CBSE Class 9 Science Structure of atom Sure Shot Questions Set 03

CBSE Class 9 Science Structure of atom Sure Shot Questions C. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

ASSIGNMENT QUESTIONS SET - 3
CHAPTER - 4
STRUCTURE OF ATOMS

1. Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2

Answer: (b) 2, 8, 2

1. Magnesium has atomic number 12, so a neutral atom has 12 electrons.
2. Shells fill in the order K = 2, L = 8, M = remaining electrons.
3. Distribution = 2, 8, 2.
4. Hence, the correct option is (b).

Teacher's Note:
a) Fill K first with 2 electrons, then L with 8, and put the rest in M; options starting with 3, 1 or 8 break this order.
b) The last shell holds 2 electrons, so magnesium has valency 2.

2. Rutherford's 'alpha (α) particles scattering experiment' resulted in to discovery of
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass

Answer: (c) Nucleus in the atom

1. Most α-particles passed straight through the gold foil, and a few were deflected through large angles.
2. This showed that the positive charge and most of the mass are packed in a tiny central region.
3. That region is the nucleus. Hence, the correct option is (c).

Teacher's Note:
a) Electrons were found by J.J. Thomson and protons by Goldstein, so (a) and (b) do not belong to this experiment.
b) Link the result to the key observation: a very small number of α-particles bounced back.

3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
(a) ³¹₁₅X
(b) ³¹₁₆X
(c) ¹⁶₁₅X
(d) ¹⁵₁₆X

Answer: (a) ³¹₁₅X

1. In a neutral atom, number of protons = number of electrons = 15, so atomic number = 15.
2. Mass number = protons + neutrons = 15 + 16 = 31.
3. The representation puts the mass number on top and the atomic number at the bottom: ³¹₁₅X.
4. Hence, the correct option is (a).

Teacher's Note:
a) Superscript is the mass number (protons + neutrons) and subscript is the atomic number (protons).
b) Do not swap the two numbers; the larger number is always the mass number.

4. Dalton's atomic theory successfully explained
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)

Answer: (d) (i), (ii) and (iv)

1. Dalton's theory said atoms are indivisible and are neither created nor destroyed in a reaction, which explains the law of conservation of mass.
2. Atoms of a given element are identical and combine in fixed ratios, which explains the law of constant composition and the law of multiple proportion.
3. Radioactivity involves the breaking of atoms, which Dalton's indivisible atom cannot explain.
4. Hence, the correct option is (d).

Teacher's Note:
a) Radioactivity was discovered after Dalton and contradicts the idea of an indivisible atom.
b) Any option containing (iii) can be rejected at once.

5. Which of the following statements about Rutherford's model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the α-particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson's model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)

Answer: (a) (i) and (iii)

1. Statement (i) is correct: the nucleus carries the positive charge.
2. Statement (iii) is correct: electrons revolve around the nucleus like planets around the Sun.
3. Statement (ii) is not a feature of the model; the mass of an α-particle was already known.
4. Statement (iv) is wrong because Rutherford's model replaced Thomson's model.
5. Hence, the correct option is (a).

Teacher's Note:
a) Rutherford's model is called the nuclear or planetary model.
b) Do not choose (iv); the model was proposed because Thomson's model failed to explain the scattering results.

6. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)

Answer: (d) (ii) and (iv)

1. Mass number = protons + neutrons, so (ii) is true.
2. In a neutral atom, atomic number = protons = electrons, so (iv) is true.
3. Atomic number is only the number of protons, so (i) is false.
4. Protons and neutrons are not always equal, so (iii) is false.
5. Hence, the correct option is (d).

Teacher's Note:
a) Atomic number counts only protons; adding electrons is a common slip.
b) Neutrons equal protons only in some atoms, for example carbon-12, not in all.

7. In the Thomson's model of atom, which of the following statements are correct?
(i) the mass of the atom is assumed to be uniformly distributed over the atom
(ii) the positive charge is assumed to be uniformly distributed over the atom
(iii) the electrons are uniformly distributed in the positively charged sphere
(iv) the electrons attract each other to stabilise the atom
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)

Answer: (a) (i), (ii) and (iii)

1. Thomson pictured the atom as a positively charged sphere with electrons embedded in it, like seeds in a watermelon.
2. Mass and positive charge are spread uniformly over the sphere, and the electrons are spread uniformly inside it. So (i), (ii) and (iii) are correct.
3. Electrons repel each other; they are held in place by the positive sphere, so (iv) is wrong.
4. Hence, the correct option is (a).

