CBSE Class 9 Science Structure of atom Sure Shot Questions Set 01

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ASSIGNMENT QUESTIONS SET - 1
CLASS IX SCIENCE: CHAPTER 4
STRUCTURE OF ATOMS

1. Plum pudding Model of atom was discovered by _______________

Answer: J.J. Thomson.

Teacher's Note:
a) In this model the atom is a positively charged sphere with electrons embedded in it.
b) The electrons are spread out like raisins in a pudding.

2. Combining capacity of an atom is called _______________

Answer: Valency.

Teacher's Note:
a) Valency is the number of electrons an atom gains, loses or shares to complete its octet.
b) Valency is a whole number and has no sign.

3. Alpha - particle scattering experiment of Rutherford led to discovery of _________

Answer: The nucleus of an atom.

Teacher's Note:
a) Most alpha particles passed straight through the gold foil.
b) A few were deflected through large angles, which pointed to a small, dense, positive nucleus.

4. Number of neutrons in 3581Br is _____________

Answer: 46 (mass number 81, atomic number 35, so neutrons = 81 - 35 = 46).

Teacher's Note:
a) The number at the bottom is the atomic number (35) and the number at the top is the mass number (81).
b) Neutrons = mass number - atomic number, so always subtract.

5. __________ are atoms having the same mass number but different atomic number.

Answer: Isobars.

Teacher's Note:
a) Isobars belong to different elements because their atomic numbers differ.
b) Example: argon-40 and calcium-40.

6. The charge on the electron is found to be ________ coulombs.

Answer: -1.6 × 10⁻¹⁹ coulomb.

Teacher's Note:
a) The magnitude of the electron's charge is 1.6 × 10⁻¹⁹ coulomb.
b) The sign is negative, so the electron is negatively charged.

7. The electronic configuration of silicon is ___. (Atomic number = 14)

Answer: 2, 8, 4

Teacher's Note:
a) Fill K shell with 2 electrons, L shell with 8 electrons, and the rest (4) in M shell.
b) The total of the shell electrons must equal the atomic number, 14.

8. _____ electrons are responsible for the chemical properties of an atom.

Answer: Valence electrons (the electrons in the outermost shell).

Teacher's Note:
a) Chemical reactions involve only the outermost electrons.
b) Inner shell electrons are not involved in bonding.

9. Rutherford's model of an atom was modified by _______.

Answer: Niels Bohr.

Teacher's Note:
a) Bohr added fixed circular orbits (shells) for electrons around the nucleus.
b) This solved the problem of electrons falling into the nucleus.

10. ____ is an isotope of carbon used in determining the age of dead plants.

Answer: Carbon-14 (C-14).

Teacher's Note:
a) Carbon-14 is radioactive and decays slowly.
b) Its ratio in dead plant material shows how long ago the plant died.

11. An atom with 3 protons and 4 neutrons will have a valency of _________.

Answer: 1

Teacher's Note:
a) The atomic number is 3, so the electronic configuration is 2, 1.
b) It has one electron in its outermost shell, so its valency is 1.

12. What do you understand by valency of an element? What is valency of boron?

Answer:
1. Valency of an element is the combining capacity of its atom, that is, the number of electrons it gains, loses or shares to complete its octet.
2. Boron has atomic number 5, with electronic configuration 2, 3.
3. Boron has 3 valence electrons, so its valency is 3.

Teacher's Note:
a) Valency is found from the outermost shell electrons.
b) Boron's valency is 3 because it can share three electrons, not because it gains five.

13. List the features of Rutherford's nuclear model of atom.

Answer:
1. An atom has a small, dense, positively charged nucleus at its centre.
2. Most of the mass of the atom is concentrated in the nucleus.
3. Electrons revolve around the nucleus in circular orbits.
4. Most of the space inside an atom is empty.

Teacher's Note:
a) The nucleus is extremely small compared with the whole atom.
b) The electrons are held by electrostatic attraction of the nucleus.

14. What are the postulates of Bohr Model of an atom?

Answer:
1. Electrons revolve around the nucleus only in certain fixed circular orbits, called shells.
2. An electron in a fixed orbit does not radiate energy.
3. Energy is absorbed or emitted only when an electron jumps from one orbit to another.

Teacher's Note:
a) The orbits are labelled K, L, M and so on, starting from the nucleus.
b) Energy change happens only in jumps, never in between orbits.

