Ans: The limitations of Rutherford’s atomic model are as follows: According to Rutherford’s model, an electron revolving in a circular orbit should continuously accelerate, and an accelerating charged particle should emit energy. As a result, the electron would lose energy and eventually fall into the nucleus. If this happened, atoms would be highly unstable and matter would not exist in the form we observe. However, we know that atoms are stable.
4. Describe Niels Bohr’s atomic model.
Ans: The postulates of Bohr’s atomic model are:
(i) Electrons revolve in certain permitted discrete orbits within the atom.
(ii) While revolving in these orbits, electrons do not radiate energy. These orbits or shells are called energy levels. They are denoted by K, L, M, N, … or by numbers n = 1, 2, 3, …
5. Compare the atomic models given in this chapter.
Ans: Compare the atomic models given in this chapter.
6. Briefly write the rules for distribution of electrons in different shells for the first eighteen elements.
Ans: Briefly write the rules for distribution of electrons in different shells for the first eighteen elements.
7. Define valency taking silicon and oxygen as examples.
Ans: The total number of electrons in an atom is called valency, and the electrons present in the outermost shell are called valence electrons. The electronic configuration of silicon is 2.8.4. It has 4 electrons in its outermost shell and shares these electrons to form covalent bonds to complete the octet rule. The electronic configuration of oxygen is 2.6. It has 6 electrons in its outermost shell and gains 2 electrons to complete the octet.
8. Explain with examples:
(i) Atomic number
(ii) Mass number
(iii) Isotopes
(iv) Isobars
(v) Write any two uses of isotopes.
Ans: Here is a short and clear explanation without charts:
(i) Atomic Number: The atomic number is the number of protons present in the nucleus of an atom. It determines the identity of an element. In a neutral atom, it is equal to the number of electrons. Example: Oxygen has an atomic number of 8.
(ii) Mass Number: The mass number is the total number of protons and neutrons in the nucleus.
Example: Sodium-23 has 11 protons and 12 neutrons, so its mass number is 23.
(iii) Isotopes: Isotopes are atoms of the same element with the same atomic number but different mass numbers due to different numbers of neutrons.
Example: Carbon-12 and Carbon-14.
(iv) Isobars: Isobars are atoms of different elements that have the same mass number but different atomic numbers.
Example: Carbon-14 and Nitrogen-14.
(v) Two Uses of Isotopes
a. Medical Treatment: Cobalt-60 is used in cancer radiotherapy, and Iodine-131 is used to treat thyroid disorders.
b. Archaeology: Carbon-14 is used to determine the age of fossils and ancient objects (radiocarbon dating).
9. Explain why K and L shells are completely filled in Na?.
Ans: The atomic number of sodium is 11. Therefore, a neutral sodium atom has 11 electrons with configuration 2.8.1. But Na? has 10 electrons. These 10 electrons are distributed in K and L shells as 2 and 8, respectively. Thus both K and L shells are completely filled.
10. If Z = 3, what will be the valency of the element? Write its name.
Ans: Z = 3 means the atomic number is 3. The electronic configuration is 2.1. Therefore, the valency is 1. The element is lithium.
11. Write T for true and F for false statements:
(a) J. J. Thomson proposed that the nucleus contains only nucleons.
(b) An electron and a proton combine to form a neutron; hence it is neutral.
(c) The mass of an electron is approximately equal to that of a proton.
(d) An isotope of iodine is used to prepare tincture iodine, which is used as a medicine.
Ans:
(a) F
(b) F
(c) F
(d) F
12. Ernest Rutherford’s alpha particle scattering experiment led to the discovery of:
(a) Atomic nucleus
(b) Electron
(c) Proton
(d) Neutron
Ans: (a) Atomic nucleus
13. Isotopes of an element have:
(a) Same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers
Ans: (c) Different number of neutrons
14. Number of valence electrons in Clion:
(a) 16
(b) 8
(c) 17
(d) 18
Ans: (b) 8
18. Which is the correct electronic configuration of sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
Ans: (d) 2, 8, 1
Additional Questions & Answers
1. What were the postulates of J. J. Thomson’s atomic model?
Ans:
(i) An atom is a positively charged sphere with electrons embedded in it.
(ii) The positive and negative charges are equal, so the atom is electrically neutral.
2. What were the observations of Rutherford’s alpha particle experiment?
Ans:
(i) Most alpha particles passed straight through the gold foil.
(ii) Some were deflected by small angles.
(iii) Very few (about 1 in 12000) were deflected back.
3. What conclusions did Rutherford draw from the experiment?
Ans:
(i) Most of the atom is empty space.
(ii) Positive charge occupies very little space.
(iii) Almost all mass and positive charge are concentrated in a small central nucleus.
4. What are the features of Rutherford’s nuclear model?
Ans:
(i) Atom has a positively charged nucleus containing most of the mass.
(ii) Electrons revolve around the nucleus in fixed paths.
(iii) The nucleus is very small compared to the atom.
5. What are the postulates of Niels Bohr’s atomic model?
Ans:
(i) Electrons revolve only in permitted orbits.
(ii) Electrons do not radiate energy while moving in these orbits.
6. What are valence electrons?
Ans: Electrons present in the outermost shell of an atom are called valence electrons.