Teacher's Note:
a) The model is also called the plum pudding or watermelon model.
b) Electrons carry like charges, so they repel and never attract each other.

8. Rutherford's α-particle scattering experiment showed that
(i) electrons have negative charge
(ii) the mass and positive charge of the atom is concentrated in the nucleus
(iii) neutron exists in the nucleus
(iv) most of the space in atom is empty
Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv)

Answer: (b) (ii) and (iv)

1. Most α-particles went straight through the foil, so most of the space inside an atom is empty - statement (iv).
2. A few were deflected or bounced back, so the mass and positive charge are concentrated in a tiny nucleus - statement (ii).
3. The negative charge of the electron was shown by Thomson and the neutron was discovered later by Chadwick, so (i) and (iii) do not come from this experiment.
4. Hence, the correct option is (b).

Teacher's Note:
a) Match each statement with the observation it needs; deflection means a nucleus, undeflected passage means empty space.

9. Identify the Mg²⁺ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively
(a)
(b)
(c)
(d)

[Figure: Fig. 4.1 shows four atomic models. (a) nucleus n = 12, p = 12 with three shells holding 2, 8 and 1 electrons; (b) nucleus n = 12, p = 10 with three shells; (c) nucleus n = 10, p = 12 with two shells holding 2 and 8 electrons; (d) nucleus n = 12, p = 12 with two shells holding 2 and 8 electrons]

Answer: (d)

1. Magnesium has atomic number 12, so every Mg atom or ion has 12 protons (p = 12).
2. Mg²⁺ has lost 2 electrons, so it has 12 - 2 = 10 electrons arranged as 2, 8 in two shells.
3. Figure (b) has only 10 protons, and figure (a) shows a three-shell atom; both are ruled out.
4. Figures (c) and (d) both have p = 12 and 10 electrons; the common magnesium atom has mass number 24, that is 12 neutrons, which matches figure (d) with n = 12.
5. Hence, the correct option is (d).

Teacher's Note:
a) Count protons first, then electrons; an ion differs from its atom only in the number of electrons.
b) The usual printed key for this question is (c), but magnesium-24 has 12 neutrons, so figure (d) is the better match; figure (c) shows n = 10.

10. In a sample of ethyl ethanoate (CH₃COOC₂H₅) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.

Answer: (c) The two oxygen atoms are isotopes

1. Same number of electrons means the same number of protons, that is the same atomic number.
2. Different number of neutrons means different mass numbers.
3. Atoms of the same element with the same atomic number but different mass numbers are isotopes.
4. Hence, the correct option is (c).

Teacher's Note:
a) Isotopes: same atomic number, different mass number. Isobars: same mass number, different atomic number.
b) Gaining electrons would change the charge of the atom, not its neutron count.

11. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16

Answer: (b) 10

1. Number of protons = mass number - neutrons = 27 - 14 = 13.
2. The neutral atom has 13 electrons.
3. The ion has 3 positive charges, so it has lost 3 electrons: 13 - 3 = 10.
4. Hence, the correct option is (b).

Teacher's Note:
a) A positive ion always has fewer electrons than protons; the number of protons never changes.
b) Option (a) is the number of electrons in the neutral atom (aluminium), a common trap.

12. Elements with valency 1 are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals

Answer: (c) either metals or non-metals

1. Sodium and lithium are metals with valency 1.
2. Hydrogen and chlorine are non-metals with valency 1.
3. So valency 1 occurs in both metals and non-metals.
4. Hence, the correct option is (c).

Teacher's Note:
a) One counter-example is enough to reject words such as "always".
b) Check both a metal and a non-metal before choosing.

13. The first model of an atom was given by
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson

Answer: (d) J.J. Thomson

1. The order of atomic models is Thomson, then Rutherford, then Bohr.
2. The first model of the atom, with electrons embedded in a positive sphere, was proposed by J.J. Thomson.
3. Hence, the correct option is (d).

Teacher's Note:
a) Remember the order T-R-B for Thomson, Rutherford, Bohr.

14. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4

Answer: (c) 1

1. 3 protons mean 3 electrons in the neutral atom.
2. Electron distribution = 2, 1.
3. The atom loses its single outer electron to attain the stable duplet, so valency = 1.
4. Hence, the correct option is (c).

Teacher's Note:
a) Valency depends on electrons in the outermost shell, not on the number of neutrons or the mass number.
b) The mass number 7 in option (b) is a distractor.

15. The electron distribution in an aluminium atom is
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8

Answer: (a) 2, 8, 3

1. Atomic number of aluminium = 13, so it has 13 electrons.
2. K shell takes 2, L shell takes 8, the remaining 13 - 10 = 3 go to the M shell.
3. Distribution = 2, 8, 3.
4. Hence, the correct option is (a).

Teacher's Note:
a) Total of the numbers must equal the atomic number: 2 + 8 + 3 = 13.
b) Inner shells are filled completely before outer ones.

16. Which of the following in Fig. 4.2 do not represent Bohr's model of an atom correctly?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)

[Figure: Fig. 4.2 shows four atoms, each with a central nucleus and two shells. (i) 2 electrons in the first shell and 5 in the second; (ii) 4 electrons in the first shell and 8 in the second; (iii) 2 electrons in the first shell and 8 in the second; (iv) 2 electrons in the first shell and 9 in the second]

Answer: (c) (ii) and (iv)

1. According to Bohr, the first shell holds a maximum of 2 electrons and the second shell a maximum of 8.
2. Figure (ii) has 4 electrons in the first shell, which is more than 2, so it is wrong.
3. Figure (iv) has 9 electrons in the second shell, which is more than 8, so it is wrong.
4. Figures (i) and (iii) obey the limits.
5. Hence, the correct option is (c).

Teacher's Note:
a) Check each shell against its limit 2n² (2 for K, 8 for L).
b) Counting dots carefully is the whole skill in this question.

17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.

Answer: (a) An atom has equal number of electrons and protons.

1. An atom is electrically neutral, so the positive charge of protons equals the negative charge of electrons.
2. Hence the numbers of electrons and protons are always equal.
3. Neutron number varies, for example ordinary hydrogen has none, so (b), (c) and (d) are not always true.
4. Hence, the correct option is (a).

Teacher's Note:
a) The word "always" is the clue; only the neutrality of the atom is guaranteed.

18. Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order
(i) Rutherford's atomic model
(ii) Thomson's atomic model
(iii) Bohr's atomic model
(a) (i), (ii) and (iii)
(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)

Answer: (c) (ii), (i) and (iii)

1. Thomson's model came in 1904.
2. Rutherford's model followed in 1911.
3. Bohr's model came in 1913.
4. Hence, the order is (ii), (i), (iii) and the correct option is (c).

Teacher's Note:
a) Each model was built to remove a defect of the earlier one.
b) The years 1904, 1911 and 1913 help to fix the order.

19. Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Answer: Yes, it is possible. The element is hydrogen. Its most common isotope, protium (¹₁H), has 1 proton, 1 electron and no neutron.

Teacher's Note:
a) Hydrogen is the only element whose common atom has no neutron.
b) Its other isotopes, deuterium and tritium, have 1 and 2 neutrons respectively.

20. Write any two observations which support the fact that atoms are divisible.

Answer:
1. J.J. Thomson found that cathode rays are streams of negatively charged particles (electrons) that come out of atoms, so atoms contain smaller particles.
2. Goldstein found canal rays (positively charged particles, later called protons), and Rutherford's α-particle scattering showed a nucleus inside the atom; both prove that an atom has parts.

Teacher's Note:
a) Dalton said atoms cannot be divided; the discovery of electrons and protons disproved this.
b) Name the discoverer along with each observation to earn full marks.

21. Will ³⁵Cl and ³⁷Cl have different valencies? Justify your answer.

Answer:
1. No, they will not have different valencies.
2. ³⁵Cl and ³⁷Cl are isotopes of chlorine, both with atomic number 17.
3. Both have the same electron distribution 2, 8, 7, so both have valency 1.

Teacher's Note:
a) Valency depends on electrons, not on neutrons.
b) Isotopes differ only in mass number, so their chemical properties are identical.