15. Define Valency. What is the valency of chlorine, sulphur and magnesium?

Answer:
1. Valency is the combining capacity of an atom of an element.
2. Valency of chlorine = 1, valency of sulphur = 2, valency of magnesium = 2.

Teacher's Note:
a) Chlorine (2, 8, 7) needs one electron, so its valency is 1.
b) Sulphur (2, 8, 6) needs two electrons, so its valency is 2.
c) Magnesium (2, 8, 2) loses two electrons, so its valency is 2.

16. Cl^- has completely filled K&L shells. Explain.

Answer:
1. A chlorine atom has the configuration 2, 8, 7.
2. It gains one electron to form Cl^-, with the configuration 2, 8, 8.
3. The K shell has 2 electrons and the L shell has 8 electrons, so both are completely filled.

Teacher's Note:
a) The gain of one electron completes the octet in the outermost M shell.
b) The ion has a negative charge because it has one extra electron.

17. Na⁺ is possible but Cl⁺ is not possible. What is the reason?

Answer: Sodium has one electron in its outermost shell, which it loses easily to form Na⁺. A chlorine atom would have to lose seven electrons to form Cl⁺, which is energetically not possible; it gains one electron instead to form Cl^-.

Teacher's Note:
a) Atoms lose electrons when the number is small, and gain electrons when the number is close to eight.
b) Removing seven electrons from chlorine needs far too much energy.

18. What are isotopes? Give two examples .

Answer:
Isotopes are atoms of the same element having the same atomic number but different mass numbers.
Examples: Hydrogen-1 and Hydrogen-2 (deuterium); Carbon-12 and Carbon-14.

Teacher's Note:
a) Isotopes differ only in the number of neutrons.
b) They have the same chemical properties because their electronic configuration is the same.

19. What were the observations of Rutherford's Alpha particles scattering experiment?

Answer:
1. Most of the alpha particles passed straight through the gold foil without any deflection.
2. A small number of alpha particles were deflected through small angles.
3. A very few alpha particles (about one in twelve thousand) were deflected through angles of nearly 180°.

Teacher's Note:
a) The straight-through result shows that most of the atom is empty space.
b) The large deflections show that the positive charge is concentrated in a very small nucleus.

20. What are the drawbacks of Rutherford's model of atom?

Answer:
1. It could not explain the stability of the atom, because a revolving electron should lose energy and fall into the nucleus.
2. It did not explain how the electrons are arranged around the nucleus.

Teacher's Note:
a) Accelerating electrons radiate energy, according to classical physics.
b) Bohr's model later corrected both drawbacks.

21. From what observations do you derive the following inferences?
(i) The most of the space inside the atom is empty.
(ii) The volume of the nucleus is very small.
(iii) Anode rays consist of positively charged particles.

Answer:
(i) Most alpha particles passed straight through the gold foil without deflection.
(ii) Only a very few alpha particles were deflected through large angles, which shows that the positive charge is concentrated in a very small volume.
(iii) In the discharge tube experiment, anode rays were deflected towards the negative plate of an electric field, which shows that they are positively charged.

Teacher's Note:
a) The inference (i) is based on the straight-through result.
b) The inference (ii) is based on the rare large deflections.
c) Anode rays bend in the direction opposite to cathode rays, which confirms their positive charge.

22. Name the fundamental particle not present in the nucleus of hydrogen atom

Answer: Neutron.

Teacher's Note:
a) The ordinary hydrogen nucleus contains only one proton.
b) Neutrons are found in heavier isotopes such as deuterium.

23. Who discovered electrons?

Answer: J.J. Thomson (1897).

Teacher's Note:
a) Thomson studied cathode rays and showed that they are made of electrons.
b) He also measured the charge to mass ratio of the electron.

24. Who discovered protons?

Answer: Eugen Goldstein (1886), who observed canal rays; the proton was named by Rutherford.

Teacher's Note:
a) Goldstein observed positively charged particles in the discharge tube.
b) Rutherford gave these particles the name proton.

25. Who discovered neutrons?

Answer: James Chadwick (1932).

Teacher's Note:
a) Chadwick showed that neutrons have no charge and a mass close to that of a proton.
b) Neutrons were predicted earlier by Rutherford.

26. Who discovered nucleus of an atom?

Answer: Ernest Rutherford (1911).

Teacher's Note:
a) The discovery came from the alpha particle scattering experiment on gold foil.
b) For this reason Rutherford is called the father of nuclear physics.

27. What are canal rays?

Answer: Canal rays are positively charged rays of particles, produced in a discharge tube, that travel from the anode towards the cathode through holes in the cathode.

