SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Journey Inside the Atom

33 questions · 19 still being checked

Revise, Reflect, Refine 8.11–8.15 (part 4 of 4)

  1. Exercise 8.11

    An atom 70\displaystyle 70  X has 31\displaystyle 31 electrons. How many neutrons are there in its nucleus?
    NCERT’s answer
    Neutrons $\displaystyle 39$
    $\displaystyle 39$ neutrons.
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-11
    The atom is neutral, so protons \(\displaystyle =\) electrons \(\displaystyle = 31\), giving \(\displaystyle Z = 31\).
    The superscript is the mass number, \(\displaystyle A = 70\).
    \(\displaystyle \text{neutrons} = A - Z = 70 - 31 = 39\)

  2. Exercise 8.12

    An atom has 79\displaystyle 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
    NCERT’s answer
    (i)
    Neutrons $\displaystyle 118$ (ii) Electrons $\displaystyle 79$
    (i) $\displaystyle 118$ neutrons (ii) $\displaystyle 79$ electrons
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-12
    Protons \(\displaystyle = 79\), so \(\displaystyle Z = 79\); mass number \(\displaystyle A = 197\).
    (i) \(\displaystyle \text{neutrons} = A - Z = 197 - 79 = 118\)
    (ii) The atom is electrically neutral, so electrons \(\displaystyle =\) protons \(\displaystyle = 79\).
    (\(\displaystyle Z = 79\) is gold, symbol Au from the Latin aurum.)

  3. Exercise 8.13

    Complete the Table 8.5\displaystyle 8.5: Atomic number 5\displaystyle 5 - - 15\displaystyle 15 -
    NCERT’s answer
    Atomic Mass Number of Number of Number of Name of the number number neutrons protons electrons element $\displaystyle 5$ $\displaystyle 11$ $\displaystyle 6$ $\displaystyle 5$ $\displaystyle 5$ Boron $\displaystyle 7$ $\displaystyle 14$ $\displaystyle 7$ $\displaystyle 7$ $\displaystyle 7$ Nitrogen $\displaystyle 12$ $\displaystyle 24$ $\displaystyle 12$ $\displaystyle 12$ $\displaystyle 12$ Magnesium $\displaystyle 15$ $\displaystyle 31$ $\displaystyle 16$ $\displaystyle 15$ $\displaystyle 15$ Phosphorus $\displaystyle 1$ $\displaystyle 1$ $\displaystyle 0$ $\displaystyle 1$ $\displaystyle 1$ Hydrogen
    Completed table:
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-13
    Atomic numberMass numberNeutronsProtonsElectronsElement
    $\displaystyle 5$$\displaystyle 11$$\displaystyle 6$$\displaystyle 5$$\displaystyle 5$Boron
    $\displaystyle 7$$\displaystyle 14$$\displaystyle 7$$\displaystyle 7$$\displaystyle 7$Nitrogen
    $\displaystyle 12$$\displaystyle 24$$\displaystyle 12$$\displaystyle 12$$\displaystyle 12$Magnesium
    $\displaystyle 15$$\displaystyle 31$$\displaystyle 16$$\displaystyle 15$$\displaystyle 15$Phosphorus
    $\displaystyle 1$$\displaystyle 1$$\displaystyle 0$$\displaystyle 1$$\displaystyle 1$Hydrogen
    Two relations do all the work: \(\displaystyle Z = \text{protons} = \text{electrons}\) in a neutral atom, and \(\displaystyle A = \text{protons} + \text{neutrons}\).
    Row $\displaystyle 1$ — \(\displaystyle Z = 5\) gives $\displaystyle 5$ protons and $\displaystyle 5$ electrons; \(\displaystyle A = 5 + 6 = 11\); \(\displaystyle Z = 5\) is boron.
    Row $\displaystyle 2$ — $\displaystyle 7$ electrons gives $\displaystyle 7$ protons and \(\displaystyle Z = 7\); \(\displaystyle \text{neutrons} = 14 - 7 = 7\); this confirms nitrogen.
    Row $\displaystyle 3$ — $\displaystyle 12$ protons gives \(\displaystyle Z = 12\) and $\displaystyle 12$ electrons; \(\displaystyle \text{neutrons} = 24 - 12 = 12\); \(\displaystyle Z = 12\) is magnesium.
    Row $\displaystyle 4$ — \(\displaystyle Z = 15\) gives $\displaystyle 15$ protons and $\displaystyle 15$ electrons; \(\displaystyle A = 15 + 16 = 31\); \(\displaystyle Z = 15\) is phosphorus.
    Row $\displaystyle 5$ — \(\displaystyle A = 1\) with $\displaystyle 0$ neutrons leaves $\displaystyle 1$ proton, so \(\displaystyle Z = 1\) and $\displaystyle 1$ electron; this is hydrogen, the only atom whose nucleus has no neutron.

