SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Journey Inside the Atom

33 questions · 19 still being checked

Revise, Reflect, Refine 8.8–8.9 (part 14 of 23)

  1. Exercise 8.8

    Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24\displaystyle 24 and atomic number 12\displaystyle 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
    NCERT’s answer
    (i)
    Protons $\displaystyle 12$ (ii) Neutrons $\displaystyle 12$ (iii) Electrons $\displaystyle 12$; Electronic configuration $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 2$
    (i) protons \(\displaystyle = 12\) (ii) neutrons \(\displaystyle = 12\) (iii) electrons \(\displaystyle = 12\)
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-8
    (i) Atomic number \(\displaystyle Z = 12\), and \(\displaystyle Z\) is the number of protons → $\displaystyle 12$ protons.
    (ii) \(\displaystyle \text{neutrons} = A - Z = 24 - 12 = 12\) → $\displaystyle 12$ neutrons.
    (iii) A magnesium atom is electrically neutral, so electrons \(\displaystyle =\) protons \(\displaystyle = \) $\displaystyle 12$ electrons.
    Electronic configuration: fill K first (maximum \(\displaystyle 2\times1^{2} = 2\)), then L (maximum \(\displaystyle 2\times2^{2} = 8\)), and the $\displaystyle 2$ left over go into M → $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 2$.
    What your drawing must contain:
    a small circle at the centre for the nucleus, labelled inside with "$\displaystyle 12$ p, $\displaystyle 12$ n";
    three concentric circles drawn around it, labelled K (n = $\displaystyle 1$), L (n = $\displaystyle 2$) and M (n = $\displaystyle 3$), with K nearest the nucleus;
    $\displaystyle 2$ dots on the K circle, $\displaystyle 8$ dots on the L circle, $\displaystyle 2$ dots on the M circle — check that \(\displaystyle 2 + 8 + 2 = 12\);
    the $\displaystyle 2$ electrons on the outermost M shell marked as the valence electrons;
    a title, "Magnesium, \(\displaystyle ^{24}_{12}\text{Mg}\)".

  2. Exercise 8.9

    Find the following information for the elements shown in Fig. 8.17\displaystyle 8.17: (i) Name of the element (ii) Symbol (iii) Total number of electrons (iv) Number of valence electrons (v) Valency of the element (vi) Number of protons (vii) Atomic number (a)NCERT_Question_Class9_Science_Ch8_RRR_Q8-9
    NCERT’s answer
    (i)
    Lithium; (ii) Li; (iii) $\displaystyle 3$; (iv) $\displaystyle 1$; (v) $\displaystyle 1$; (vi) $\displaystyle 3$; (vii) $\displaystyle 3$ (b) (i) Nitrogen; (ii) N; (iii) $\displaystyle 7$; (iv) $\displaystyle 5$; (v) $\displaystyle 3$; (vi) $\displaystyle 7$; (vii) $\displaystyle 7$ (c) (i) Aluminium; (ii) Al; (iii) $\displaystyle 13$; (iv) $\displaystyle 3$; (v) $\displaystyle 3$; (vi) $\displaystyle 13$; (vii) $\displaystyle 13$ (d) (i) Fluorine; (ii) F; (iii) $\displaystyle 9$; (iv) $\displaystyle 7$; (v) $\displaystyle 1$; (vi) $\displaystyle 9$; (vii) $\displaystyle 9$
    The four shell diagrams in Fig. $\displaystyle 8.17$ carry $\displaystyle 3$, $\displaystyle 7$, $\displaystyle 13$ and $\displaystyle 9$ electrons respectively, and everything asked follows from those counts.
    (a) Lithium — (i) lithium; (ii) Li; (iii) $\displaystyle 3$ electrons, arranged $\displaystyle 2$, $\displaystyle 1$; (iv) $\displaystyle 1$ valence electron; (v) valency $\displaystyle 1$; (vi) $\displaystyle 3$ protons; (vii) atomic number $\displaystyle 3$.
    (b) Nitrogen — (i) nitrogen; (ii) N; (iii) $\displaystyle 7$ electrons, arranged $\displaystyle 2$, $\displaystyle 5$; (iv) $\displaystyle 5$ valence electrons; (v) valency $\displaystyle 3$; (vi) $\displaystyle 7$ protons; (vii) atomic number $\displaystyle 7$.
    (c) Aluminium — (i) aluminium; (ii) Al; (iii) $\displaystyle 13$ electrons, arranged $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 3$; (iv) $\displaystyle 3$ valence electrons; (v) valency $\displaystyle 3$; (vi) $\displaystyle 13$ protons; (vii) atomic number $\displaystyle 13$.
    (d) Fluorine — (i) fluorine; (ii) F; (iii) $\displaystyle 9$ electrons, arranged $\displaystyle 2$, $\displaystyle 7$; (iv) $\displaystyle 7$ valence electrons; (v) valency $\displaystyle 1$; (vi) $\displaystyle 9$ protons; (vii) atomic number $\displaystyle 9$.
    How the valencies were fixed: fewer than four valence electrons → the atom loses them (Li loses $\displaystyle 1$, so valency $\displaystyle 1$; Al loses $\displaystyle 3$, so valency $\displaystyle 3$).
    More than four valence electrons → the atom gains enough to complete its octet (N gains \(\displaystyle 8 - 5 = 3\), so valency $\displaystyle 3$; F gains \(\displaystyle 8 - 7 = 1\), so valency $\displaystyle 1$).
    Protons and atomic number are equal to the electron count in each case, because a neutral atom has as many protons as electrons.