Teacher's Note:
a) Canal rays are also called anode rays.
b) Their nature depends on the gas inside the tube.

28. What are the properties of anode rays?

Answer:
1. They are positively charged.
2. Their charge to mass ratio depends on the gas filled in the discharge tube.
3. They are deflected by electric and magnetic fields, in the direction opposite to cathode rays.

Teacher's Note:
a) The mass of the particles depends on the gas used.
b) They are not always protons, because the particles come from the gas atoms.

29. What are cathode rays?

Answer: Cathode rays are streams of fast-moving electrons that travel from the cathode to the anode in a discharge tube at low pressure.

Teacher's Note:
a) They are negatively charged and move in straight lines.
b) They are deflected towards the positive plate of an electric field.

30. Who discovered X-Rays?

Answer: Wilhelm Röntgen (1895).

Teacher's Note:
a) X-rays were discovered while studying cathode rays.
b) They are also called Röntgen rays.

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

Answer: No, the atom carries no charge.

Teacher's Note:
a) The electron has charge -1 and the proton has charge +1.
b) Equal and opposite charges cancel, so the atom is electrically neutral.

32. Complete the following table.

Answer: The completed table is given above.

Teacher's Note:
a) Relative charge is measured in units of the proton's charge.
b) The electron's mass is very small, so it is taken as almost zero in most calculations.

33. What is the mass of proton as compared to electron?

Answer: A proton is about 1836 times heavier than an electron.

Teacher's Note:
a) Mass of proton = 1.67 × 10⁻²⁷ kg and mass of electron = 9.1 × 10⁻³¹ kg.
b) The ratio is about 1836 : 1.

34. Describe briefly Thomson's model of an atom.

Answer:
1. The atom is a sphere of positive charge.
2. The negatively charged electrons are embedded in this positive sphere, like seeds in a watermelon.
3. The total positive charge equals the total negative charge, so the atom is neutral.

Teacher's Note:
a) This model is also called the plum pudding model.
b) It was later disproved by Rutherford's experiment.

35. Write the limitations of J.J. Thomson's model of an atom.

Answer:
1. It could not explain the results of Rutherford's alpha particle scattering experiment.
2. It did not explain the line spectrum of atoms.

Teacher's Note:
a) A uniform positive sphere cannot deflect alpha particles through large angles.
b) The model had no concept of a nucleus.

36. What are α-particles?

Answer: Alpha particles are positively charged particles, each made of two protons and two neutrons, that is, the nucleus of a helium atom.

Teacher's Note:
a) They are emitted by some radioactive substances.
b) Their mass is about four times that of a hydrogen atom.

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

Answer: Protons.

Teacher's Note:
a) The nucleus carries the positive charge of the atom.
b) Neutrons were found in the nucleus later by Chadwick.

38. Who is known as 'Father of Nucleus Physics'?

Answer: Ernest Rutherford.

Teacher's Note:
a) Rutherford discovered the nucleus and the concept of nuclear model.
b) He also explained radioactive decay and gave the names alpha and beta rays.

39. What were the observations of Rutherford's α-scattering experiment?

Answer:
1. Most alpha particles passed straight through the gold foil.
2. Some alpha particles were deflected through small angles.
3. A very few alpha particles were deflected through very large angles, nearly 180°.

Teacher's Note:
a) The result is the same as the answer to Question 19.
b) The experiment used a very thin gold foil.

40. What were the important features of atomic model based on Rutherford's scattering experiment?

Answer:
1. The atom has a positively charged nucleus at its centre.
2. The nucleus contains almost all the mass of the atom.
3. Electrons revolve around the nucleus, and most of the atom is empty space.

Teacher's Note:
a) The nucleus is very small compared with the atom.
b) The electrostatic force of the nucleus holds the electrons in orbit.

41. What are the limitations of Rutherford's model of the atom?

Answer:
1. It could not explain the stability of the atom, since a revolving electron would radiate energy and collapse into the nucleus.
2. It did not describe how electrons are arranged in the atom.

Teacher's Note:
a) This drawback was removed by Bohr's model.
b) The model gave no information about electron shells.

42. Draw a sketch of Bohr's model of an atom with three shells.

[Figure: diagram to be added. A central nucleus marked with the protons and neutrons, surrounded by three concentric circles labelled K, L and M. Two small dots marked as electrons on the K circle, eight dots on the L circle, and one dot on the M circle, showing the sodium atom (2, 8, 1)]

Answer: The sketch shows a nucleus at the centre with three shells K, L and M. The K shell holds 2 electrons, the L shell holds 8 electrons and the M shell holds 1 electron, as in sodium.