  4. Exercise 8.14

    Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35\displaystyle 35 and contains 18\displaystyle 18 neutrons. Based on this information, answer the following questions: (i) How many electrons and protons does element X have? (ii) What is its atomic number? (iii) Identify the element X. (iv) Write its electronic configuration. (v) How many valence electrons does it have? (vi) What will be the mass number if two neutrons are added to its nucleus? (vii) What will be the relation of X with the new atom?
    NCERT’s answer
    (i)
    Electrons $\displaystyle 17$; Protons $\displaystyle 17$ (ii) Atomic number $\displaystyle 17$ (iii) Chlorine (iv) Electronic configuration $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 7$ (v) Valence electrons $\displaystyle 7$ (vi) Mass number $\displaystyle 37$ (vii) Isotopes
    (i) $\displaystyle 17$ electrons and $\displaystyle 17$ protons
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-14
    \(\displaystyle \text{protons} = A - \text{neutrons} = 35 - 18 = 17\); the atom is neutral, so electrons \(\displaystyle =\) protons \(\displaystyle = 17\).
    (ii) Atomic number \(\displaystyle = 17\) — the atomic number is simply the number of protons.
    (iii) X is chlorine (Cl) — the atomic number identifies the element, and \(\displaystyle Z = 17\) is chlorine.
    (iv) Electronic configuration: $\displaystyle 2$, $\displaystyle 8$, $\displaystyle 7$ — K takes $\displaystyle 2$, L takes $\displaystyle 8$, and the remaining \(\displaystyle 17 - 10 = 7\) go into M.
    (v) $\displaystyle 7$ valence electrons — the outermost shell is M, and it holds 7.
    (vi) New mass number \(\displaystyle = 37\) — adding neutrons does not change the proton count: \(\displaystyle A = 17 + (18 + 2) = 37\).
    (vii) The two are isotopes — same atomic number $\displaystyle 17$, different mass numbers $\displaystyle 35$ and 37. Being isotopes, they have the same electronic configuration and therefore the same chemical properties.

  5. Exercise 8.15

    In an atom, there are 12\displaystyle 12 protons and 12\displaystyle 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500\displaystyle 500 times heavier. What effect will this replacement have on the atom’s: (i) Atomic number (ii) Atomic mass (iii) Mass number (iv) Overall charge

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    (i) Atomic number: unchanged, still 12.
    NCERT_Solution_Class9_Science_Ch8_RRR_Q8-15
    The atomic number counts protons in the nucleus, and the nucleus was not touched — the substitution happened outside it.
    (ii) Atomic mass: it increases — the atom becomes measurably heavier.
    Normally the $\displaystyle 12$ electrons can be ignored in mass calculations because an electron's mass is almost negligible.
    Here the $\displaystyle 12$ replacement particles together carry \(\displaystyle 12 \times 500 = 6000\) times an electron's mass, which is no longer something you may throw away.
    (iii) Mass number: unchanged, still 24.
    Mass number counts nucleons only: \(\displaystyle A = \text{protons} + \text{neutrons} = 12 + 12 = 24\). Particles outside the nucleus never enter this count, however heavy they are.
    (iv) Overall charge: unchanged — the atom is still neutral, total charge zero.
    The hypothetical particles carry the same charge as electrons, so $\displaystyle 12$ of them still cancel the \(\displaystyle +12\) of the protons exactly.
    Charge and mass are independent properties: making a particle heavier does not make it more negative.