7. Maximum number of electrons in the outermost shell:
Ans: 8
Multiple Choice Questions (MCQ)
1. Who discovered the electron?
(a) J. J. Thomson
(b) Ernest Rutherford
(c) Niels Bohr
(d) James Chadwick
Ans: (a)
2. Who discovered the nucleus of an atom?
(a) J. J. Thomson
(b) Ernest Rutherford
(c) Niels Bohr
(d) James Chadwick
Ans: (b)
3. Who discovered the neutron?
(a) J. J. Thomson
(b) Ernest Rutherford
(c) James Chadwick
(d) Niels Bohr
Ans: (c)
4. The charge of an electron is:
(a) Positive
(b) Negative
(c) Neutral
(d) Variable
Ans: (b)
5. The charge of a proton is:
(a) Positive
(b) Negative
(c) Neutral
(d) Zero
Ans: (a)
6. The mass of neutron is approximately equal to:
(a) Electron
(b) Proton
(c) Both (a) and (b)
(d) None
Ans: (b)
7. Atomic number represents:
(a) Number of neutrons
(b) Number of protons
(c) Sum of protons and neutrons
(d) Number of electrons only
Ans: (b)
8. Mass number is equal to:
(a) Protons + Electrons
(b) Protons + Neutrons
(c) Electrons + Neutrons
(d) Only protons
Ans: (b)
9. Isotopes have:
(a) Same atomic number
(b) Same mass number
(c) Same neutrons
(d) Same electrons only
Ans: (a)
10. Isobars have:
(a) Same atomic number
(b) Same mass number
(c) Same electrons
(d) Same protons
Ans: (b)
11. Rutherford’s experiment used:
(a) Beta particles
(b) Alpha particles
(c) Gamma rays
(d) Electrons
Ans: (b)
12. Most of the atom is:
(a) Solid
(b) Empty space
(c) Liquid
(d) Dense matter
Ans: (b)
13. The nucleus of an atom is:
(a) Negatively charged
(b) Positively charged
(c) Neutral
(d) Variable
Ans: (b)
14. Electrons revolve around the nucleus in:
(a) Random paths
(b) Fixed orbits
(c) Straight lines
(d) Zig-zag paths
Ans: (b)
15. Who proposed fixed energy levels in atoms?
(a) Ernest Rutherford
(b) J. J. Thomson
(c) Niels Bohr
(d) James Chadwick
Ans: (c)
16. Maximum number of electrons in K shell:
(a) 2
(b) 8
(c) 18
(d) 32
Ans: (a)
17. Maximum electrons in L shell:
(a) 2
(b) 8
(c) 18
(d) 32
Ans: (b)
18. Formula for maximum electrons in a shell is:
(a) n²
(b) 2n²
(c) 3n²
(d) n³
Ans: (b)
19. Valence electrons are present in:
(a) Inner shell
(b) Outermost shell
(c) Nucleus
(d) Proton
Ans: (b)
20. Valency is:
(a) Number of neutrons
(b) Combining capacity
(c) Number of shells
(d) Atomic mass
Ans: (b)
21. Sodium atomic number is:
(a) 10
(b) 11
(c) 12
(d) 13
Ans: (b)
22. Electronic configuration of Na is:
(a) 2,8
(b) 2,8,1
(c) 8,2,1
(d) 2,1,8
Ans: (b)
23. Number of valence electrons in oxygen:
(a) 2
(b) 4
(c) 6
(d) 8
Ans: (c)
24. An atom is electrically neutral because:
(a) Protons = Neutrons
(b) Electrons = Neutrons
(c) Protons = Electrons
(d) All equal
Ans: (c)
25. Cathode rays consist of:
(a) Protons
(b) Electrons
(c) Neutrons
(d) Alpha particles
Ans: (b)
26. The nucleus contains:
(a) Electrons only
(b) Protons and neutrons
(c) Only neutrons
(d) Only protons
Ans: (b)
27. Atomic mass unit is defined as:
(a) Mass of proton
(b) Mass of electron
(c) 1/12 mass of carbon atom
(d) Mass of neutron
Ans: (c)
28. Cl-ion has valence electrons:
(a) 7
(b) 8
(c) 6
(d) 5
Ans: (b)
29. Bohr’s model explains:
(a) Stability of atom
(b) Shape of nucleus
(c) Mass of atom
(d) Neutron discovery
Ans: (a)
30. Rutherford used which metal foil?
(a) Silver
(b) Copper
(c) Gold
(d) Iron
Ans: (c)
31. Electrons are negatively charged particles:
(a) True
(b) False
(c) Sometimes
(d) None
Ans: (a)
32. Proton mass is approximately:
(a) Equal to electron
(b) Much larger than electron
(c) Zero
(d) Negative
Ans: (b)
33. Neutron has:
(a) Positive charge
(b) Negative charge
(c) No charge
(d) Double charge
Ans: (c)
34. Shells are also called:
(a) Energy levels
(b) Neutrons
(c) Nuclei
(d) Isotopes
Ans: (a)
35. Maximum electrons in outermost shell:
(a) 2
(b) 4
(c) 8
(d) 16
Ans: (c)
36. Lithium atomic number is:
(a) 2
(b) 3
(c) 4
(d) 5
Ans: (b)
37. Electronic configuration of Li:
(a) 2,1
(b) 1,2
(c) 2,2
(d) 1,1,1
Ans: (a)
38. Isotopes differ in:
(a) Protons
(b) Neutrons
(c) Electrons
(d) Atomic number
Ans: (b)
39. Mass number =
(a) Protons + Electrons
(b) Protons + Neutrons
(c) Electrons only
(d) Neutrons only
Ans: (b)
40. The central part of atom is called:
(a) Orbit
(b) Shell
(c) Nucleus
(d) Electron cloud
Ans: (c)