Teacher's Note:
a) The shells are drawn as concentric circles around the nucleus.
b) The first shell holds at most 2 electrons, and the second shell holds at most 8.

43. Describe Bohr's model of the atom.

Answer:
1. Electrons revolve around the nucleus in fixed circular orbits called shells, labelled K, L, M and so on.
2. An electron in a given orbit does not lose energy.
3. An electron can move to a higher orbit by absorbing energy, and it returns to a lower orbit by emitting energy.

Teacher's Note:
a) The energy of each orbit is fixed.
b) The energy emitted or absorbed matches the difference between the two orbits.

44. 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: Most alpha particles would still pass straight through the foil, and a few would be deflected. The extent of deflection would differ because it depends on the positive charge of the nucleus of the metal used.

Teacher's Note:
a) The basic result, a tiny dense nucleus, does not depend on the metal.
b) A metal with a larger nuclear charge would deflect alpha particles more strongly.

45. Write a short note on Nucleus.

Answer: The nucleus is the small, dense, positively charged central part of an atom. It contains protons and neutrons and holds almost all the mass of the atom.

Teacher's Note:
a) The nucleus was discovered by Rutherford.
b) Its size is about one ten-thousandth of the size of the atom.

46. What is Atomic Number? Who coined this term?

Answer:
Atomic number is the number of protons present in the nucleus of an atom of an element. It was introduced by Henry Moseley.

Teacher's Note:
a) In a neutral atom, the atomic number is also the number of electrons.
b) The atomic number identifies the element.

47. What is mass number?

Answer: Mass number is the total number of protons and neutrons present in the nucleus of an atom.

Teacher's Note:
a) Mass number = number of protons + number of neutrons.
b) Electrons are not counted because their mass is negligible.

48. If A = 23 and Z = 11 for Na atom, how many protons, electrons and neutrons present in Na atom?

Answer:
1. Protons = Z = 11.
2. Electrons = 11, since the atom is neutral.
3. Neutrons = A - Z = 23 - 11 = 12.

Teacher's Note:
a) A is the mass number and Z is the atomic number.
b) Electrons equal protons in a neutral atom.
c) Neutrons are found by subtracting the atomic number from the mass number.

49. What is ionization energy?

Answer: Ionization energy is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom.

Teacher's Note:
a) Atoms with a low ionization energy form positive ions easily.
b) It is measured in electron volts or kilojoules per mole.

50. In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons. What can be the reason for it?

Answer: The two oxygen atoms are isotopes of oxygen. They have the same atomic number (8), so the same number of protons and electrons, but different mass numbers because they contain different numbers of neutrons.

Teacher's Note:
a) Isotopes of oxygen include oxygen-16, oxygen-17 and oxygen-18.
b) Isotopes have the same chemical behaviour, so both oxygen atoms bond in the same way.

51. In the atom of element X, 6 electrons are present in the outermost shell. If it requires an octet configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed?

Answer:
1. The outermost shell has 6 electrons, so it needs 8 - 6 = 2 electrons to complete the octet.
2. On accepting 2 electrons, the ion has 2 extra electrons.
3. The charge on the ion is -2, written as X²⁻.

Teacher's Note:
a) Gaining electrons gives a negative ion.
b) Each extra electron adds one unit of negative charge.

52. Why do helium, neon & argon have a zero valency?

Answer: Helium, neon and argon are noble gases. Their outermost shells are completely filled (a duplet for helium and an octet for neon and argon), so they are very stable and neither gain, lose nor share electrons. Their valency is therefore zero.

Teacher's Note:
a) Helium has 2 electrons in its only shell, which is a complete duplet.
b) Neon and argon have 8 electrons in their outermost shell.

53. Write the atomic number and the symbol of an element which has mass number 32 and the number of neutrons 16 in the nucleus.

Answer:
1. Atomic number = mass number - number of neutrons = 32 - 16 = 16.
2. The element with atomic number 16 is sulphur, with symbol S.
3. The atom is written as ³²₁₆S.

Teacher's Note:
a) Atomic number is the number of protons, here 16.
b) Sulphur is the element with atomic number 16.

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

Answer:
1. Mass number = 4.
2. Protons = 2.
3. Neutrons = 4 - 2 = 2.

