SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Journey Inside the Atom

33 questions · 19 still being checked

Pause and Ponder 8.14–8.15 (part 21 of 23)

  1. Exercise 8.14

    Identify the number of electrons in the outermost shell of the following elements: (i) 12\displaystyle 12 6\displaystyle 6 C (ii) 19\displaystyle 19 9\displaystyle 9 F (iii) 28\displaystyle 28 14\displaystyle 14 Si
    NCERT’s answer
    (i)
    $\displaystyle 4$ (ii) $\displaystyle 7$ (iii) $\displaystyle 4$
    (i) $\displaystyle 4$ (ii) $\displaystyle 7$ (iii) $\displaystyle 4$
    NCERT_Solution_Class9_Science_Ch8_PP_Q8-14
    Shells fill from the nucleus outward — K first (maximum \(\displaystyle 2\times1^{2}=2\)), then L (maximum \(\displaystyle 2\times2^{2}=8\)), then M.
    (i) Carbon, \(\displaystyle Z = 6\): $\displaystyle 2$ electrons in K, the remaining $\displaystyle 4$ in L → configuration $\displaystyle 2$, $\displaystyle 4$ → outermost shell has $\displaystyle 4$ electrons.
    (ii) Fluorine, \(\displaystyle Z = 9\): $\displaystyle 2$ in K, the remaining $\displaystyle 7$ in L → configuration $\displaystyle 2$, $\displaystyle 7$ → outermost shell has $\displaystyle 7$ electrons.
    (iii) Silicon, \(\displaystyle Z = 14\): $\displaystyle 2$ in K, $\displaystyle 8$ in L, the remaining $\displaystyle 4$ in M → configuration $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 4$ → outermost shell has $\displaystyle 4$ electrons.
    The subscript is the atomic number, which equals the number of electrons in a neutral atom; the superscript (mass number) plays no part here.

  2. Exercise 8.15

    Write the electronic configuration of the elements having atomic numbers 12\displaystyle 12, 16\displaystyle 16 and 18.
    NCERT’s answer
    $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 2$; $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 6$; $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 8$
    \(\displaystyle Z = 12\): $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 2$
    NCERT_Solution_Class9_Science_Ch8_PP_Q8-15
    \(\displaystyle Z = 16\): $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 6$
    \(\displaystyle Z = 18\): $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 8$
    Rule used: a shell holds at most \(\displaystyle 2n^{2}\) electrons — K \(\displaystyle = 2\times1^{2} = 2\), L \(\displaystyle = 2\times2^{2} = 8\), M \(\displaystyle = 2\times3^{2} = 18\) — and shells are filled in order K, L, M.
    \(\displaystyle 12 = 2 + 8 + 2\) (magnesium), \(\displaystyle 16 = 2 + 8 + 6\) (sulfur), \(\displaystyle 18 = 2 + 8 + 8\) (argon).
    For \(\displaystyle Z = 18\) the M shell stops at $\displaystyle 8$, not $\displaystyle 18$, because the outermost shell can never hold more than $\displaystyle 8$ electrons — argon therefore has a complete octet.