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NCERT Exemplar · Class 9 Science Structure of the Atom

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Multiple Choice Questions 11–18 (part 2 of 5)

  1. Exercise 11

    In a sample of ethyl ethanoate (CH3\displaystyle CH_{3}COOC2\displaystyle COOC_{2}H5\displaystyle H_{5}) 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.

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    NCERT’s answer
    (c)
    (c) Equal electrons \(\displaystyle \Rightarrow\) equal protons \(\displaystyle \Rightarrow\) the same element; differing only in neutron number is the definition of isotopes, not isobars (equal mass number) or an electron/neutron gain. Answer: (c)

  2. Exercise 12

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

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    NCERT’s answer
    (c)
    (c) either metals or non-metals. \[\text{Na}(2,8,1) \xrightarrow{-1e^-} \text{Na}^+(2,8) \] \[\text{Cl}(2,8,7) \xrightarrow{+1e^-} \text{Cl}^-(2,8,8) \] Both reach a stable octet by exchanging just one electron -- one a metal, one a non-metal.

  3. Exercise 13

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

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    NCERT’s answer
    (d)
    (d) J.J. Thomson. His plum-pudding model ($\displaystyle 1904$) preceded Rutherford's nuclear model ($\displaystyle 1911$) and Bohr's model ($\displaystyle 1913$).

  4. Exercise 14

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

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    NCERT’s answer
    (c)
    (c) 1. \[Z=p=3 \Rightarrow \text{Li: shells } 2,1 \] $\displaystyle 3$ protons fix the element as lithium; its single outer-shell electron gives valency 1. NCERT_Solution_Class9_Science_Exemplar_Ch4_Q14

  5. Exercise 15

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

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    NCERT’s answer
    (a)
    (a) $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 3$\[Z_{\text{Al}} = 13 \] \[2n^2:\ K_{\max}=2,\ L_{\max}=8,\ M_{\max}=18 \] \[K=2,\quad L=8,\quad M = 13-2-8 = 3 \]NCERT_Solution_Class9_Science_Exemplar_Ch4_Q15Shells fill K then L to capacity before the remainder goes to M.

  6. Exercise 16

    Which of the following in Fig. 4.2\displaystyle 4.2 do not represent Bohr’s model of an atom
    correctly?
    (iii)
    (iv)
    (i)
    (ii)(a)
    (i)
    and (ii)
    (ii)
    and (iii)
    (ii)
    and (iv)
    (i)
    and (iv)
    NCERT_Question_Class9_Science_Exemplar_Ch4_Q16

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    NCERT’s answer
    (c)
    (c) (ii) and (iv)
    (ii)
    's first shell holds $\displaystyle 4$ electrons — above the \(\displaystyle 2n^2=2\) K-shell limit. (iv)'s second shell holds $\displaystyle 9$ electrons — above the \(\displaystyle 2n^2=8\) L-shell limit. (i) and (iii) fill each shell within its limit, in order.
    Answer: (c) (ii) and (iv)

  7. Exercise 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.

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    NCERT’s answer
    (a)
    (a) An atom has equal number of electrons and protons.A neutral atom's electrons always equal its protons (atomic number); neutron count varies between isotopes, so (b)–(d) fail.Answer: (a)

  8. Exercise 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
    (i)
    , (ii) and (iii)
    (ii)
    , (iii) and (i)
    (ii)
    , (i) and (iii)
    (iii)
    , (ii) and (i)

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    NCERT’s answer
    (c)
    (c) (ii), (i) and (iii)\[\text{Thomson (1904)} \rightarrow \text{Rutherford (1911)} \rightarrow \text{Bohr (1913)} \]Discovery dates fix the order: Thomson's model came first, Rutherford's nuclear model next, Bohr's shell model last.