Teacher's Note:
a) The mass of the atom is the mass number, which is the sum of protons and neutrons.
b) The helium nucleus has two protons and two neutrons.

55. 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? (ii) what is the charge on the atom?

Answer:
(i) Atomic number = number of protons = 8.
(ii) The charge on the atom is zero, since the numbers of protons and electrons are equal.

Teacher's Note:
a) The element with atomic number 8 is oxygen.
b) Equal numbers of protons and electrons give a neutral atom.

56. What are the limitations of J. J. Thomson's model of the atom?

Answer:
1. It could not explain the scattering of alpha particles through large angles.
2. It did not explain the stability of the atom or the arrangement of electrons.

Teacher's Note:
a) This is the same limitation as in Question 35.
b) Rutherford's experiment showed the nucleus, which Thomson's model lacked.

57. Why was Thomson's model of atom discarded and replaced by Rutherford's model? Why is Rutherford's model also called the nuclear model of atom

Answer:
1. Rutherford's alpha particle scattering experiment showed that the positive charge and most of the mass are concentrated in a small nucleus, which Thomson's model could not explain.
2. Rutherford's model is called the nuclear model because it places a small, dense, positively charged nucleus at the centre of the atom.

Teacher's Note:
a) Thomson's model predicted that alpha particles would be deflected only slightly.
b) The nucleus is the defining feature of Rutherford's model.

58. The average atomic mass of a sample of an element X is 16.2u. What are the percentage of isotopes ¹⁶₈X and ¹⁸₈X in the sample?

Answer:
1. Let the percentage of ¹⁶₈X be x, so the percentage of ¹⁸₈X is (100 - x).
2. Average atomic mass = [16 × x + 18 × (100 - x)] / 100 = 16.2.
3. 16x + 1800 - 18x = 1620, so -2x = -180, and x = 90.
4. Percentage of ¹⁶₈X = 90%, and percentage of ¹⁸₈X = 10%.

Teacher's Note:
a) Element X is oxygen, since its atomic number is 8.
b) The average of the two masses is weighted by abundance, so the heavier isotope has the smaller share.
c) Check: 0.9 × 16 + 0.1 × 18 = 16.2.

59. What are the salient features of Bohr's Atomic Model? How is it advantageous over Rutherford's Nuclear Model?

Answer:
1. Electrons revolve in fixed circular orbits called shells, and they do not radiate energy while in these orbits.
2. Energy is absorbed or emitted only when an electron jumps between orbits.
3. The model explains the stability of the atom and the line spectrum of hydrogen, which Rutherford's model could not explain.

Teacher's Note:
a) Bohr's model keeps Rutherford's nucleus but fixes the electron orbits.
b) The fixed orbits remove the problem of electrons falling into the nucleus.
c) Quantised energy levels are the key idea of the model.

60. What is the charge and mass of the anode rays emitted when, hydrogen gas is enclosed in the discharge tube experiment? What is the name of these particles?

Answer:
1. Charge = +1.6 × 10⁻¹⁹ coulomb (one unit of positive charge).
2. Mass = 1.67 × 10⁻²⁷ kg, which is the mass of a proton.
3. The particles are called protons.

Teacher's Note:
a) Hydrogen atoms lose their electron in the discharge tube, leaving a proton.
b) These positive particles are the anode rays of hydrogen.

61. Do the anode rays always consist of protons whatever be the gas enclosed in the discharge tube? Explain.

Answer: No. Anode rays are positively charged ions of the gas filled in the tube. Their mass depends on the gas. Protons are obtained only when hydrogen gas is used.

Teacher's Note:
a) Each gas gives positive ions of a different mass.
b) Hydrogen is the lightest gas, so its ions are the lightest anode rays.

62. What is the other name of X-rays?

Answer: Röntgen rays.

Teacher's Note:
a) They are named after Wilhelm Röntgen, who discovered them.
b) X-rays are high-energy electromagnetic waves.

63. How are x-rays produced? Why was this name given?

Answer:
1. X-rays are produced when fast-moving electrons (cathode rays) strike a metal target of high atomic mass in a discharge tube.
2. The name X was given because their nature was unknown at the time of discovery.

Teacher's Note:
a) "X" stands for unknown in mathematics and science.
b) Heavy metal targets such as tungsten are used to produce X-rays.

64. How are x-rays made tools in diagnostic purposes in the medical field?

Answer: X-rays pass easily through soft tissues but are absorbed by bones and denser parts. The shadow of bones on a photographic plate shows fractures, dental problems and other conditions, so X-rays are used in diagnosis.

