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NCERT Exemplar · Class 9 Science Structure of the Atom

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Multiple Choice Questions 1–10 (part 1 of 5)

  1. Exercise 1

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

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    NCERT’s answer
    (b)
    (b) $\displaystyle 2$, $\displaystyle 8$, 2. \[Z_{\text{Mg}} = 12 \] \[\text{Shell capacity } 2n^2 \Rightarrow K,L,M \le 2,8,18 \] \[K=2,\ L=8,\ M=12-2-8=2 \] Bohr–Bury filling gives shells $\displaystyle 2$, $\displaystyle 8$, 2. NCERT_Solution_Class9_Science_Exemplar_Ch4_Q1

  2. Exercise 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

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    NCERT’s answer
    (c)
    (c) Nucleus in the atom. \(\displaystyle \text{few } \alpha \text{ bounce back} \Rightarrow \text{small, dense, positively charged core}\) Most particles passed straight through the empty atom.

  3. Exercise 3

    The number of electrons in an element X is 15\displaystyle 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?
    (a)
    1531X\displaystyle ^{31}_{15}\text{X}
    (b)
    1631X\displaystyle ^{31}_{16}\text{X}
    (c)
    1516X\displaystyle ^{16}_{15}\text{X}
    (d)
    1615X\displaystyle ^{15}_{16}\text{X}

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    NCERT’s answer
    (a)
    (a) \(\displaystyle ^{31}_{15}\text{X}\). \[Z = e^{-} = 15 \] \[A = Z + n = 15 + 16 = 31 \] Answer: (a) \(\displaystyle ^{31}_{15}\text{X}\).

  4. Exercise 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
    (i)
    , (ii) and (iii)
    (i)
    , (iii) and (iv)
    (ii)
    , (iii) and (iv)
    (i)
    , (ii) and (iv)

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    NCERT’s answer
    (d)
    (d) (i), (ii) and (iv). Dalton's theory rests on conservation of mass, constant composition and multiple proportions; radioactivity needs sub-atomic particles, which his indivisible-atom idea denied.

  5. Exercise 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
    (i)
    and (iii)
    (ii)
    and (iii)
    (i)
    and (iv)
    (d)
    only (i)

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    NCERT’s answer
    (a)
    (a) (i) and (iii). Positive charge sits in a central nucleus with electrons revolving around it, like planets round the Sun; the model directly contradicted, not agreed with, Thomson's model.

  6. Exercise 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
    (i)
    and (ii)
    (i)
    and (iii)
    (ii)
    and (iii)
    (ii)
    and (iv)

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    NCERT’s answer
    (d)
    (d) (ii) and (iv). \[A = p+n \] \[Z = p = e^- \] Atomic number is the proton (= electron) count, never a sum; mass number is the proton+neutron sum.

  7. Exercise 7

    In the Thomson’s model of atom, which of the following statments are correct?
    (i)
    the mass of the atom is assumed to be uniformaly distributed over the atom
    (ii)
    the positive charge is assumed to be uniformaly distributed over the atom
    (iii)
    the electrons are uniformaly distributed in the positively charged sphere
    (iv)
    the electrons attract each other to stabilise the atom
    (i)
    , (ii) and (iii)
    (i)
    and (iii)
    (i)
    and (iv)
    (i)
    , (iii) and (iv)

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    NCERT’s answer
    (a)
    (a) (i), (ii) and (iii). Thomson pictured the atom as a uniformly positive sphere of the atom's mass with electrons embedded evenly inside it; electrons repel, not attract, each other.

  8. Exercise 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?
    (i)
    and (iii)
    (ii)
    and (iv)
    (i)
    and (iv)
    (iii)
    and (iv)

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    NCERT’s answer
    (b)
    (b) (ii) and (iv). \(\displaystyle \text{no deflection} \Rightarrow \text{atom mostly empty space (iv)}\) \(\displaystyle \text{rare large-angle bounce} \Rightarrow \text{mass, +charge concentrated in nucleus (ii)}\) The experiment says nothing about electron charge or neutrons.Answer: (b) (ii) and (iv).

  9. Exercise 9

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

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    NCERT’s answer
    (b)
    (b) 10. \[p = A-n = 27-14 = 13 \] \[e^-_{\text{ion}} = p - \text{charge} = 13-3 = 10 \] Losing $\displaystyle 3$ electrons from a neutral $\displaystyle 13$-electron atom leaves 10.

  10. Exercise 10

    Identify the Mg2+\displaystyle Mg^{2+} ion from the Fig.4.1 where, n and p represent the number
    of neutrons and protons respectively
    (a)
    (b)
    (d)
    (c)
    NCERT_Question_Class9_Science_Exemplar_Ch4_Q10

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    NCERT’s answer
    (d)
    (d) Neutral Mg (\(\displaystyle Z=12\), mass number $\displaystyle 24$) has \(\displaystyle p=12,\ n=12\), electrons \(\displaystyle 2,8,2\). \(\displaystyle \text{Mg}^{2+}\) loses its $\displaystyle 2$ valence electrons: \(\displaystyle p=12,\ n=12\) unchanged, electrons now \(\displaystyle 2,8\) — only (d) pairs these p, n values with a $\displaystyle 10$-electron, two-shell ion. Answer: (d)