22. Why did Rutherford select a gold foil in his α-ray scattering experiment?

Answer:
1. Gold is highly malleable, so it can be beaten into an extremely thin foil, about one atom thick in places, which lets α-particles pass through.
2. Gold atoms are heavy and have a large positive nucleus, so the deflection of α-particles is easy to observe.

Teacher's Note:
a) A thick foil would stop the α-particles and no scattering pattern would be seen.
b) Mention both thinness and the heavy nucleus of gold.

23. Find out the valency of the atoms represented by the Fig. 4.3 (a) and (b).

[Figure: Fig. 4.3 shows two atoms. (a) 2 electrons in the first shell, 8 in the second shell and 8 in the third shell; (b) 2 electrons in the first shell and 6 in the second shell]

Answer:
1. Atom (a) has the distribution 2, 8, 8. Its outermost shell is complete with 8 electrons, so its valency is 0.
2. Atom (b) has the distribution 2, 6. It needs 8 - 6 = 2 more electrons to complete its octet, so its valency is 2.

Teacher's Note:
a) Valency is the number of electrons gained, lost or shared to reach a full outermost shell.
b) Atom (a) is argon, an inert gas; atom (b) is oxygen.

24. What information do you get from the Fig. 4.4 about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form.

[Figure: Fig. 4.4 shows three atoms. X: nucleus 5 p and 6 n, shells with 2 and 3 electrons; Y: nucleus 8 p and 10 n, shells with 2 and 6 electrons; Z: nucleus 15 p and 16 n, shells with 2, 8 and 5 electrons]

Answer:
1. Atomic number = number of protons, and mass number = protons + neutrons.
2. Valency is found from the outermost shell: X has 3 outer electrons, Y has 6 and Z has 5.

Teacher's Note:
a) Read the numbers from the nucleus labels and the outer shell from the drawing.
b) For 5 or more outer electrons, valency = 8 - number of outer electrons.

25. In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

Answer:
1. No, I do not agree with the statement.
2. In a neutral atom the number of protons is always equal to the number of electrons, so protons cannot be greater than electrons.
3. The number of neutrons is equal to or greater than the number of protons in most atoms, so the order stated by the student is wrong.

Teacher's Note:
a) Atoms are neutral, so protons = electrons.
b) Hydrogen has no neutron and many heavier atoms have more neutrons than protons.

26. Calculate the number of neutrons present in the nucleus of an element X which is represented as ³¹₁₅X

Answer:
1. Mass number = 31 and atomic number = 15.
2. Number of neutrons = mass number - atomic number = 31 - 15 = 16.

Teacher's Note:
a) Neutrons = A - Z.
b) The element is phosphorus.

27. One electron is present in the outer most shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outer most shell?

Answer:
1. The atom is neutral with equal protons and electrons.
2. Removing 1 electron leaves one proton more than electrons.
3. So the ion is positively charged, a cation, with a charge of +1 (X⁺).

Teacher's Note:
a) Loss of electrons gives positive ions; gain of electrons gives negative ions.
b) Metals such as sodium behave in this way.
c) The size of the charge equals the number of electrons lost.

28. Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17).

Answer:
1. Chlorine has 17 electrons.
2. Electron distribution = K 2, L 8, M 7, that is 2, 8, 7.
3. The L shell has 8 electrons.

Teacher's Note:
a) The L shell is the second shell and is full with 8 electrons.
b) The outer shell has 7 electrons, so chlorine easily gains 1 electron.

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

Answer:
1. A stable outermost shell holds 8 electrons.
2. The atom needs 8 - 6 = 2 more electrons.
3. Gaining 2 electrons gives 2 extra negative charges, so the ion is X^2- with a charge of -2.

Teacher's Note:
a) Gain of electrons produces a negative ion (anion).
b) Oxygen and sulphur are examples with 6 outer electrons.

30. Match the names of the Scientists given in column A with their contributions towards the understanding of the atomic structure as given in column B

Answer:
(a) Ernest Rutherford - (iii) Concept of nucleus
(b) J.J.Thomson - (iv) Discovery of electrons
(c) Dalton - (i) Indivisibility of atoms
(d) Neils Bohr - (ii) Stationary orbits
(e) James Chadwick - (vi) Neutron
(f) E. Goldstein - (vii) Canal rays
(g) Mosley - (v) Atomic number

Teacher's Note:
a) Learn each scientist with one key word: nucleus, electron, indivisible, orbits, neutron, canal rays, atomic number.
b) Rutherford and Bohr are often confused; Rutherford gave the nucleus and Bohr gave the fixed orbits.

31. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?

Answer:
1. Calcium and argon have different atomic numbers (20 and 18) but the same mass number (40).
2. Atoms of different elements with the same mass number are called isobars.

Teacher's Note:
a) Isobars: same mass number, different atomic number. Isotopes: same atomic number, different mass number.

32. Complete the Table 4.1 on the basis of information available in the symbols given below
(a) ³⁵₁₇Cl   (b) ¹²₆C   (c) ⁸¹₃₅Br

Answer:
1. Number of protons n_p = atomic number and number of neutrons n_n = mass number - atomic number.
2. For Cl: n_p = 17 and n_n = 35 - 17 = 18.
3. For C: n_p = 6 and n_n = 12 - 6 = 6.
4. For Br: n_p = 35 and n_n = 81 - 35 = 46.

Teacher's Note:
a) The subscript of the symbol gives the protons; subtract it from the superscript to get neutrons.
b) Check: protons + neutrons gives back the mass number in each row.

33. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain.

Answer:
1. Helium has only one shell, the K shell, and this shell can hold a maximum of 2 electrons.
2. With 2 electrons the K shell is already complete, so helium neither gains, loses nor shares electrons.
3. Hence its valency is zero, not 2.

Teacher's Note:
a) Valency is the combining capacity needed to complete the outermost shell, not simply the number of valence electrons.
b) Helium is stable with a duplet, whereas other noble gases have an octet.

34. Fill in the blanks in the following statements
(a) Rutherford's α -particle scattering experiment led to the discovery of the ______
(b) Isotopes have same ______but different______.
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be______and______respectively.
(d) The electronic configuration of silicon is ______and that of sulphur is ______.

Answer:
(a) nucleus
(b) Isotopes have same atomic number but different mass number.
(c) Neon (2, 8) has valency 0 and chlorine (2, 8, 7) has valency 1; the valencies will be 0 and 1 respectively.
(d) The electronic configuration of silicon (14 electrons) is 2, 8, 4 and that of sulphur (16 electrons) is 2, 8, 6.

Teacher's Note:
a) Write the electron configuration first, then read off the valency.
b) In (c), a full outer shell means valency 0.

35. An element X has a mass number 4 and atomic number 2. Write the valency of this element?

Answer:
1. Atomic number 2 means 2 protons and 2 electrons, so the element is helium.
2. Its electron distribution is 2, which completes the K shell.
3. Hence the valency of X is 0.

Teacher's Note:
a) Mass number does not decide valency; only the electron arrangement does.

36. Why do Helium, Neon and Argon have a zero valency?

Answer:
1. Helium has the distribution 2, neon 2, 8 and argon 2, 8, 8.
2. In each of them the outermost shell is completely filled (a duplet for helium and an octet for neon and argon).
3. They have no tendency to gain, lose or share electrons, so their valency is zero.

Teacher's Note:
a) A complete outermost shell means a very stable, unreactive atom.
b) These elements belong to the group of noble or inert gases.

37. The ratio of the radii of hydrogen atom and its nucleus is ~ 10⁵. Assuming the atom and the nucleus to be spherical, (i) what will be the ratio of their sizes? (ii) If atom is represented by planet earth 'R_e' = 6.4 × 10⁶ m, estimate the size of the nucleus.

Answer:
1. (i) Size of a sphere is taken as its radius, so the ratio of the sizes of atom and nucleus = 10⁵ : 1.
2. (ii) Radius of the nucleus = radius of the atom ÷ 10⁵.
3. Radius of the nucleus = 6.4 × 10⁶ m ÷ 10⁵ = 6.4 × 10¹ m = 64 m.
4. So the nucleus would be about 64 m across in radius, the size of a small stadium.

Teacher's Note:
a) Divide the earth's radius by the given ratio to scale down to the nucleus.
b) The powers in the printed question were missing from the PDF text and are restored as 10⁵ and 10⁶.
c) The answer shows how much empty space an atom has.