Teacher's Note:
a) Photographic film records the X-ray shadow.
b) Over-exposure to X-rays can be harmful, so doses are kept low.

65. What is phosphorescence? How does it differ from fluorescence?

Answer:
1. Phosphorescence is the property of a substance to continue emitting light for some time after the source of radiation is removed.
2. Fluorescence is the property of a substance to emit light only while it is being exposed to radiation.
3. The difference is that phosphorescence lasts for a longer time after the exposure stops.

Teacher's Note:
a) Glow-in-the-dark stickers show phosphorescence.
b) Fluorescent tubes stop glowing as soon as they are switched off.

66. Who discovered the nuclear model of the atom?

Answer: Ernest Rutherford.

Teacher's Note:
a) Rutherford proposed the nuclear model in 1911.
b) It was based on the alpha particle scattering experiment.

67. Explain how Rutherford's atomic structure cannot explain the stability of atom?

Answer:
1. According to classical physics, an electron moving in an orbit around the nucleus is accelerated.
2. An accelerated electron radiates energy, so it loses energy continuously.
3. As it loses energy, the electron would spiral inwards and fall into the nucleus, which would make the atom collapse.

Teacher's Note:
a) The model predicted an unstable atom, which does not match observation.
b) Bohr solved this by stating that electrons in fixed orbits do not radiate energy.

68. Explain how Bohr's atomic model explains the emission and absorption of radiation.

Answer:
1. An electron in a fixed orbit has a fixed energy.
2. When it absorbs energy, it jumps to a higher orbit, and when it returns to a lower orbit it emits energy as radiation.
3. The energy emitted or absorbed equals the difference between the energies of the two orbits, which gives the characteristic lines in the spectrum.

Teacher's Note:
a) Absorption moves an electron outwards, and emission moves it inwards.
b) Because the orbit energies are fixed, only certain wavelengths appear.

69. What is the charge and mass of a β-particle? From which part of the atom are β-particles emitted?

Answer:
1. A β-particle is an electron, with charge -1.6 × 10⁻¹⁹ coulomb and mass 9.1 × 10⁻³¹ kg (about 1/1836 u).
2. It is emitted from the nucleus, when a neutron changes into a proton.

Teacher's Note:
a) Beta particles come from the nucleus, not from the electron cloud.
b) They are more penetrating than alpha particles.

70. The two elements, A and B have 17 electrons each in their atom. Element A has 35 nucleons while element B has 33 nucleons (nucleons are the total number of protons and neutrons in the nucleus). What is the relationship between these two elements? Explain. The element A reacts with hydrogen in diffused light. Does the element B also react with hydrogen?

Answer:
1. Both elements have 17 electrons, so both have atomic number 17 and the same electronic configuration (2, 8, 7).
2. Element A has mass number 35 and element B has mass number 33. Since they have the same atomic number but different mass numbers, they are isotopes of the same element.
3. Isotopes have the same electronic configuration, so element B also reacts with hydrogen in the same way as element A.

Teacher's Note:
a) Atomic number 17 identifies the element as chlorine.
b) Chemical reactions depend on electrons, not on the number of neutrons.

71. One isotope of carbon with atomic mass 12, occupies group 14 in the 2nd period in the long form periodic Table. Predict the position of another radioactive isotope of carbon with atomic mass 14?

Answer: Carbon-14 also occupies group 14 in period 2 of the long form periodic table.

Teacher's Note:
a) Position in the periodic table depends on the atomic number, which is 6 for all carbon isotopes.
b) Isotopes differ only in mass, so they share the same place.

72. What is the name given to the rays, traveling from cathode to anode?

Answer: Cathode rays.

Teacher's Note:
a) Cathode rays are streams of electrons.
b) They travel in straight lines from cathode to anode.

73. Do the cathode rays always consist of electrons only, whatever be the gas enclosed in the discourage tube?

Answer: Yes. Cathode rays always consist of electrons, whatever gas is enclosed in the discharge tube.

Teacher's Note:
a) The word "discourage" in the question is printed in place of "discharge".
b) Electrons are emitted from the cathode, so the gas inside does not change their nature.

74. Why are electrons in the outermost shell known as valence electrons?

Answer: Electrons in the outermost shell are called valence electrons because they take part in the formation of chemical bonds, and so they decide the valency of the atom.

Teacher's Note:
a) The word valence comes from the idea of combining power.
b) Inner shell electrons are held tightly and are not involved in bonding.