38. Enlist the conclusions drawn by Rutherford from his α -ray scattering experiment.

Answer:
1. Most of the space inside an atom is empty, because most α-particles passed straight through the gold foil.
2. Very few particles were deflected by large angles, so the positive charge of the atom occupies a very small volume.
3. A very small number of particles bounced back, so the atom has a tiny, dense and positively charged centre called the nucleus.
4. The size of the nucleus is very small compared with the size of the atom, and almost all the mass of the atom is in the nucleus.

Teacher's Note:
a) Link each observation to its conclusion: undeflected means empty space, rebound means a dense nucleus.
b) The electrons move around the nucleus in circular paths, which Rutherford also stated.
c) Mention that the nucleus is positively charged.
d) Four distinct points earn full credit here.

39. In what way is the Rutherford's atomic model different from that of Thomson's atomic model?

Answer:
1. Thomson's model has a positively charged sphere with electrons embedded in it and has no nucleus; Rutherford's model has a small, dense, positively charged nucleus at the centre.
2. In Thomson's model the positive charge and mass are spread uniformly; in Rutherford's model they are concentrated in the nucleus.
3. In Thomson's model electrons are fixed in the sphere; in Rutherford's model electrons revolve around the nucleus in circular orbits.

Teacher's Note:
a) The main difference is the nucleus.
b) Thomson's model explains neutrality, while Rutherford's explains the scattering results.
c) Use a two-column difference in the exam if the question asks for a table.

40. What were the drawbacks of Rutherford's model of an atom?

Answer:
1. According to Rutherford, electrons revolve around the nucleus in circular orbits.
2. A revolving charged particle is accelerated and, by electromagnetic theory, should radiate energy continuously.
3. The electron would lose energy and spiral into the nucleus, so the atom would collapse; but atoms are stable.
4. The model could not explain the stability of the atom.

Teacher's Note:
a) The key word is stability.
b) Bohr's model fixed this by proposing stationary orbits in which electrons do not radiate energy.
c) State that the electron would fall into the nucleus.

41. What are the postulates of Bohr's model of an atom?

Answer:
1. Only certain special orbits, known as discrete orbits of electrons, are allowed inside the atom.
2. While revolving in these discrete orbits, the electrons do not radiate energy.
3. These orbits or shells are called energy levels and are represented by the letters K, L, M, N or by the numbers n = 1, 2, 3, 4.
4. An electron can move from one orbit to another only by absorbing or emitting a fixed amount of energy.

Teacher's Note:
a) Bohr's orbits are also called stationary orbits.
b) Link postulate 2 to the drawback of Rutherford's model.
c) Name the shells K, L, M, N with n = 1, 2, 3, 4.
d) Capacity of a shell is 2n², so 2, 8, 18 and 32.

42. Show diagramatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Answer:
1. The atomic number of sodium is 11, and it is the same for the sodium atom and the sodium ion, because the number of protons does not change.
2. Sodium atom (Na): 11 protons and 11 electrons, electron distribution 2, 8, 1 as in the first diagram.
3. Sodium ion (Na⁺): it loses the single outer electron, so it has 11 protons and 10 electrons, electron distribution 2, 8 as in the second diagram.

Sodium atom (Na): atomic number 11, electron distribution 2, 8, 1

Sodium ion (Na⁺): atomic number 11, electron distribution 2, 8

Teacher's Note:
a) The atomic number is fixed by protons, so it does not change when an ion forms.
b) Show the third shell empty or absent in Na⁺.
c) Na⁺ has the same electron arrangement as neon.
d) Label the nucleus with the number of protons.

43. In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford's model of an atom, ~ 1.00% of the α -particles were found to deflect at angles > 50°. If one mole of α -particles were bombarded on the gold foil, compute the number of α -particles that would deflect at angles less than 50°.

Answer:
1. One mole of α-particles = 6.022 × 10²³ particles.
2. About 1.00% deflect at angles greater than 50°, so the remaining 99.00% deflect at angles less than 50°.
3. Number of particles = 99% of 6.022 × 10²³ = 0.99 × 6.022 × 10²³ = 5.96 × 10²³.

Teacher's Note:
a) Subtract the 1% from 100% to get the share that deflects by smaller angles.
b) One mole always contains 6.022 × 10²³ particles (Avogadro number).
c) Keep three significant figures in the final answer.

Please click the link below to download CBSE Class 9 Science Structure of atom Sure Shot Questions C.